Module 3: C5 - Work, Energy, And Power

Question 1

Name 8 Energy stores

Answer 1

• Thermal
• Kinetic
• Nuclear
• Elastic Potential
• Gravitational Potential
• Chemical Potential
• Magnetic
• Electrostatic

Question 2

Name 6 Types of Energy Transfers

Answer 2

• Heating
• Light (Radiation)
• Nuclear (Radiation)
• Sound (Radiation)
• Electrical Work
• Mechanical Work

Question 3

What is Work Done in Physics (+ Equation)

Answer 3

Work Done is when a force acts on something and makes it move as a result.

Work Done = Force x Distance
W = Fd
(The distance in the direction that the force is applied)

(Therefore you are only working when you are moving something when applying a force)

Question 4

Energy Definition

Answer 4

Energy is defined as the ability or capacity to do work

Question 5

Equation for Mechanical Power

Answer 5

Power = Force x Velocity
P = Fv

Question 6

Equation for Work Done by a Force at an angle to the direction of motion

Answer 6

W = Fx cosΘ

Question 7

Example Question:

A toy car is pulled along by a piece of string which is at 30° to the horizontal. Calculate the work done in pulling the toy if the tension in the string is 10 N, and it is pulled along 5 m.

Answer 7

10 x cos(30) = 8.66

8.66 x 5 = 43.3 J

Question 8

Example Question:

Calculate the work done, in kJ, by a dog pulling a sled horizontally with a constant horizontal force of 100N, moving it by half a kilometre.

Answer 8

W = Fx cos Θ

W = 100 x 500 x cos (0)

W = 100 x 500 x 1 = 50,000J, or 50kJ

Question 9

What is the Equation for Impulse

Answer 9

Impulse = Force x Time
I = Ft

Impulse = Change in Momentum
I = mv - mu

Ft = m(v-u)
F = m(v-u) / t
F = ma

Question 10

How do you calculate the work done from force-distance graphs

Answer 10

W = Fd

It is the area under the force-distance graph.

Question 11

Equation for Work Done

Answer 11

Work Done = Force x Distance moved in the direction of the force

W = Fx

Question 12

How are Work Done and Energy Transferred related

Answer 12

Work Done = Energy Transferred

Question 13

Conservation of Energy Defintion

Answer 13

Energy cannot be created or destroyed. It can only be converted from one form to another

Question 14

Efficiency Definition

Answer 14

Efficiency is a measure of how much useful energy you get out of an object from the energy you put into it.

Question 15

Equations for Efficiency

Answer 15

Efficiency = (Useful Energy Out / Total Energy In) x 100

Efficiency = (Useful Work Done / Total Energy Input) x 100

Efficiency = (Useful Energy Output / Total Power Output) x 100

Question 16

Example Question:

A laptop can convert 400W of electrical power into 240W of light and sound power. What is its efficiency? Where does the rest of the energy go?

Answer 16

(240/400) x 100 = 60% Efficiency

The rest of the energy could be thermal energy, dissipating as heat (into the surroundings as excess heat).

Question 17

What is Useful Energy and Wasted Energy

Answer 17

Useful Energy:
Useful energy is energy transferred for a purpose.

Wasted Energy:
In any machine, energy is wasted due to friction, heat, sound etc..

Question 18

Example Question:

A machine supplies a power of 500W which raises a weight of 150N by 6.0m in 10s

Answer 18

W = Fd
159 x 6 = 900

900/10 = 90

(90/500) x 100 = 18

Question 19

Is it possible to have a Perpetual Motion Machine, and if it isn’t, why not?

Answer 19

Perpetual Motion Machines are not possible. None of them work and all eventually stop. Perpetual Motion is impossible according to the principle of conservation of energy.

Question 20

What is Energy

Answer 20

Energy is the capacity for doing work. It is a scalar quantity, with magnitude but no direction. The SI unit for energy is the joule (J), the same unit as for work done.

Question 21

What is the Principle of Conservation of Energy

Answer 21

The principle of conservation of energy states that the total energy of a closed system remains constant: energy can never be created or destroyed, but it can be transferred from one form to another.

Question 22

Equation for Kinetic Energy (+ what is it?)

Answer 22

Kinetic energy is associated with an object as a result of its motion. You can calculate the KE of an object in linear motion using the equation:

Ek = 1/2 mv^2

Question 23

Equation for Gravitational Potential Energy (+ what is it?)

Answer 23

Gravitational potential energy is the capacity for doing work as a result of an object’s position in a gravitational field. You can calculate the change in GPE Ep of an object in a uniform gravitational field by using the equation:

Ep = mgh

Question 24

When is GPE gained and lost

Answer 24

GPE is gained when an object gets higher, and is lost when an object gets lower.

Question 25

Worked Example: Skydiver A skydiver with a mass of 80kg falls through a distance of 2.0km at a terminal velocity of 45.0ms^-1. Calculate the loss of GPE and explain what happens to the energy lost.

