Lesson 5: Law of Conservation of energy

Question 1

What is a closed system?

Answer 1

A closed system is an object or group of objects that energy cannot leave or enter.

Question 2

What does The Law of the Conservation of energy state?

Answer 2

Energy cannot be created or destroyed, only transferred usefully, stored or dissipated.
Total energy before transfer = total energy after transfer

Question 3

An apple in a tree has a gravitational store of 10 J. As it falls, it accelerates constantly until it hits the ground. What is the apple’s maximum kinetic store?

Answer 3

kinetic - 10J
The law of conservation of energy states that energy cannot be created or destroyed. Therefore, once the apple reaches the max KE, all of the GPE will be transferred.

Question 4

Explain the energy stores/ transfers in a pendulum.

Answer 4

1.) When it is at its highest, it is highest in gravitational potential energy.
2.) When you let go, it has kinetic energy.
3.) When it goes to the other side, its gravitational potential energy build up again.
4.) Some energy is dissipated as thermal energy into the surroundings (through heating pathway.)

Question 5

What causes energy dissipation in a pendulum?

Answer 5

• Friction at fixed point leads to increase in thermal energy.
• Some energy is dissipated as thermal energy into the surroundings.

Question 6

When does KE equal GPE?

Answer 6

If all the energy has been conserved in a falling object.

Question 7

Explain why height is the only factor that affects the maximum speed of an object

Answer 7

• If two balls are dropped at same height but one is on a slope, the two objects will still move at the EXACT SAME SPEED as the vertical height is the same.

Question 8

What are the 2 factors maximum speed depends on?
(of an object dropping to the ground.)

Answer 8

• Gravitational field strength
• Height

Question 9

How can we reduce thermal energy dissipation in the pendulum?

Answer 9

• Reduce friction
• Use lubricant on fixed point
• Remove the air particles around it.

Question 10

Explain the energy transfers in a bungee jumper.

Answer 10

• Maximum gravitational potential energy.
• Gravitational –> kinetic energy
• When it reaches bottom = 0 kinetic energy (all in elastic potential store)
• Elastic potential –> kinetic energy
• Kinetic energy back to GPE.

Question 11

Why does the bungee jumper never return back to their original position?

Answer 11

• Energy is dissipated as thermal energy through:
• Friction with air particles
• Thermal energy dissipated in stretching and recoiling (not fully elastic.)

Question 12

Why does the bungee chord, not being fully elastic mean that thermal energy is dissipated?

Answer 12

• Not all energy (from initial point) is transferred to EPE.
• Some of it is transferred as thermal energy to the surroundings.

Question 13

What causes thermal energy to be dissipated from a rope?

Answer 13

• Molecules sliding over each other: friction releases thermal energy.
• Also, friction of rope against air particles.