EXPLANATION- Form 4
Chapter 2: Force and Motion
Implusive Force
In two separate boxing ring, a kick-boxer Thailand fights without wearing boxing gloves whereas a boxer from Malaysia fights with a pair of boxing gloves. Explain who will cause more injury to their opponent if both boxers punch with the same force. (3 marks)
Answer:
- Thai boxer
- Without wearing boxing gloves, the time for change momentum is shorter.
- The impulsive force is larger.
Gravitational Force
Explain why it is easier to walk on a rough surface than on an oily surface (4 marks)
Answer:
- A rough surface produce more friction.
- Therefore better grip and less slipping between the soles and the surface. Thus easier to walk.
- An oily surface reduces contact between the soles and the surface no opposing force will act
- Therefore difficult to walk.
Energy
A stone which released from a tall building falls down in the air. During the fall, the stone experiences an energy change. Explain the energy changes in the stone. (4 marks)
Answer:
- Total energy at any time is constant
- As the stone falls, gravitational potential energy convert to kinetic energy
- Gravitational potential is maximum at the highest point
- Kinetic energy converts to heat and sound when it hits the ground.
Elasticity
Explain how the forces between the molecules cause the elasticity
Answer:
- The forces between the molecules are attractive forces and repulsive forces
- When the solid is stretched, the molecules to be displaced away from each other
- Attractive intermolecular forces act to oppose the stretching
- When the solid is compressed, the molecules are being displaced closer to each other. Repulsive forces are acting to oppose the compression.
Chapter 3: Force and Pressure
Atmospheric pressure
Explain why places at high altitude the temperature is colder compare to places at low altitude which is more warmer.
Answer:
- high altitude low density of air
- less collision of molecules with surface
- low altitude high density of air
- more collision of molecules with surface
- if more collision of molecules with surface, heat energy are more produced. So that low collision is more warmer.
Pressure in Liquid
A small bubble of air is released from the bottom of a lake.
(i) Explain what happens to the size of bubble as it rises to the surface of the water.
[2 marks]
(ii) Explain how this change affects the upward motion of the bubble.
[2 marks] Answer:
(i) - The pressure of water increases with the depth of the water
- The bubble expands upon reaching the surface of the water//The volume of air bubble increases as the depth of water decreases
(ii) - Buoyant force increases as the volume of the bubble increases
- The air bubble moving with increasing acceleration
Siphon
Explain how a siphon works to transfer liquid from one container to another. (4 marks)
Answer:
- The siphon works based on pressure difference
- Air pressure in the inner tube is less than the atmospheric pressure outside the tube.
- The atmospheric pressure pushes the surface of liquid
- Causes the liquid to flow into the column of the tube. (to the other side of the tube into the other container)
Buoyant Force
1. Explain why a ship may sink when it sails from sea water to fresh water. ( 3 marks)
Answer:
1. In the sea, buoyant force > weight of the ship
2. When the ship in the river, density of water< density of sea, so buoyant force decrease
3. The weight of the ship > the buoyant force ,so the ship will sink
2. Explain why a balloon filled with helium gas rises up in the air. (4 marks)
Answer:
- The balloon acted by two forces: upthrust and the weight of the balloon
- The density of helium gas is less than the density of surrounding air.
- Upthrust equal to the weight of the air displaced by the balloon
- Upthrust is higher than the weight of the balloon.
5. Explain why the sinking levels of the boats are different in the river and in the sea.
[3 marks]
Answer:
- Sea water is denser
- Boat displaced less sea water and gain the same upthrust .
- Therefore boat sinks less in sea water
6. The submarine is able to submerge in the sea.
Explain how a submarine on the sea surface submerges and float still on the sea surface.
Answer:
- When the ballast tank is filled with water,
- The buoyant force is smaller than the weight of the submarine. It will
sink/submerges
- When the ballast tank is filled with air / when the water pumps out,
- the buoyant force is larger than the weight of the submarine, and it will float.
