P9. Motion

Question 1

Does a distance quantity require a specific direction? i.e. Is it a scalar or vector quantity

Answer 1

No specific direction is required so it is a scalar quantity

Question 2

If an object moves 3 metres to the left and then 3 metres back to its initial position, what is the object’s total displacement?

Answer 2

The object has zero displacement
Displacement is a vector quantity so it also involves direction
The object starts and ends at the same point

Question 3

State a typical value for the speed of the sound

Answer 3

330m/s

Question 4

State the equation linking distance, speed and time

Answer 4

Distance = Speed × time

Question 5

Why can an object travelling at a constant speed in a circle do not have a constant velocity

Answer 5

Speed is a scalar quantity
Velocity is a vector quantity which means it can only ne constant if the direction is constant
In circular motion, the direction is continuously charging

Question 6

How can speed be calculated from a distance-time graph?

Answer 6

The speed is equal to the gradient of the graph

Question 7

What must be done to calculate speed at a given time from a distance-time graph for an accelerating object?

Answer 7

Drawing a tangent to the curve at the required time
Calculating the gradient of the tangent

Question 8

State the equation for the average acceleration of an object. Give appropriate units

Answer 8

Acceleration = Change in velocity/time taken

Question 9

How can the distance travelled by an object be calculated from a velocity-time graph?

Answer 9

It is equal to the area under the graph

Question 10

Give an approximate value for the acceleration of an object in free fall under the gravity near the Earth’s surface

Answer 10

9.8m/s²

Question 11

What can be said about the resultant force acting on an object it is falling at terminal velocity?

Answer 11

The resultant force is zero
When at terminal velocity, the object is moving at a constant speed and so isn’t accelerating

Question 12

State Newton’s first law for a stationary object

Answer 12

If the resultant force on a stationary object is zero, the object will remain at rest

Question 13

State Newton’s first law for a moving object

Answer 13

If the resultant force on a moving object is zero, the object will remain at constant velocity (same speed in same direction)

Question 14

What can be said about the braking forces and driving forces when a car is travelling at constant velocity?

Answer 14

The braking forces are equal to the driving forces

Question 15

If an object changes direction but remains at a constant speed, is there a resultant force?

Answer 15

Since there is a change in direction, there is a change in velocity and so there must be a resultant force

Question 16

What is inertia?

Answer 16

The tendency of an object to continue in its state of rest or uniform motion

Question 17

State the defining equation for Newton’s Second Law

Answer 17

Resultant force = Mass × Acceleration

F = ma

Question 18

State Newton’s Second Law in words.

Answer 18

An object’s acceleration is directly proportional to the resultant force acting on it and inversely proportional to its mass

Question 19

What is inertial mass?

Answer 19

A measure of how difficult it is to change a given object’s velocity
The ratio of force over acceleration

Question 20

State Newton’s Third Law

Answer 20

Whenever two objects interact, the forces that they exert on each other are always equal and opposite

Question 21

What is the stopping distance of a vehicle equal to?

Answer 21

The sum of thinking distance and braking distance

Question 22

For a given braking distance, if the vehicle’s speed is increased, what can be said about its stopping distance?

Answer 22

The stopping distance is increased with an increase in speed

Question 23

Give a typical range of values for human reaction time

Answer 23

0.2 seconds - 0.9 seconds

Question 24

Give three factors which can affect a driver’s reaction time

Answer 24

1. Tiredness
2. Drugs
3. Alcohol

Question 25

Give two factors which may affect braking distance

Answer 25

1. Adverse (wet/icy) road conditions
2. Poor tyre/brake conditions

Question 26

Describe the energy transfers that take place when a car applies its brakes

Answer 26

Work is done by the friction force between the brakes and wheel
Kinetic energy of the wheel is converted to heat and is dissipated to the surroundings through the brake discs

Question 27

To stop a car in a given distance, if its velocity is increased, what must happen to the braking force applied?

Answer 27

The braking force must also be increased

Question 28

State two consequences of a vehicle undergoing very large decelerations

Answer 28

1. Kinetic energy converted to heat is very high causing brakes to overheat
2. Loss of control of the vehicle

Question 29

State the equation used to calculate an object’s momentum

Answer 29

Momentum = Mass × Velocity

Question 30

What is the unit used for momentum?

Answer 30

kg m/s

Question 31

In a closed system, what can be said about the momentum before and after a collision?

Answer 31

The total momentum before is equal to the total momentum afterwards

Question 32

State an equation linking change in momentum, force and time?

Answer 32

Force × Time = Change in Momentum

Question 33

What quantity is equal to the force experienced in a collision?

Answer 33

The rate of change of momentum

Question 34

If an object’s change of momentum is fixed, what is the only way tk reduce the force that the object experiences?

Answer 34

Increase the length of time over which the change of momentum occurs

Question 35

Explain a seatbelt improves a passenger’s safety during a collision

Answer 35

Passenger must decelerate from the vehicle’s velocity at impact to zero, meaning they undergo a fixed change of momentum
The force they experience is equal to the rate of change of momentum
Seatbelts increase the time over which force is applied, reducing the rate experienced