Energy and Power

Monday we reviewed a bit of what we did on wednesday, we covered calculating work from a graph and when work is done by pulling something on a angle. We went over how work is produced: the force must be in the same direction as displacement to have work done.

What we learned on Monday was that when work is done, energy is transferred from one object to another. If the energy of an object increases, the work done on it will be positive. If the energy of an objects decreases, (ex. slowly bringing an object to the floor.) the work done on it will be negative.

Time has no effect when calculating work, but it does matter when we calculate power.

Power is the rate at which work is done and the rate at which energy is used. We calculate power in watts (W).... 1 watt is 1 J/s. Since watts are really small, we use kilowatts too (kW). To describe engines, the term horsepower is used. 1 horsepower equals 746 watts.

Things to Remember:

• P=W/t
• work is the product of force and the objects displacement (same direction)
• the less amount of time doing work, the more power being used (same displacement)
• to calculate kilowatt hours, multiply number of kilowatts by the number of hours used
• work is measured in joules (J)

Ex. 2. (in notes)

How much power is developed in lifting 82 kg of concrete to a height of 21 meters in 12 seconds?

m= 82kg
h= 21m (distance)
t= 12s
P=?
W=?
F=?

F= mg
F= 82kg x 9.80 m/s²
F= 803.6 N

W= Fd
W= 803.6N x 21m
W= 16 875.6J

P= W/t
P= 16 875.6J/12s
P= 1400W