4.1.1 Energy changes in a system, and the ways energy is stored before and after such changes

Question 1

What is the equation for kinetic energy?
Provide units for quantities involved.

Answer 1

1/2 m v^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? Provide units for quantities involved.

Answer 2

1/2 k x^2

1/2 x Spring constant x (Extension)^2

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

Question 3

What is the equation for gravity potential energy? Provide units for 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

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. Provide 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 it’s 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 transfers 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 lowest point, 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 (eg. Wood, Coal, Petrol)
• Batteries

Question 14

State 4 different stores of energy

Answer 14

1. Kinetic energy
2. Gravitational Potential energy
3. Elastic potential energy
4. Chemical Energy