P2 - Forces

Question 1

Explain why making a car more streamlined has an effect on its top speed.

Answer 1

Air resistance will decrease so a higher speed is reached before the resultant force is zero.

Question 2

What two forces cause a motor boat to move?

Answer 2

The force from the water on the boat in the forward direction and the force from the propeller of the boat on the water in the opposite direction.

Question 3

What is meant by the term resultant force?

Answer 3

A single force that has the same effect as all the forces combined/ the overall force

Question 4

When an aircraft moves along the runway to take off, it’s acceleration decreases although the push of the engine remains constant. Why?

Answer 4

As speed increases, air resistance increases reducing the resultant force

Question 5

When the motor inside a toy is switched off, the toy starts to accelerate downwards. What happens to the momentum of the toy and why?

Answer 5

Increases as the velocity increases.

Question 6

Explain why a toy accelerates upwards when the fan inside the toy rotates faster.

Answer 6

Because there is a greater change in momentum as the velocity of the air increases so the upward force increases and is greater than the downward force.

Question 7

Explain why objects are wrapped in polystyrene for protection.

Answer 7

When the object is dropped the polystyrene increases the time to stop which decreases the rate of change of momentum and therefore the force on the object.

Question 8

How is velocity different from speed?

Answer 8

Velocity includes direction, velocity is a vector

Question 9

When a tube is filled with air, a coin inside the tube will fall faster than a piece of paper. Why?

Answer 9

Air resistance has a greater effect on the paper.

Question 10

The forward force on a tractor is exactly balanced by the resisting forces on the tractor. Describe the motion of the tractor.

Answer 10

Tractor is moving at constant speed.

Question 11

Describe how the horizontal forces acting on a car change during the first 2 seconds of acceleration.

Answer 11

driving force increases, friction forces increase, the driving force is bigger than friction

Question 12

Using the idea of forces explain why a parachutist reaches terminal velocity.

Answer 12

Leaving plane: weight only force. Air resistance increases. Weight > air resistance so accelerates downwards. When air resistance = weight, terminal velocity reached.

Question 13

Using the idea of forces explain what happens when a parachutist opens his parachute.

Answer 13

The open parachute increases the surface area so air resistance increases as well. Air resistance is now > weight, so the velocity decreases. When weight = air resistance again a new, lower terminal velocity has been achieved.

Question 14

Define braking distance.

Answer 14

The distance a vehicle travels before stopping, once the brakes are applied

Question 15

Name two resistive forces that act on a vehicle.

Answer 15

Air resistance, friction between tyres and the road

Question 16

State factors that affect thinking distance.

Answer 16

Tiredness, alcohol, drugs, speed, age, using a mobile phone, visibility (weather)

Question 17

State factors that affect braking distance.

Answer 17

Icy or wet road, worn tyres, road surface, mass of car, speed of car, brakes are in bad condition

Question 18

Why does applying the rakes increase the temperature of the brakes?

Answer 18

Friction between brakes and the wheel transfers kinetic energy to thermal energy

Question 19

State & explain the benefits of a regenerative braking system (system that slows car down and recharges the car battery in a hybrid car).

Answer 19

the range of the car is increased, the efficiency of the car

is increased as the decrease in kinetic energy is not converted to thermal energy but work is done to charge the battery

Question 20

When you slide down a slide, your speed at the bottom of the slide is much less than the calculated value. Why?

Answer 20

Work is done against friction as the slide is not smooth. Kinetic energy is therefore transferred to thermal energy.

Question 21

Explain why the top speed of a car is higher than the top speed of a van.

Answer 21

Top speed is reached when forward force = drag force (air resistance/friction). The drag force of a car is smaller due to it being more streamlined. Drag force = forward force for the car at higher speed.

Question 22

How can the velocity of a car change although the speed remains constant?

Answer 22

Because the direction is changing

Question 23

During a collision the front end of a car becomes buckled. Why is such a collision described as inelastic?

Answer 23

In an I elastic collision, kinetic energy is lost. Here it does the work to crumple the car.

Question 24

A car cannot accelerate above a certain maximum speed. Why not?

Answer 24

there is a maximum forward force when you push the accelerator pedal. Air resistance increases with speed until it is equal to the forward force so the is no net force.

Question 25

In terms of force and deceleration, what would happen if a climber, who used a non- elastic rope, fell?

Answer 25

deceleration would be great, because force = mass x

acceleration. The force on the climber would be great. The rope might exceed its elastic limit.

Question 26

Define direct proportionality.

Answer 26

Straight line through the origin

Question 27

Bent metal ruler, stretched elastic band, springs on a playground ride, moulded plastic model- which objects are storing elastic potential energy? Explain your answer.

Answer 27

Elastic band and springs because they will go back to their original shape

Question 28

How much momentum does a car have when it stops at a traffic light?

Answer 28

Zero, as it’s velocity is zero

Question 29

Crash test dummies are fitted with electronic sensors. Why?

Answer 29

To measure the forces exerted on the dummies during a collision

Question 30

What is meant by the phrase ‘ momentum is conserved’?

Answer 30

Momentum before a collision = momentum after a collision

Question 31

In a collision, momentum is not always conserved. Why?

Answer 31

An external force acts on the colliding objects

Question 32

Explain why air bags reduce the risk of the driver sustaining serious head injuries.

Answer 32

air bags reduce the time taken for the head to stop so

decrease the change in momentum and reduce the force on the head

Question 33

If the speed of a car doubles, the amount of energy transferred in a collision quadruples. Why?

Answer 33

greater speed means greater kinetic energy (1/2 mass x velocity squared). 2 squared = 4, so the value for the kinetic energy quadruples

Question 34

Define momentum

Answer 34

product of mass and velocity

Question 35

Why is it easier to drag an object up a ramp instead of lifting it?

Answer 35

You only work against a component of gravity, not the direct force.

Question 36

Could a skydiver ever hover in calm weather conditions?

Answer 36

no, as terminal velocity cannot be eliminated, only reduced.