New
New
Year 10
OCR
Higher

# Calculating energy changes - with complex examples (E)

I can use the principle of conservation of energy to calculate changes in energy of moving objects.

New
New
Year 10
OCR
Higher

# Calculating energy changes - with complex examples (E)

I can use the principle of conservation of energy to calculate changes in energy of moving objects.

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## Lesson details

### Key learning points

1. Energy cannot be created or destroyed.
2. In an ideal situation, no energy is dissipated.
3. As an object changes height, change in gravitational potential energy = change in kinetic energy.

### Common misconception

Energy can be lost or created in physical processes.

Always discuss how energy is dissipated to the surroundings when discussing energy transformations, even when dealing with ‘ideal situations’ in which there is no dissipation.

### Keywords

• Conservation of energy - The principle of conservation of energy states that the total amount of energy at the end of a process is always the same as the total amount of energy at the beginning.

• Dissipate - Friction or air resistance can cause energy to dissipate into the surroundings as heat.

• Gravitational potential energy - The energy an object has in its gravitational store is referred to as the object’s gravitational potential energy.

• Kinetic energy - The energy an object has in its kinetic store is referred to as the object’s kinetic energy.

### Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

## Starter quiz

### 6 Questions

Q1.
Match each symbol in the equations \$\$E = mgh\$\$ and \$\$E = ½mv^2\$\$ with the quantity it represents.
Correct Answer:\$\$E\$\$,gravitational potential energy or kinetic energy

gravitational potential energy or kinetic energy

mass

gravitational field strength

height

speed

Q2.
Match the following quantities to their units.

joules (J)

kilograms (kg)

metres per second (m/s)

Correct Answer:gravitational field strength,newtons per kilogram (N/kg)

newtons per kilogram (N/kg)

metres (m)

Q3.
Which of the following statements are correct?
Correct answer: Kinetic energy is directly proportional to an object's mass.
Kinetic energy is directly proportional to an object's speed.
Correct answer: Gravitational potential energy is directly proportional to an object's mass.
Gravitational potential energy is directly proportional to an object’s speed.
Q4.
An eagle with a mass 4.0 kg flies at a speed 15 m/s. Which of the following is the amount of energy in its kinetic store?
30 J
60 J
240 J
900 J
Q5.
A book of mass 1.6 kg falls off a shelf onto the floor. The shelf is 2.0 m above the floor. Which of the following is the change in energy in the gravitational store? Use \$\$g\$\$ = 10 N/kg.
3.2 J
13.6 J
16 J
20 J
Q6.
An apple of mass 200 g falls from a branch that is 150 cm above the ground. Which of the following is the change in energy in the gravitational store? Use \$\$g\$\$ = 10 N/kg.
30 J
300 J
3000 J
300 000 J

## Exit quiz

### 6 Questions

Q1.
The principle of states that total amount of energy at the end of a process is always the same as the total amount of energy at the beginning.
Q2.
A plate is dropped. If no energy is dissipated during the fall, the final speed of the plate before it hits the ground is 4 m/s. If energy is dissipated, which of the following statements is correct?
Correct answer: The final speed will be less than 4 m/s.
The final speed will still be 4 m/s.
The final speed will be more than 4 m/s.
It is impossible to say whether the final speed will be more or less than 4 m/s.
Q3.
A 0.20 kg ball is thrown vertically upwards and reaches a maximum height of 2.1 m. Assuming that no energy is dissipated, the ball’s kinetic energy at the start is J. Use \$\$g\$\$ = 10 N/kg.
Correct Answer: 4.2, 4.20, 4.2J, 4.2 J, 4.20J
Q4.
A child sledges down a hill of height 12 m. The total mass of child and sledge is 30 kg. Assuming no energy is dissipated, the kinetic energy at the bottom of the hill is J. Use \$\$g\$\$ = 10 N/kg.
Correct Answer: 3600, 3600J, 3600 J, 3600joules, 3600 joules
Q5.
A mountain goat with a mass of 52 kg jumps vertically upwards, leaving the ground at speed 6.0 m/s. The maximum height that the goat can reach is m. Use \$\$g\$\$ = 10 N/kg.
Correct Answer: 1.8, 1.80, 1.8m, 1.8 m, 1.80m
Q6.
A frog with a mass of 0.025 kg jumps vertically upwards and reaches a height of 0.40 m. Which of the following is the frog’s speed at the moment it jumps? Use \$\$g\$\$ = 10 N/kg.
2.0 m/s