Hydrometer
A hydromater is used to measure the density of acid a car battery. Using a suitable physisc principle, explain how the hyromater works. (4marks)
Answer:
- related principle is the Archimedes’ Principle.
- Draw a diagram of hyrometer correctly ( small and long stem, with big bulb with lead or steell balls)
- When a hydromater floats in a liquid, the buoyant force is equal to the weight of the hydromater
- Bouyant force = rVg / depends on density of liquid
- The smalleer the density of the liquid, the more liquid it displaces and the deeper the hydromater will sink
Bernoulli Principle
The aircraft obtains its lift when flying due to a difference in pressure of the air between the upper and bottom sides of the wings. Explain how this difference in pressure is produced.
[3 marks]
Answer:
- The shape of cross section of the wing causes the speed of airflow
- Above the wings to be higher than the speed of airflow below
- When the speed of moving air is higher, the pressure is lower
- Hence air pressure below the wings is higher compare to above the wings
Pascal Principle
Using a concept of pressure in liquid, explain why the hydraulic system in this machine is known as a force multiplier. (4 marks)
Answer:
1. When the small piston is pulled up, the hydraulic oil is drawn from the reservoir into the small piston
2. When the small piston is pushed down , the hydraulic oil is exerted with force and experienced a pressure
3. The pressure is transmitted uniformly from the small piston to the bigpiston.
4. The forced produced raised the big piston / The system can convert a small input force into a bigger output force.
Chapter 4: Heat
Thermal equilibrium
1. Explain how a new thermometer can be calibrated. [4 marks]
Answer:
- Put the thermometer in melting ice, mark the lower part of mercury thread,l0
- Place the same thermometer in the boiling water, mark the top part of the mercury thread, l 100
- Divide the length between the two marks into 100 equal divisions
- Each division is now equal to 1 o C
2. According to the principle of thermal equilibrium and the working principle of a thermometer, explain how a doctor can check his patient temperature during medical treatment. [ 4 mark ]
Answer:
- Thermometer is placed in the mouth of patient,
- Heat is transferred from patient’s body to the thermometer.
- Thermal equilibrium between the thermometer and patient’s body is reached when the net rate of heat transfer is zero.
- The thermometer and the patient’s body are at the same temperature. The thermometer reading shows the temperature of the patient’s body.
Specific heat capacity
1. Explain why water is normally used as a cooling agent or cooling system in a car engine radiator. ( 4 marks)
Answer:
- It doesn’t react to the inner part of the engine
- it take a long time to heat up water because high specific heat capacity
- can absorb large amount of heat energy
- cheap and easily available
Specific Latent Heat
(i) Your body sweats when you are feeling hot. How does sweating helps to cool down your body? [ 2 marks ]
(ii) Explain why a scald from steam is more serious than the one from boiling water?
[ 2 marks ]
Answer:
(i) - Water evaporates from the skin when we sweat.
- for water to evaporate it use heat from the body
(ii) - Steam has more heat than boiling water
- due to latent heat of vaporization
A student standing near the beach could feel the cool breeze from the sea on a hot day. Explain how the cool breeze from the sea occurs. [3 marks ]
Answer:
- The increase in temperature for land is higher
- The air above land is heated up
- Cooler air from the sea moves towards land
Chapter 5:Light
Reflection of light
Optical Fibre
(ii) With the help of a diagram, explain the working principle of an optical fibre. [4 marks]
Answer:
- Diagram shows a curved optical fibre (with outer and internal layer) with at least two total internal reflection
- The refractive index of the internal layer of the optical fibre is greater than that of the external layer or air
- Angle of incidence, i > critical angle, c
- Total internal reflection occur
Describe how you estimate the focal length of a convex lens. [4 marks]
Answer:
- The convex lens with its holder is aimed at a distant object like a tree.
- Adjust a screen until you see a very sharp and clear image of the distant
object.
- When parallel light rays from a distant object pass through a convex lens, they
will converge to a point called focal point/ label on a diagram
- Measure the distance between the convex lens and screen./ label on a diagram
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