1.1 Energy Changes in a System

Question 1

what is the equation for kinetic energy?

Answer 1

1/2 m v^2
1/2 x mass x (velcoity)^2
energy (J), mass (kg), velocity (m/s)

Question 2

what is the equation for elastic potential energy?

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?

Answer 3

mgh
mass x gravitational field strength x height
energy (J), mass (kg), gravitational field strenght (N/kg), height (m)

Question 3

define the ‘specific heat capacity’ of a substance.

Answer 3

the amount of energy needed to raise the temperature of 1 kilogram of a substance by 1 degree celsius.

Question 4

state the units for specific heat capacity

Answer 4

J/°C/kg
Joules/degree celsius/kilogram

Question 5

what is the definition of ‘power’?

Answer 5

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

Question 6

state two equations for power.

Answer 6

power = energy transferred/time
power = work done/time
energy (J), work done (J), time (s)

Question 7

what is the unit of power?

Answer 7

watt (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, zero KE
• downwards: GPE is converted to KE
  KE (kinetic energy), GPE (gravitational potential energy)

Question 10

describe the energy transfers for a bungee jumper.

Answer 10

• 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 11

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

Answer 11

• 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 12

give examples of chemical energy stores.

Answer 12

• food
• fuel (e.g. wood, coal, petrol)
• batteries

Question 13

state 4 different stores of energy.

Answer 13

1. kinetic energy
2. gravitational potential energy
3. elastic potential energy
4. chemical energy