1.1 Energy changes in a system

Question 1

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

Answer 1

1/2mv^2

1/2 x mass x velocity^2

Energy (J), Mass (kg), velocity (m/s)

Question 2

What is the equation for elastic potential energy ? Give SI units for all quantities involved.

Answer 2

1/2kx^2

1/2 x spring constant x extension^2

Energy (J), Spring constant (N/m), Extension (m)

Question 3

What is the equation for gravitational potential energy? Give SI units for all quantities involved.

Answer 3

mgh

Mass x gravitational field strength x height

Energy (J), mass (kg), gravitational field strength (N/kg), height (m)

Question 4

Define the ‘specific heat capacity’ of a substance

Answer 4

The amount of energy needed to raise the temperature of 1kg of a substance by 1°C

Question 5

What is the definition of ‘power’?

Answer 5

The rate at which energy is transferred (on rate at which work is done)

Question 5

State the units for specific heat capacity

Answer 5

J/°C/kg

Question 6

State two equations for power. Give SI units for all quantities involved.

Answer 6

power = energy transferred / time

power = work done / time

Energy (J), Work done (J), Time (s)

Question 7

What is the unit for power?

Answer 7

Watts (W)

Question 8

Two motors lift the same mass through the same height. Motor A does this in half the time of Motor B. Which dissipates the most power?

Answer 8

Motor A

The energy transferred is the same but the time taken is less (P=E/t)

Question 9

Describe the energy changes involved when a ball is thrown upwards and then returns to its starting position. Ignore air resistance.

Answer 9

Upwards: KE is converted to GPE

Peak: Maximum GPE, 0 KE

Downwards: GPE is converted to KE

Question 10

Describe the energy transfers for a bungee jumper

Answer 10

1. When falling, the GPE is converted to KE of jumper
2. As the chord tightens, KE is converted and stored as EPE
3. At lowest point, the jumper’s initial GPE equals the EPE stored in the cord

Question 11

Explain why a bungee jumper slows down once the cord begins to stretch

Answer 11

KE decreases since it is converted to EPE. Since KE is proportional to velocity squared, as KE decreases, so does velocity.

Question 12

Give examples of chemical energy stores

Answer 12

Food, Fuel (wood, coal petrol), Batteries

Question 13

State four different stores of energy

Answer 13

● Kinetic energy

● Gravitational potential energy

● Elastic potential energy

● Chemical energy