Momentum PPQs Flashcards
Both ends of the nail exert pressure when the nail goes into the wood. Explain why the nail exerts more pressure on the wood than it does the hammer. (2 marks)
- pressure = force/area
- pressure is inversely proportional to area
- the forces on the wood and hammer are equal
- therefore, since a smaller area of the nail is in contact with the wood, it exerts more pressure on the wood
The skater wears soft knee pads that compress easily. Explain how the pads protect her knees when she falls on ice. (3 marks)
- force is the rate of change in momentum
- knee pads increase the time of impact
- this means the same change in momentum
- which results in reduced force on the knee
Some cars have a pedestrian airbag for safety. If a pedestrian is hit and lands on the front of the car, the airbag inflates. Use ideas about momentum to explain how this airbag can reduce injuries to pedestrians. (4 marks)
- the momentum is reduced
- by the same amount
- over a longer period of time
- therefore, the force is reduced
- because force is the rate of change of momentum
- less force means less damage/injuries
Using ideas of momentum and force, explain how a seat belt reduces the risk of serious injury in a car crash. (4 marks)
- seatbelts increase the time of impact
- for the same momentum change
- this leads to a reduced rate of momentum change
- which results in a reduced force
- the seat belt stretches during the collision
- which increases the area over which the force acts
- hence the pressure on the driver is reduced
Suggest why a full-body harness is used in a racing car, instead of an ordinary seatbelt. (1 mark)
- there is a larger change in momentum
- there is a larger force during impact
- straps have a greater area over which the force acts
- larger area of straps reduces the pressure on the driver
A car has crashed into a concrete wall. State what happens to the momentum of the car during the crash. (1 mark)
- the momentum of the car and dummy reduces to zero
Newton’s Cradle consists of a set of identical solid metal balls hanging by threads from a frame so that they are in contact with each other. The student releases balls A and B together. Predict what will happen to the other balls after the collision and give a reason for your answer. (2 marks)
- two balls at opposite ends of the cradle moves up
- because momentum is still conserved in the collision
A lorry stops suddenly and the load inside slides to the front. Force A and force B are upward forces from the road on the lorry.
i. Use ideas about momentum to explain why the load slides to the front when the lorry stops suddenly.
ii. Use ideas about moments to explain why force B increases when the load slides to the front.
(5 marks)
i.
- the load has momentum
- the lorry stops in a shorter time because of the brakes
ii.
- the centre of gravity of the load and lorry moves closer to the front of the lorry
- the clockwise and anticlockwise moments are equal
- therefore, the force on B must increase to balance the moments
A stationary rocket in deep space ignites its engines, propelling exhaust gases at high speed away from the rocket. Identify the pair of equal and opposite forces that show how momentum is conserved in this situation. (2 marks)
- the rocket exerts a backwards force on the exhaust gases
- the exhaust gases exert a force equal in magnitude and opposite in direction on the rocket
Suggest how rocket engineers could use the idea of momentum to design safety features to protect the crew when the rocket ignites its engines. (1 mark)
- harnesses/restraints could be used to reduce the forces experienced by astronauts due to the rocket’s change in momentum
- shock absorbers in seats that would extend the time interval over which the momentum changes, reducing the forces exerted on the astronauts
- protective padding around the astronauts that reduces impact by increasing the time interval over which the forces act
