Energy changes in a system

Question 1

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

Answer 1

1⁄2mv²
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

1⁄2kx²
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 1 kilogram of a substance by 1 degree Celsius.

Question 5

State the units for Specific Heat Capacity

Answer 5

J/kg°C

Joules/Degree celsius/Kilogram

Question 6

What is the definition of ‘Power’?

Answer 6

The rate at which energy is transferred (or rate at which work is done).

Question 7

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

Answer 7

Power = energy transferred/time
Power = work done/time
Energy (J), Work done (J), Time (s).

Question 8

What is the unit of power?

Answer 8

Watt (W).

Question 9

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 9

Motor A.

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

Question 10

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

Answer 10

• Upwards: KE is converted to GPE.
• Peak: Maximum GPE, zero KE.
• Downwards: GPE is converted to KE.

Question 11

Describe the energy transfer for a bungee jumper

Answer 11

• When falling, the GPE is converted to KE of jumper.
• As the cord tightens, KE is converted and stored as elastic potential energy (EPE).
• At the lowest point (when cord if fully tight), the jumper’s initial GPE equals the EPE stored in the cord.

Question 12

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

Answer 12

• Kinetic energy decreases since it is converted to elastic potential energy.
• Since KE is proportional to (velocity)². as KE decreases so does velocity.

Question 13

Give examples of chemical energy stores

Answer 13

• Food.
• Fuel such as wood, coal, petrol.
• Batteries.

Question 14

State 4 different stores of energy

Answer 14

• Kinetic energy.
• Gravitational potential energy.
• Elastic potential energy.
• Chemical energy.