3.3 Work, Energy and Power Flashcards
How is work done defined in physics?
Work Done (Nm or J) = Force (N) x Distance Moved in Direction of Force (m)
Why is work done when you lift an object off the ground?
You exert a force on the object but there is no movement. Energy is transferred into GPE and back into kinetic when the object falls.
How is energy defined?
The capacity or ability to do work.
How do you calculate work done when the force applied is at an angle to the direction in which the object moves?
W = Fxcosθ
How is efficiency calculated?
Efficiency (%) = (useful output energy/total input energy) x 100
What does it mean if an object has a high efficiency?
There is a greater percentage of input energy converted.
How is power defined in words?
Power is the rate of work done.
How is power defined as an equation?
Power (W) = Work Done (J)/Time (s)
What are the two units used for power?
- Watts (W)
- Joules per Second (Js-1)
What is an example where a constant force has to be applied on an object to maintain a constant speed?
A constant force must be applied in a car to maintain speed on a level surface.
What is an expression relating power and constant speed?
Power (W) = Force (N) x Constant Speed (ms-1)
Why is the braking distance of a car directly proportional to the initial speed²?
The more initial kinetic energy/speed there is, the more energy needs to be converted to thermal energy in braking, which therefore increases braking distance.
What is gravitational potential energy?
The capacity for doing work as a result of an object’s position in a gravitational field.
How is gravitational potential energy calculated?
GPE (J) = Mass (kg) x Gravitational Field Strength x Change in Height (m)
Why does the GPE equation only work for objects in a uniform field?
Mass and air resistance aren’t considered, meaning all objects will only have the gravitional field considered.
