Quick Revision

Physical units and quantities

1.State the property of a vector that distinguishes it from a scalar.
A vector quantity contains both magnitude and direction while a scalar quantity contains magnitude only.

2. Suggest a method to measure accurately the diameter of a wire using a metre rule to measure.
Wind the wire closely around a pencil for 100 rounds.
The diameter of the wire can be found more accurately by dividing the length of the wire windings (measured using the metre rule) by 100 rounds.

3. Describe how it is possible to produce, from 2 forces of 5 N, (i) a resultant of 0 N,
(ii) a resultant of 10 N, (iii) a resultant of 5 N.
(i) The 2 forces of 5N acts at opposite direction to produce a resultant of 0 N.
(ii) The 2 forces of 5N acts in the same direction to produce a resultant of 10 N.
(iii) The 2 forces of 5N at at a direction 120° to each other.
(The resultant vector diagram should form an equilateral triangle.)

Kinematics

1. An athlete runs at a 100 m race at an average speed of 8 m/s.
Explain why, for part of the race, the athelete must have been running faster than 8 m/s.
Since the athlete starts from rest, at certain part of the race, he must be running less than 8 m/s.
Hence, to clock an average speed of 8 m/s, he must have run faster than
8 m/s at certain part of the race.

2. Explain why a box is travelling at constant velocity even though it is pushed by a constant forward force.
The forward force exerted on the box is equal to friction and air resistance opposing the forward force.
Hence, resultant force = 0 N and the box moves with zero acceleration
(constant velocity),

3. A car travels round a circular track at a constant speed of 10 m/s. (i) Explain why the car’s velocity is not constant.(ii) Explain whether the car is accelerating.
(i) Since velocity is defined as speed in a specified direction, the car’s velocity is not constant as direction of the car is constantly changing.
(ii) Acceleration is defined as the rate of change of velocity. Since velocity of the car is changing with time (as direction of car is constantly changing), the car is accelerating.

4. Explain why happens to the motion of an object dropped from rest from a height of 200 m.
The object will first free fall at constant acceleration due to gravity.
As the object free falls, air resistance acting against the object builds up. The object hence drops with decreasing acceleration till it approaches constant (terminal) velocity, where the air resistance acting on object is equal to its weight.
The object will fall at terminal velocity till it hits the ground.
(Note: Decreasing acceleration does not meant that speed is decreasing. It just meant that speed is increasing at a slower rate.)

5. Describes what happens when an astronaut in moon drops a feather and a coin from a height of 10m.
The feather and the coin will both land on the ground at the same time. As air resistance is absent, both objects will fall at constant acceleration due to the gravitational field of the moon.

6. A ball is thrown vertically upwards. State the magnitude and
direction of (i) velocity, (ii) acceleration of the ball at its highest point.
At the highest point, the velocity of the ball is zero.
The acceleration of the ball is 10 m/s2 acting downwards (due to gravity).

7. A car is travelling along a straight road with constant power provided by the car engine. Explain why the acceleration of the car decreases while its speed increases.
As the car travels along the road, air resistance acting against the car’s motion increases while the driving force of car provided by the engine remains constant. This causes the resultant force of the car to decrease. Hence, the acceleration of the car decreases while its speed increases.
(Note: This will continue till the acceleration of car becomes zero and the car will travel at constant speed.)

8. describe the motion of bodies with constant weight falling with or without air resistance, including reference to terminal velocity
Without air resistance (in vacuum)
all objects , whether heavy or light, drop with same acceleration, g. g = 10m/s2 near surface of Planet Earth. Object increases speed with uniform acceleration of 10m/s-2
With air resistance
as object falls in air, speed increases, Air resistance increases. Resultant force acting downwards = weight – air resistance force. Resultant force decreases, acceleration of object decreases
 When the air-resistance = weight, resultant force = 0N, acceleration = 0 ms-2. object stops accelerating and maintain at top speed , called terminal velocity.

Dynamics

1.An object is pushed at constant velocity by a force of 10 N on a straight rough ground.State the value of the frictional force acting on the object. Give an explanation for your answer.
Frictional force = 10 N (opposite to the push force).
As the object is moving at constant velocity, the resultant force of the object is 0 N (Fresultant = ma, a = 0 m s-2). Hence, frictional force have the same magnitude as the push force, but acts in the opposite direction.

