C5 (Work, Energy and Power) Flashcards
Work done equation
W= force x distance moved in the direction of the force
What are the units for work done
1 Nm= 1 J
What is work done equal to
Energy transferred (Energy is defined as the capacity to do work)
Define energy
The capacity for doing work
It’s a scalar quantity (magnitude no direction)
SI units- Joules
The energy of systems with mass can be classified as…
Kinetic and gravitational potential energy
Kinetic energy
Energy due to the movement of an object
Gravitational potential energy
The energy due to the position of an object in the Earth’s gravitational field
Name some other forms of energy
Chemical Elastic potential Electrical potential Nuclear Radiant (electromagnetic energy) Sound Internal (heat or thermal)
Principle of conservation of energy
Energy can be converted from one form to another
(The total initial energy is always equal to the total final energy)
The total energy in a closed system remains constant, energy can never be created or destroyed, but can be transferred from one form to another.
Equation for kinetic energy
Ek= 1/2 mv^2
What is KE directly proportional to
The mass and square of it’s speed
When can we find out the gravitational potential energy
When an object is in a uniform gravitational field
Equation for gravitational potential energy
Ep=mgh
Where g is the acceleration of free fall
When is GPE gained and lost
When an object gets higher GPE is gained
When an object gets lower GPE is lost
There are many situations where KE and GPE are exchanged
Give an example
As an object falls
mgh=1/2mv^2
Where v is the final velocity of the object
The mass on both side cancel out
So v= √2gh
This equation is valid only if there are no resistive forces involved.
What’s power
Equation
The rate of work done
P=W/t
What is power measured in
In joules pers second Js^-1 or in watts (W)
1Wis equal to one joule per second
How can you find the power of a car from its velocity
P equals W over t and so P equals Fx over t, and x over t is equal to speed. Therefore P=Fv
Expression for efficiency
Efficiency= useful output energy/ total input energy x 100
Contact forces (examples):
Drag (air resistance, friction, upthrust, buoyancy) Thrust Tension Compression Lift
Non contact forces
Weight Magnetism Electromagnetic Electrostatic (Last three all have potential fields).
What are the 4 fundamental forces
Gravitational (W=mg)
Electromagnetic/ Electrostatic
Nuclear (weak and strong)
How can you detect changes in gravity
Using a gravimeter (used to map tunnelling, caves, mines and volcanic activity).
When can we say work is done?
When a force acts on something and makes it move (no motion, no work).
Equation for work done
W= F x change in s (displacement)
What is energy
The capacity/ ability to get work done (measured in joules).
Useful Energy
Only a fraction of the energy can be considered “useful energy”
What is work time equal to in a force-distance time graph
The area underneath
What is efficiency
Is the measure of the amount of useful energy
Efficiency equation
Efficiency (%) = useful energy output divided by total energy output
Useful work done divided by total energy input
Momentum equation
p= m x x
mass x velocity
What is the rate of change of momentum of an object proportional to?
To the resultant force acting on the object
F directly proportional change p/ change t
Conservation of momentum
The total momentum in a close system (no external forces) is equal to the total momentum after
In elastic collisions what is conserved
Kinetic energy
Equation for kinetic energy
1/2 mv^2
If a constant force acts on an object, the energy transferred (kinetic energy) is equal to…
The work done
Two equations for power
P= change W/ change t
Therefore P= F x v
