work, energy and power

energy

an object’s capacity to do work (unit: Joules (J)

mechanical energy

comes from position/motion

gravitational potential energy

comes from change in height

kinetic energy

comes from motion

energy can be expressed specifically by using the term _____

work

work

transfer of energy; done whenever force is applied through distance; when done → there is a transfer of energy

work =

scalar dot product between force and displacement

scalar

quantity with NO direction

work formula

Fxcosθ where F is the force applied, x is the displacement, and θ is the angle between the force and displacement vectors.

F and x must be…

parallel

cos (0) = 1

cos (180) = -1

cos (90) = 0

what component causes the box to move?

only horizontal component; the vertical component (Fsinθ) does no work because it isn’t parallel to the displacement

W = 0J

A man pushes against a car stuck in a snow bank while his
date sits nervously behind the steering wheel trying not to make
the tires spin. He applies a 3000 N force, while the car and his
date together weigh 4500 N. However, the car does not move.
How much work did he do on the car?

Sally carries a text book under her arm along a horizontal path, covering 10
meters. The book’s mass is 4.5 kg. How much work was done on the text
book?

None, since the force Sally exerted against gravity is
perpendicular to the distance the book moved. (cos 90° = 0 so W = 0).
It may help to think of it as there being no energy being transferred to the
book in this example.