What is a Force?
- A force is a push or pull
- Acts on an object
- Requires an agent
- Is a vector
- Is a contact force or a long range force
Newton’s First Law: Consider an object with no force acting on it. If it is at rest, It will remain at rest; if it is moving, it will continue to move in a straight line at a constant speed.
What is Power?
Power is the way we measure how quickly energy is being changed. When we look at the power of a moving object, we are really looking at how fast work is happening.
A lift motor has to move a fully laden lift 4m between floors in 1.5s.
The lift has a mass of 1850 kg (ignore friction).
a) Calculate the weight of the fully laden lift.
W = mg = 1850 x 10 = 18500N
b) What is the upward force in the cable when the lift is moving at a constant speed?
At constant speed, forces must be balanced. Upward force = downward force (weight) Upward force = 18500 N
c) What is the work done by the motor?
WD = F x d = 18500 x 4 so WD = 74000 J
d) What is the minimum power of the motor to raise the lift at a steady speed?
Steady speed means forces are balanced. ! Power = work done time = 7400 1.5
So Power = 49333 W
Difference between force and power:
|Unit||watt = joules/second||newton|
|Definition||Power is the rate at which work is done, or energy is transmitted.||A push or a pull resulting from an interaction between objects.|
|Derivations from other quantities||P = w/t (power = work divided by time)||F = m a (force = mass multiplied by acceleration)|
|Everyday Example||Horsepower (1 horsepower = 750 Watts).||Gravity, friction, magnetism.|
|Relation to “Work”||Rate at which work is performed.||Force applied over a distance creates work.|
|Named After||James Watt||Isaac Newton|