P10 Forces and Motion

Question 1

A car driver sees a fallen tree lying across the road ahead and makes an emergency stop.
The braking distance of the car depends on the speed of the car.
For the same braking force, explain what happens to the braking distance if the speed
doubles.
You should refer to kinetic energy in your answer (4)

Answer 1

doubling speed increase the kinetic energy
• kinetic energy increases by a factor of 4
• work done (by brakes) to stop the car increases
• work done increases by a factor of 4
• work done is force × distance and braking force is constant
• so if work done increases by 4 then the braking distance must increase by 4

Question 2

Using the idea of forces, explain why the parachutist reaches a terminal velocity and why
opening the parachute reduces the terminal velocity. (6)

Answer 2

on leaving the plane the only force acting is weight (downwards)
accept gravity for weight throughout
• as parachutist falls air resistance acts (upwards)
accept drag / friction for air resistance
• weight greater than air resistance
or
resultant force downwards
• (resultant force downwards) so parachutist accelerates
• as velocity / speed increases so does air resistance
• terminal velocity reached when air resistance = weight

to explain second lower terminal velocity

opening parachute increases surface area
• opening parachute increases air resistance
• air resistance is greater than weight
• resultant force acts upwards / opposite direction to motion
• parachutist decelerates / slows down
• the lower velocity means a reduced air resistance
air resistance and weight become equal but at a lower (terminal) velocity

Question 3

Explain how a train can accelerate without changing speed

Answer 3

If the direction changes the velocity changes, and acceleration is the rate of change of velocity

Question 4

What is meant by a resultant force?

Answer 4

A single force representing all combined forces

Question 5

Explain why a driver tends to be thrown towards the windscreen in a car crash

Answer 5

Driver has forward momentum
Which is conserved
Giving drive relative forward speed to car

Question 6

During a car crash the front end of the car becomes crumpled and buckled. Explain why such a collision is described as “inelastic”.

Answer 6

Inelastic = EPE lost

Here EPE does work crumpling car

Question 7

Cars don’t accelerate above a certain maximum speed. Explain the reasons for this in terms of the forces acting on the car.

Answer 7

there is a (maximum) forward force
drag/friction/resistance (opposes motion) (not pressure)
increases with speed
till forward and backward forces equal
so no net force/acceleration

Question 8

A miner accelerates down a slide. Mayowa uses the kinetic energy equation to calculate the miner’s speed at the end of the slide. Explain why the calculated velocity is greater than the actual value [4]

Answer 8

work is done

(against) friction (between the miner and slide)

(due to the) slide not (being perfectly) smooth

causing (kinetic) energy to be transferred as heat/internal energy of surroundings

Question 9

There are two vehicles, A and B, with the same mass, and engine. Vehicle A is streamlined while B is a mess of a design. Explain why they have different top speeds

Answer 9

A and B will reach top speed when the forward
force equals the drag force

the drag force at any speed is smaller for A than
for B

as the A is more streamlined

therefore the A’s drag force will equal the forward force
at a higher speed