Answer 25

Step 1: Identify the equation needed. Loss in GPE = Ep = mgh Step 2: Substitute the values into the equation and calculate the answer. Ep = 80 x 9.81 x 2000 = 1.6x10^6J. There is no change in the KE of the skydiver, so the GPE is transferred to the thermal energy of the surrounding air.

Question 26

How do Kinetic Energy and Gravitational Potential Energy relate

Answer 26

There are many situation where KE and GPE are exchanged, like when an object falls (it’s GPE decreases and its KE increases) From the principle of conservation of energy, the object will gain an equal amount of KE. Therefore: mgh = 1/2mv^2 (therefore, v = √2gh) *The equation is only valid if there are no resistive forces.

Question 27

What is Power (+equation for it)

Answer 27

Power is the rate of work done. (It is measured in Js^-1 or W) P = W/t (Since work done is equal to energy transfer, we can also define power as the rate of energy transfer).

Question 28

Worked Example: Body Power A 60kg person runs up a flight of steps in a time of 7.2s. The gain in vertical height in this time interval is 5.0m. Calculate the rate of work done against the force of gravity.

Answer 28

Step 1: Calculating the rate of work done against gravity is the same as calculating the power P. Also, the work done against the force of gravity is the same as the gain in the gravitational potential energy Ep of the person. Power = Work Done / Time P = Ep/t = mgh/t Step 2: Substitute the values into the equation and calculate the answer. P = 60x9.81x5.0 / 7.2 = 410W (2sf) The rate of work done against the force of gravity is about 410W. This is the same as 410J per second.

Question 29

How to derive the Equation P = Fv (Power = Force x Velocity)

Answer 29

P = W/t P = Fd/t (The speed of the car is d/t, therefore:) P = Fv

Question 30

Kinetic Energy - Description - Examples

Answer 30

Description: Energy due to motion of an object with mass Examples: - Moving car - Moving atoms

Question 31

Gravitational Potential Energy - Description - Examples

Answer 31

Description: Energy of an object due to its position in a gravitational field. Examples: - Child at the top of a slide - Water held in clouds

Question 32

Chemical Energy - Description - Examples

Answer 32

Description: Energy contained with the chemical bonds between atoms - it can be released when the atoms are rearranged. Examples: - Energy stored within a chemical - Energy stored in petroleum and released when it is burnt

Question 33

Elastic Potential Energy - Description - Examples

Answer 33

Description: Energy stored in an object as a result of reversible change in its shape Examples: - A stretched guitar string - A squashed spring

Question 34

Electrical Potential Energy - Description - Examples

Answer 34

Description: Energy of electrical charges due to their position in an electric field. Examples: - Electrical charges on a thundercloud - Static charge on a charged balloon

Question 35

Nuclear Energy - Description - Examples

Answer 35

Description: Energy within the nuclei of atoms - it can be released when the particles within the nucleus are rearranged Examples: - Energy from fusion processes in the Sun - Energy from nuclear fission reactors

Question 36

Electromagnetic Energy (or Radiant Energy) - Description - Examples

Answer 36

Description: Energy associated with all electromagnetic waves, stored within the oscillating electric and magnetic fields. Examples: - Energy from the hot Sun - Energy from an LED

Question 37

Sound Energy - Description - Examples

Answer 37

Description: Energy of mechanical waves due to the movement of atoms. Examples: - Energy emitted when you clap - Output energy from your headphones

Question 38

Internal (Heat or Thermal) - Description - Examples

Answer 38

Description: The sum of random potential and kinetic energies of atoms in a system. Examples: - A hot cup of tea has more thermal energy than a cold one

Question 39

Formula for Kinetic Energy

Answer 39

Kinetic Energy = 1/2 x Mass x Velocity^2 KE = 1/2mv^2 (J = kg x ms^-1)

Question 40

How is Work Done related to the Gain in Kinetic Energy

Answer 40

Work Done = Gain in Kinetic Energy ‘If a constant force acts on a object over a certain distance, the work done by the force is equal to the gain in the kinetic energy of the object.’

Question 41

Equation for Gravitational Potential Energy

Answer 41

Gravitational Potential Energy = Mass x Gravity x Height Ep = mgh

Question 42

Example Question: Calculate the minimum braking distance for a car of mass 1000kg travelling at 11ms^-1. The car can provide a maximum force of 8500N.

Answer 42

E2 = E1 - Fx 0 = 1/2 x 1000 x 11^2 - 8500x 8500x = 60500 x = 7.12m

Question 43

What is Power?

Answer 43

Power is measured in Watts (W) where 1 watt is equal to an energy of 1 joule transferred each second. Power is defined as the rate of transfer of energy.

Question 44

Equation for Power and Derived Equation for power involving Force and Velocity

Answer 44

Power = Work Done / Time P = W/t P = Fs/t Therefore: P = Fv