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.
When are GPE and KE exchanged?
When an object falls, GPE decreases whilst KE increases.
What is an expression for the final speed of an object that is dropped from a height?
Final Speed (ms-1) = √(2 x Gravitational Field Strength x Height (m))
Assuming there are no resistive forces acting on the object.
What is kinetic energy?
The energy associated with an object as a result of it’s motion.
How is kinetic energy calculated?
For objects travelling at the same speed, how is KE related to mass?
Kinetic energy is directly proportional to the mass.
What are the two general classifications of energy for systems with mass?
- KE = energy due to movement of an object.
- GPE = energy due to position of an object in the Earth’s gravitational field.
What is the principle of conservation of energy?
Energy cannot be created or destroyed, the total energy of a closed system remains constant, energy can only be transferred.
What does the term potential mean?
It means that energy is stored.
What are two examples of potential energy?
- Elastic potential energy
- Electric potential energy
Is energy scalar or vector?
Scalar.