2. The wheel of a moving car is driven by the engine. The car is accelerating in the direction shown.

In which direction does the frictional force acts on the wheel. Give an explanation for your answer.
The frictional force acts towards the left (as shown in diagram) on the wheel.
The wheel exerts a force acting on the right direction on the ground. Hence friction force exerts an equal and opposite reaction force acting on the wheel. (Newton’s third law).
3. Explain why a person standing in a bus falls forward when the bus stops suddenly.
The person falls forward due to inertia, which is a tendency for the person to continue in its state of motion even though the bus has already stopped.

Mass, weight, density

1. Describe and explain how the density of a small cube of metal changes with the increase of temperature.
As temperature increases, the volume of the small cube of metal increases due to expansion. Since density = mass/volume and mass of the metal cube remains constant, density of the metal cube decreases as temperature increases.

2. Explain why a beam balance, not a spring balance, is able to measure the mass of an object accurately, both on earth and on moon.
A beam balance compares the mass of the object on one side of the beam balance with standard known masses on the other side of the beam balance. Since gravity acts on both sides of the balance, its effect is cancelled.
A spring balance takes into account the effects of gravity, hence it will give different readings on earth and on the moon.

3. The figure above shows the molecular structure of gas (left) and solid (right) of the same substance.
Explain why the density of gas is lower than solid.
For a given volume, there are more molecules in solid than in gas. Hence the mass of solid for a given volume is higher than that of gas. Since density = mass/volume, hence density of gas is lower than solid.

Moments

Video to explain moments


1. Paint cans are usually opened by using a lever. For example, a screw driver may be used. Explain briefly, with the aid of a diagram, how the use of the screw driver enables a large force to be applied to the lid.
As the perpendicular distance between the effort on the screw driver
and the pivot is much longer than the perpendicular distance between the load (lid) and pivot, a small effort exerted produces a big moment (Moment = force x perpendicular distance) on the lid and hence a large force applied to the lid to open it.

2. The diagram below shows two similar objects on a flat table.
Using concepts of moments, explain why object B is more stable than object A.
Object B has a lower centre of gravity and a wider base compared to object A.
When tilted, the weight exerted by object B will create a moment about the point of tilt to stabilize object B.

However, when object A is tilted, the weight exerted by object A will
create a moment to cause object A to collapse.

3 How do you balance a book on one finger. Explain your answer.
By placing the one finger on the centre of gravity of the book to support the book, it is possible to balance a book on one finger.
As the weight of the book acts on the pivot (finger), there is no moments created about the pivot. Hence the book becomes balanced.

Work power energy

1. A ball dropped from a height onto a table bounces repeatedly.
Explain why the height of the bounce decreases after each bounce.
Energy is lost to the table and air as heat and sound energy upon each impact with the table. The ball hence has lesser mechanical energy
(GPE + KE) upon each bounce and the height of the bounce reduces after each bounce.

2a Describe the energy changes which take place when a pendulum is oscillating.
When the pendulum is oscillating, it possesses maximum gravitational potential energy when it swings to its highest point, the potential energy will be converted to kinetic energy as the pendulum swings from the highest point to the lowest point, where it possesses maximum
kinetic energy. The kinetic energy will then be converted back to potential energy as it swings back to its highest point and this cycle continues.

2b A pendulum oscillating will eventually stop moving. Explain what has happened to the energy of the pendulum.
As the pendulum oscillates, the mechanical energy of the pendulum is converted to heat due to work done against air resistance. Hence the mechanical energy which consist of gravitational potential and kinetic energy of the pendulum will get lesser till the pendulum stops.

Pressure

1. Explain each of the following.
(a) When you do your handstand, the pressure exerted on your hand is greater the pressure on your feet when you stand upright.
- The contact area on the hands is smaller than the area on the feet.
- Since the weight exerted is the same, the pressure exerted on your hands when you do a handstand is greater as pressure = force / area.

(b) A sharp knife cuts meat more easily than a blunt one.
- The contact area of the blade of a sharp knife on the meat is smaller
compared to a blunt knife.
- Less force is needed to create the same pressure
(pressure = force / area), hence it is easier to cut with a sharp knife.

2. Explain how a small downward force exerted at piston L will create a large upward force at piston K.

As liquid is incompressible, the downward pressure exerted by the small downward force at piston L is equal to the upward pressure transmitted to piston K.
Since pressure = force / area, and the area of piston K is larger than that of piston L, a small downward force exerted at piston L will create a large upward force at piston K.

