Module 3: C5 - Work, Energy, And Power Flashcards
Name 8 Energy stores
- Thermal
- Kinetic
- Nuclear
- Elastic Potential
- Gravitational Potential
- Chemical Potential
- Magnetic
- Electrostatic
Name 6 Types of Energy Transfers
- Heating
- Light (Radiation)
- Nuclear (Radiation)
- Sound (Radiation)
- Electrical Work
- Mechanical Work
What is Work Done in Physics (+ Equation)
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)
Energy Definition
Energy is defined as the ability or capacity to do work
Equation for Mechanical Power
Power = Force x Velocity
P = Fv
Equation for Work Done by a Force at an angle to the direction of motion
W = Fx cosΘ
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.
10 x cos(30) = 8.66
8.66 x 5 = 43.3 J
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.
W = Fx cos Θ
W = 100 x 500 x cos (0)
W = 100 x 500 x 1 = 50,000J, or 50kJ
What is the Equation for Impulse
Impulse = Force x Time
I = Ft
Impulse = Change in Momentum
I = mv - mu
Ft = m(v-u)
F = m(v-u) / t
F = ma
How do you calculate the work done from force-distance graphs
W = Fd
It is the area under the force-distance graph.
Equation for Work Done
Work Done = Force x Distance moved in the direction of the force
W = Fx
How are Work Done and Energy Transferred related
Work Done = Energy Transferred
Conservation of Energy Defintion
Energy cannot be created or destroyed. It can only be converted from one form to another
Efficiency Definition
Efficiency is a measure of how much useful energy you get out of an object from the energy you put into it.
Equations for Efficiency
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
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?
(240/400) x 100 = 60% Efficiency
The rest of the energy could be thermal energy, dissipating as heat (into the surroundings as excess heat).
What is Useful Energy and Wasted Energy
Useful Energy:
Useful energy is energy transferred for a purpose.
Wasted Energy:
In any machine, energy is wasted due to friction, heat, sound etc..
Example Question:
A machine supplies a power of 500W which raises a weight of 150N by 6.0m in 10s
W = Fd
159 x 6 = 900
900/10 = 90
(90/500) x 100 = 18
Is it possible to have a Perpetual Motion Machine, and if it isn’t, why not?
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.
What is Energy
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.
What is the Principle of Conservation of Energy
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.
Equation for Kinetic Energy (+ what is it?)
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
Equation for Gravitational Potential Energy (+ what is it?)
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
When is GPE gained and lost
GPE is gained when an object gets higher, and is lost when an object gets lower.
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.
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.
How do Kinetic Energy and Gravitational Potential Energy relate
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.
What is Power (+equation for it)
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).
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.
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.
How to derive the Equation P = Fv
(Power = Force x Velocity)
P = W/t
P = Fd/t
(The speed of the car is d/t, therefore:)
P = Fv
Kinetic Energy
- Description
- Examples
Description:
Energy due to motion of an object with mass
Examples:
- Moving car
- Moving atoms
Gravitational Potential Energy
- Description
- Examples
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
Chemical Energy
- Description
- Examples
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
Elastic Potential Energy
- Description
- Examples
Description:
Energy stored in an object as a result of reversible change in its shape
Examples:
- A stretched guitar string
- A squashed spring
Electrical Potential Energy
- Description
- Examples
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
Nuclear Energy
- Description
- Examples
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
Electromagnetic Energy (or Radiant Energy)
- Description
- Examples
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
Sound Energy
- Description
- Examples
Description:
Energy of mechanical waves due to the movement of atoms.
Examples:
- Energy emitted when you clap
- Output energy from your headphones
Internal (Heat or Thermal)
- Description
- Examples
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
Formula for Kinetic Energy
Kinetic Energy = 1/2 x Mass x Velocity^2
KE = 1/2mv^2
(J = kg x ms^-1)
How is Work Done related to the Gain in Kinetic Energy
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.’
Equation for Gravitational Potential Energy
Gravitational Potential Energy = Mass x Gravity x Height
Ep = mgh
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.
E2 = E1 - Fx
0 = 1/2 x 1000 x 11^2 - 8500x
8500x = 60500
x = 7.12m
What is Power?
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.
Equation for Power and Derived Equation for power involving Force and Velocity
Power = Work Done / Time
P = W/t
P = Fs/t
Therefore:
P = Fv
