1.1 - Energy Changes in a System

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

• ½ mv²

• ½ × mass × (velocity)²

• energy (J), mass (kg), velocity (m/s)

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

• ½ ke²

• ½ × spring constant × (extension)²

• energy (J), spring constant (N/m), extension (m)

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

• mgh

• mass × gravitational field strength × height

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

Define the “Specific Heat Capacity” of a substance.

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

State the units for Specific Heat Capacity

• J/°C/kg

• Joules/Degrees Celsius/Kilogram

What is the definition of “power”?

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

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

• power = energy transferred / time

• power = work done / time

• energy (J), work done (J), time (s)

What is the unit of power?

watt (W)

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?

• Motor A

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

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

• Upwards: KE is converted to GPE

• Peak: Maximum GPE, zero KE

• Downwards: GPE is converted to KE

Describe the energy transfers for a bungee jumper.

• 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 lowest point, the jumper’s initial GPE equals the EPE stored in the cord

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

• Kinetic energy decreases since it is converted to elastic potential

• Since KE is proportional to (velocity)², as KE decreases, so does velocity

Give examples of chemical energy stores.

• Food

• Fuel (e.g. wood, coal, petrol)

• Batteries

State 4 different stores of energy.

1. Kinetic energy

2. Gravitational Potential energy

3. Elastic Potential energy

4. Chemical energy