3Water of depth 10 m exerts a pressure equal to atmospheric pressure. An air bubble rises to the surface of a lake which is 20 m deep. When the bubble reaches the surface, its volume is 6 cm3. What was the volume of the air bubble at the bottom of the lake?
At the surface:
Pressure, P1 = 1 atm (atmospheric pressure equivalent to 10m of water), V1 = 6cm3

At the bottom of the lake:
pressure, P2 = 3 atm (atmospheric pressure + 20 m of water), V2 = unknown
Using Bolye's Law, P1V1 = P2V2
V2 = 2 cm3

4The diagram below shows the relative equilibrium positions of a piston, of negligible mass, in a container of air, at different temperatures.
Using the kinetic model of matter,
(i) explain why the piston at 25 °C does not sink to the bottom of the container.

The particles of air collide with the surface of the container, as well as the surface of the piston, and hence exerts a upward force on the piston. This prevents it from sinking to the bottom.
(ii) calculate the pressure of the gas at 25 °C
Since the piston is of negligible mass, it exerts negligible force on the air in the container.
Since the piston is in equilibrium,
pressure of gas = pressure of atmosphere
= 1.0 х 105 Pa

(iii) calculate the pressure exerted by the gas at 50 °C
Since the piston is still in equilibrium,
pressure of gas = pressure of atmosphere
= 1.0 х 105 Pa

(iv) explain the reason the piston rises to a greater equilibrium height at 50 °C
As the temperature is greater, the kinetic energy of the particles is greater. This increases the pressure of the gas, hence causing a net upwards force on the piston, causing the piston moves upwards.
As the piston moves upwards, the volume of the air increases, causing a corresponding reduction in the gas pressure according to Boyle's Law (P
1V1 = P2V2). This continues until equilibrium is reached, when pressure of the gas equals the pressure of the atmosphere.

Kinetic theory of matter

1. Explain how the brownian motion experiment relates to kinetic theory of matter.
- Brownian motion describes about the bombardment of smoke particles
(visible through microscope) by randomly moving air particles.
-It shows evidence that molecules move in random motion continuously.
-It also demonstrates that when air molecules are heated, they become more vigourous and energetic in their movement. (As shown by more vigourous movement of smoke particles.)
-Hence, it shows that when temperature increases, the air molecules have more kinetic energy.

2. Using kinetic theory of matter, explain why the temperature of a substance remains constant when the substance is melting.
-When a substance is melting, heat (known as latent heat) is used to break down the forces of attraction between the molecules as it changes state from solid to liquid.
-The molecules of the substance vibrate at the same speed during melting.
-Since the kinetic energy of the substance remains constant, and kinetic energy of molecules is proportional to temperature, the temperature (known as melting point) of substance remains constant when it is melting.

3. Using kinetic theory of matter, explain why the temperature of a substance remains constant when the substance is freezing.
-When a substance is freezing, heat is released by the substance to form back the forces of attraction between the molecules as it changes state from liquid to solid.
-The molecules of the substance vibrate at the same speed during freezing.
-Since the kinetic energy of the substance remains constant, and kinetic energy of molecules is proportional to temperature, the temperature of substance remains constant when it is freezing.

4. Using kinetic theory of matter, explain the process of evaporation.
-During process of evaporation, faster moving molecules at the surface of the liquid (which absorbed more energy from the surroundings), gain enough energy to break the forces of attraction, causing them to be gaseous particles.
-The average kinetic energy of the liquid hence lowers as the faster moving molecules leave the liquid when they evaporate.
-Hence, the overall temperature of the liquid lowers.

5. Explain, using kinetic theory of matter, the structure and movement of molecules of solids, liquids and gases.
-Solid molecules are closely packed together in a regular pattern, The molecules vibrate about fixed positions, held in position by strong intermolecular bonds.
-Liquid molecules are randomly arranged with the particles slightly further apart compared to solids. The molecules slide among one another but are contained within the vessel containing them due to attractive forces between molecules.
-Gas molecules are very far apart and move about randomly at very high speed. They have very little attraction between them.

