2.3 - Work and energy

Question 1

What does it mean if a force is said to do ‘work’

Answer 1

The force causes an object to be displaced through a distance

Question 2

Whenever work is done, energy is transferred from one place to another. The total amount of energy remains ____

Answer 2

constant

Question 3

What is the equation used to calculate work done? Give appropriate units.

Answer 3

Work done (J) = Force (N) X Distance (M)

Question 4

Work done has the same units as energy - joules. This is because energy is the ability to do work. You must have energy to do work. You do not have to do work if you have energy though - ____ energy does not do work. Specifically, in the example above, a person could not push the box (and so do work) without energy. Work done is equal to energy transferred, ie work = energy transferred.

Answer 4

potential

Question 5

How can an object possess energy?

Answer 5

As a result of its:

position (potential energy)
motion (kinetic energy)
deformation (elastic energy)

Question 6

What does the amount of GPE an object has depend on?

Answer 6

The objects:

Mass
Height above ground

Question 7

What is the equation for gravitational potential energy?

Answer 7

E = mgh

Energy (J) = Mass (kg) X Gravitational Field Strength (g) x Height (m)

Question 8

A book with a mass of 0.25 kg is lifted 2 m onto a bookshelf. If g is 10 N/kg, how much gravitational potential energy does it gain?

Answer 8

GPE = 0.25 × 10 × 2 = 5 J

Question 9

What type of energy is stored in a moving object?

Answer 9

Kinetic energy

Question 10

What occurs when work is done against frictional forces?

Answer 10

A rise in temperature of the object

This is because kinetic energy is converted to heat

Question 11

Explain the relationship between the force applied and the extension of an elastic band?

Answer 11

The extension is directly proportional to the force applied, provided that the limit of proportionality isn’t exceeded

Question 12

What is meant by an inelastic deformation?

Answer 12

A deformation which results in the object being permanently stretched
The object doesn’t return to its original shape when the force is removed.

Question 13

State the equation relating force, spring constant and extension. Give appropriate units.

Answer 13

F = kx

Force (N) = Spring constant (N/m) x extension (m)

Question 14

What type of energy is stored in a spring when it is stretched?

Answer 14

Elastic potential energy

Question 15

What is the equation for Kinetic energy? Give SI units for all quantities involved?

Answer 15

E = 1/2 m v^2

Energy (J) = 1/2 x Mass (kg) x (Velocity)^2 (m/s)

Question 16

What is the kinetic energy of a 1,000 kg car travelling at 5 m/s?

Answer 16

KE = ½ × 1,000 × 25 = 500 × 25 = 12,500J

Question 17

Assuming the limit of proportionality (elastic limit) is not exceeded, a graph of force against extension produces a straight line that passes through the origin. The gradient of the line is the spring constant, k. The greater the value of k, the ____ stiffer/relaxed the spring

Answer 17

stiffer

Question 18

The fuel consumption and range of all vehicles also depends on factors such as whether the vehicle is:

Answer 18

1) driven uphill a lot
2) carrying large loads
3) driven at high speed
4) driven over rough road surfaces
5) driven with underinflated tyres

Question 19

How can the shape of the car be altered to make it more energy efficient?

Answer 19

The more aerodynamic the shape, the less energy that is wasted to resistive forces such as air resistance
More streamline, less frontal surface area

Question 20

State two consequences of a vehicle undergoing very large decelerations.

Answer 20

1) KE converted to heat is very high causing brakes to over heat
2) loss of control of the vehicle

Question 21

What are the frictional losses on a car?

Answer 21

Air friction due to the shape and size of the vehicle. Fuel consumption is effected by the friction between the car tyres and the road surface, and the friction between the wheels and the axles of the car.

Question 22

What is rolling resistance and how can it be reduced?

Answer 22

Rolling resistance is a measure of the force necessary to move the tyre forward. It can be reduced by improved tyre tread and shoulder designs and materials used in the tyre belt and traction surfaces.

Question 23

When there is a car crash, the car, its contents and the passengers decelerate rapidly. They experience great forces because of the change in momentum, which can cause injuries. Modern cars have safety features that absorb kinetic energy in collisions. What are they?

Answer 23

Seat belts
Air bags
Crumple zones

Question 24

How do seatbelts improve a passenger’s safety during a collision?

Answer 24

1) Seat belts stop you tumbling around inside the car if there is a collision.
2) However, they are designed to stretch a bit in a collision. This increases the time taken for the body’s momentum to reach zero, and so reduces the forces on it.

Question 25

How do airbags improve a passenger’s safety during a collision?

Answer 25

Airbags increase the time taken for the head’s momentum to reach zero, and so reduce the forces on it. They also act as a soft cushion and prevent cuts.

Question 26

How do crumple zones improve a passenger’s safety during a collision?

Answer 26

Crumple zones are areas of a vehicle that are designed to crush in a controlled way in a collision. They increase the time taken to change the momentum of the driver and passengers in a crash, which reduces the force