Work, energy and momentum

Question 1

What happens when a force moves an object?

Answer 1

When a force moves an object, energy is transferred and work is done.

Question 2

Why is work done equal to energy transferred?

Answer 2

When work is done moving an object, the supplied energy is transferred to the object so the work done is equal to the energy transferred.

Question 3

What is the unit of work and energy?

Answer 3

Joules, J

Question 4

How is the work done on an object calculated?

If the distance moved is zero, what is the work done?

Answer 4

W = F x d
W is the work done in joules, J
F is the force in newtons, N
d is the distance moved in the direction of the force in metres, m

If the distance moved is zero, no work is done on the object.

Question 5

How is work done to overcome friction mainly transferred?

Answer 5

Work done to overcome friction is mainly transferred into energy by heating the surroundings.

Question 6

What does friction between the brake pads and wheel discs do when the brakes are applied to a vehicle?

Answer 6

When the brakes are applied to a vehicle, friction between the brake pads and the wheel discs opposes the motion of the vehicle.

Question 7

How is the kinetic energy of a moving vehicle transferred when the brakes are applied?

Answer 7

The kinetic energy of the vehicle is transferred into energy that heats the brake pads and the wheel discs as well as the surrounding air.

Question 8

What is gravitational potential energy?

What happens to an object as it moves vertically upwards?

Answer 8

Gravitational potential energy is energy stored in an object because of its position in the earth’s gravitational field.
Whenever an object moves vertically upwards it gains gravitational potential energy equal to the work done on it by the lifting force.

Question 9

How can change in gravitational potential energy be calculated?

Answer 9

Ep = m x g x h
Ep is the change in gravitational potential energy in joules, J
m is the mass in kilograms, kg
h is the change in height in metres, m.

Question 10

What is power?

How can power be calculated?

Answer 10

Power is the rate of energy transfer.
P = E/t
P is the power in watts, W
E is the energy transferred in joules, J
t is the time in seconds, s.

Question 11

What type of energy do all moving objects have?

How is this type of energy affected by the mass and speed of an object?

Answer 11

All moving objects have kinetic energy.

The greater the and the faster the speed of an object, the more kinetic energy it has.

Question 12

How is kinetic energy calculated?

Answer 12

Ek = 1/2 x m x v(squared)
Ek is the kinetic energy in joules, J
m is the mass in kilograms, kg
v is the speed of the object in metres per second, m/s

Question 13

An object is described as elastic if it has what properties?
What happens when work is done to an elastic object to stretch or squash it?
What happens when the object returns to its original shape?

Answer 13

An object is described as elastic if it regains its shape after being stretched or squashed.
When work is done to an elastic object to stretch or squash it, the energy transferred to it is stored as elastic potential energy.
When the object returns to its original shape, the elastic potential energy is released.

Question 14

What do all moving objects have, other than kinetic energy?

How does the mass and velocity of an object affect this?

Answer 14

Other than kinetic energy all moving objects have momentum, the greater the mass and the greater the velocity of an object, the greater its momentum.

Question 15

How can momentum be calculated?

Answer 15

p = m x v
p is the momentum in kilogram metres per second, kgm/s
m is the mass of the object in kilograms, kg
v is the velocity of the object in metres per second, m/s

Question 16

What is the law of conservation of momentum?

Answer 16

The law of conservation of momentum states that whenever objects interact, the total momentum before interaction is equal to the total momentum afterwards - provided no external forces act on them.

Question 17

In what two ways could objects interact?

How may the objects move off after the first type of interaction?

Answer 17

1) A collision
2) An explosion
After a collision the objects may move off together, or they may move apart.

Question 18

How is momentum similar to velocity?

Answer 18

Momentum has both size and direction.

Question 19

What must be done in calculations involving momentum?

Answer 19

In calculations, one momentum must be defined as positive, so momentum in the other direction is negative.

Question 20

What happens to objects at rest in an explosion? (3)

Answer 20

1) When two objects are at rest their momentum is zero.
2) In an explosion the objects move apart with equal and opposite momentum.
3) One momentum is positive and the other is negative, so the total momentum after the explosion is zero.

Question 21

Give an example of an explosion and explain momentum in the explosion.

Answer 21

Firing a bullet from a gun is an example of an explosion, the bullet moves off in one direction and the gun ‘recoils’ with equal momentum in the opposite direction.

Question 22

What happens to the momentum of an object that is able to move or is moving when a force acts on it?

Answer 22

Its momentum changes.

Question 23

How does the time over which momentum changes affect the force that acts?

Answer 23

For a particular change in momentum the longer the time taken for the change, the smaller the force that acts.

Question 24

What happens to the momentum of an object in a collision?

Answer 24

In a collision, the momentum of the object becomes zero during the impact, the object comes to rest.

Question 25

What happens to the forces on the object in a collision as the impact time increases?

Answer 25

As the impact time increases, the forces acting on the object decrease.

Question 26

What are crumple zones in a car designed to do in a collision?
Why are they designed to do this?
What other installation in cars are designed to do this?

Answer 26

Crumple zones in cars are designed to fold in a collision, this increases the impact time and so reduces the force on the car and the people in it.
Side impact bars.

Question 27

What are airbags designed to do?

Answer 27

Air bags spread the forces acting on the body across a larger area, they also increase the impact time of the drivers head and so the force acting on it is less than if it hit the steering wheel.

Question 28

What are seat belts designed to do?

Answer 28

Seat belts stop the wearer being flung forward if the car stops suddenly, the seatbelt stretches slightly increasing the impact time and reducing the force.

Question 29

What do police use to calculate the speed of vehicles before a collision?

Answer 29

After a crash the police use measurement from the scene of the crash and the conservation of momentum to calculate the speed of the vehicles before the collision.