6.A sealed flask contains a gas.
Describe the motion of the gas molecules.
The gas molecules move at a high speed in random directions.
Explain how the motion of the gas molecules results in a pressure exerted by the gas on the walls of the flask.
The moving molecules bombard all parts of the wall of the flask. This bombardment causes a force on the wall, and hence a pressure on the wall. The pressure of the gas is dependent on the frequency of collisions and the speed of the molecules.
Explain what is meant by Brownian motion.
Brownian motion is the random movement of light particles within a fluid. An example is smoke particles in air.

7The figure below shows a metal can with a tight fitting lid. The can is heated.
Use the simple kinetic theory of gases to answer the following:
(a) What happens to the pressure in the can when the temperature increases?
The pressure increases since the molecules gain kinetic energy and move faster, making more collisions per unit area per second with the walls of the container.
(b) If a little water is put into the bottom of the can before heating, explain how this would affect the pressure in the can when the temperature increases?The pressure would be greater as there will be more gas molecules due to the vapourisation of water. In the can, there will thus be higher frequency of collisions per unit area of the walls, and this results in the higher pressure.

Heat Transfer

1. What is the difference between heat and temperature?
Heat is the thermal energy that will flow from a high temperature body to a low temperature body. Temperature is the degree of hotness / coldness of an object.

2. Explain, in terms of molecular movement, why metals are better conductors than other solids.
-Metals are better conductors than other solids as they conduct heat through molecular vibration and free electron diffusion.
-Heat is transferred through metal when the kinetic energy of the heated molecule due to its vibration is passed from one molecule to the neighbouring molecules when they collide with each other.
-In addition, metals contain free electrons, which gains kinetic energy quickly when heated and diffuses quickly to other parts of the metal, increasing the speed of heat transfer greatly.

3. Explain how a thick fur coat is able to keep a person warm in cold weather.
-A thick fur coat traps air, which is a good insulator.
-Hence, there is less transfer of body heat to the surroundings. The person is able to feel warm.

4. Which feels warmer – walking on wooden floor or on a marble floor?
Explain.
-Walking on wooden floor feels warmer.
-As wood is a poor conductor, there is less transfer of body heat from the soles of the person walking to the ground, compared to walking on marble, which is a better conductor of heat.
5. One end of a copper bar and one end of an iron bar of the same size were placed side by side in a fire. The heated end of the iron bar became red hot. The heated end of the copper bar did not become red hot. What can be deduced from these observations? Which rod could be removed by the hand with less risk of being burnt at the opposite end?
-Copper is a better conductor of heat. The copper transfers heat energy away from the heated end more quickly and so the heat energy is less concentrated at the heated end which is why it doesn’t become red hot.
-We can remove the iron rod with less risk of being burnt. This is because most of the heat energy is concentrated at the heated end of the iron rod compared with the copper rod.

6. Should air conditioners be placed high or low in a room? Explain.
-They should be placed high in a room. As hot air rises (as it is less dense), cold air sinks, the air conditioner, placed high,will remove heat from the hot air.
-This will cause the rising air to cool down and sink, causing the air at the bottom to rise.
-The process continues to form efficient convection currents to cool the entire room quickly.

7. Explain how land and sea breezes are formed.
-During the day, as land is a better conductor of heat than sea, the air above land is heated up more quickly, causing it to rise (less dense).
-The air from the sea quickly rushes in towards the land to form sea breezes. This cycle repeats to create a convection current system.
-During the night, the land loses heat more quickly than the sea. The opposite effect occurs and air from the land rushes in towards the sea to form land breezes.

8. Explain, through heat transfer processes, how a pot of cold water placed on a hot stove is heated to a temperature of 80 °C.
-Water at the bottom of the pot is first heated up due to heat transfer from hot stove to pot through conduction.
-The hot water particles, which are less dense, rises, to be replaced by cold water particles that sinks.
-This process repeats to form convection currents till the water is heated to a temperature of 80 °C.

9. What colour are the cooling fins at the back of refrigerators painted?Why?
-Black.
-Black coloured cooling fins are good radiators and are able to emit radiant heat from the refrigerator better, causing the refrigerator to cool down faster.
(Note: Cooling fins also are designed to have bigger surface area to encourage more heat loss through radiation.)
10. In countries where the outdoor temperature is high, why is it more advisable to get a silver car, rather than a black car?
Silver surface is a poor absorber of infra-red radiation compared to black surface.
This reduces the heat received by the car from the sun’s infra red radiation and keeps the car as cool as possible.

Thermal properties of matter

1. Explain what is meant by the internal energy of a body.
Internal energy of a body is the combination of total kinetic energy (due to molecular motion) and potential energy (due to intermolecular forces) of the molecules in a body.

