Work, Energy and Power

Question 1

Define Power

Answer 1

The rate of transfer of energy

Question 2

What is power measured in?

Answer 2

Watts or J/S

Question 3

What do the terms stand for in the following equation:
P=F x v
When can it be used?

Answer 3

Power
Force
Velocity

When an object is moving e.g. a car.

Question 4

How is the motive power of an object calculated?

Answer 4

Motive Power = Energy wasted per second + Gain of kinetic energy per second.

Motive power = power wasted + power gained from KE

Question 5

The engine of a train travelling at a constant velocity is providing a force of 10kN, and transfers 1.08 MJ every second. There is a drag force of 1,000N acting against the train. Calculate the distance the train travels over 3 seconds.

Answer 5

Power output = E/t = 1080000/1 = 1080000
v=P/F = 1080000/(10000-1000)
d=st = 120 x 3 =360 m

Question 6

How is work calculated?

Answer 6

Work done = force x distance

Question 7

What does work stand for?

Answer 7

The energy transferred by a force.

Question 8

How is percentage efficiency calculated?

Answer 8

(useful energy / input energy) x 100

Question 9

The power input to a computer is 120W, and the computer wastes 40J over 10 seconds through heat and sound. Calculate the percentage efficiency of the computer.

Answer 9

P= ∆E / ∆t, = 40 / 10 = 4W wasted.
Useful output= 120-4 = 116W.
Efficiency = (116/120) x 100% = 96.7%.

Question 10

Name the 5 main types of renewable energy sources.

Answer 10

Wind, Tidal, Solar, Wave, Hydroelectric

Question 11

How do wave power stations produce electricity?

Answer 11

Waves push water into a capture chamber, and air is forced up through a turbine by the rising water. The turbine turns a generator to produce electricity.

Question 12

There are several different types of energy, can you describe 6 types?

Answer 12

Potential energy - energy due to height.

Kinetic energy -Energy due to motion.

Thermal Energy - energy due to the temperature of an object.

Chemical or Nuclear Energy - Energy associated with chemical or nuclear reactions.

Electrical Energy - Energy of electrically charged objects.

Elastic Energy - Energy stored in an object when it is stretched or compressed.

Question 13

What is the principle of conservation of energy?

Answer 13

Energy cannot be created or destroyed.

Question 14

When an object is moved it gains energy, what equation governs this?

Answer 14

Work done = force x distance moved in the direction of the force.

Question 15

When would you use Work done = distance x force x cos theta

Answer 15

When the force is not being applied in the direction the object is moving.

Question 16

Define Kinetic Energy, then state the equation associated with it.

Answer 16

Kinetic energy is the energy of an object due to its motion.

Ek = 1/2 mv^2

Kinetic Energy = half x mass x Velocity^2

Question 17

Define Potential Energy, then state the equation associated with it.

Answer 17

Potential energy is the energy of an object due to it’s position.

Ep= mgh

Energy = Force x Height change

Question 18

When an object is falling e.g. a roller coaster down a track, there is a relationship between Potential Energy and Kinetic Energy with air resistance and without air resistance.

Answer 18

In an ideal world that has no air resistance.

mgh = 1/2 mv^2

Potential Energy = Kinetic Energy

In a world with air resistance.

mgh = 1/2 mv^2 + Energy lost due to air resistance

Potential Energy > Kinetic Energy

This enables you to find out the velocity an object is moving given only the height through which it has fallen.