2.Describe, qualitatively, the different phrases of a heating curve when ice becomes steam.

From -20°C to 0 °C, the particles gain heat and vibrate at fixed positions with greater speed. Kinetic energy of the particles increases, potential energy of particles remains the same.


At 0 °C, heat is taken in by ice to break the intermolecular forces of attraction, causing it to melt to form liquid. Temperature remains constant . Kinetic energy of particles (i.e. speed of particles is constant during melting.) remains constant. Potential energy of the particles increases.


From 0 °C to 100 °C , the liquid molecules will slide among each other within a fixed volume with greater speed as liquid water gains heat to cause the temperature of liquid water to increase to100 °C. Kinetic energy of the particles increases, potential energy of particles remains the same.


At 100 °C, heat is taken in by water to break the intermolecular forces of attraction and push back on the surrounding atmosphere, causing it to boil to form steam. Temperature remains constant . Kinetic energy of particles (i.e. speed of particles is constant during melting.) remains constant. Potential energy of the particles increases.


From 100 °C, the particles then move freely and randomly without fixed boundaries with increasing speed as it gains heat. Kinetic energy of the particles increases, potential energy of particles remains the same.

3. Being burnt by steam at 100 °C is more serious than being burnt by hot water at 100°C. Explain why.
-When steam condensed on our skin, it will release latent heat (of vaporisation).
-This will cause the burn to be more serious compared to being burnt by hot water at 100 °C.

4. State the differences between boiling and evaporation.

5. State the factors that will affect
(a) melting and boiling point of a substance, (b) rate of evaporation of a liquid
(a) Adding impurities (such as salt) to the substance. This raises boiling point and lowers melting point.
Changing the pressure applied to the substance. Increasing pressure raises boiling point and lowers melting point.

(b) Surface area, temperature of the substance, wind, relative humidity, pressure and boiling point of liquid.

6. Why do you feel cooler after perspiration evaporates off your body?
-During the process of evaporation, the liquid molecules (perspiration) take in heat from the body in order to change to gaseous state.
-Hence when the perspiration evaporates, heat is removed from the body, making you feel more cooler.

Static electricity

1The figure below shows a person walking on a carpet and approaching a metal door which is earthed.

(a) It is found that the carpet on which the person has walked on carries positive charges. Explain briefly how these charges are formed.

(b) What kind of electric charges can be found on the body of the person?

(c) When the person is close to the metal door, sketch the distribution of electric charges on the person and the metal door in the following figure.
(d) What happens when the person touches the metal door? Explain briefly.
(a) When the person's shoes rubbed against the carpet, there is friction. Electrons are transferred from the carpet to the person. The person has an excess of electrons and is negative.
(b) Negative.

(c)


(d) The negative charges on the person's body will be discharge through the door, which is earthed. The person will feel a slight and short electric shock.

Practical electricity
1. What is a short circuit?


When the terminals of an electrical source are connected with wires without any
electrical component in the circuit, a very large current will flow through the circuit
as the wires are of very low resistance. This is called a short circuit.

2To which wire are switches, fuses and circuit breakers connected?To the live wire.

Explain why switches, fuses and circuit breakers are connected to the live wire, and not
to the neutral wire.
The live wire is at a high voltage, whereas the neutral wire is at 0 V.


If the above components are connected to the live wire, and in the event that the
switch is opened, or fuse blown, or the circuit breaker turned off, the circuit will be
completely disconnected from the high voltage.
If, however, the components are connected to the neutral wire, no current would flow
when the switch is off. However, the circuit is still “live”. Thus, a person will be
electrocuted if he touches the live wire, because a current will then flow from the live
wire, through his body, and down to earth which is at 0 V.
 4 A typical lighting circuit consists of bulbs connected in parallel. Give three advantages
that a parallel circuit has over a series circuit.
a) All the bulbs in a parallel circuit can function independently. On the other hand, all
the bulbs in a series circuit must be switched on or off at the same time.
b)If any one bulb in a parallel circuit becomes faulty, the others can still function,
unlike in the case of a series circuit.
c)Each bulb in a parallel circuit will be able to receive its operational voltage from
the mains supply, thus enabling it to light up to its normal brightness. On the other
hand, each bulb in a series circuit may not receive its operational voltage if there
are excessive numbers of bulbs connected.