Work and EnergyChapter 5 pg 168-189

Chapter 12 pg 438-441

Key terms to know• Work• Kinetic energy• Work-kinetic energy theorem• Potential energy• Gravitational potential energy

• Elastic potential energy• Mechanical energy• Power

Concept Review

•Kinematics (section 2-2)•Newton’s second law ( section 4-3)

•Force of friction (section 4-4)

What is work?• In everday sense: to do something that takes physical or mental effort

• NOT the same in physics• Def: the product of the magnitudes of the component of a force along the direction of displacement and the displacement

• Work is the transfer of energy through motion.

In order for work to take place, a force must be exerted through a distance.

The amount of work done depends on two things:

(1) the amount of force exerted

(2) the distance over which the force is applied.

When is work done?When is work done?

There are two factors to keep in mind when deciding when work is being done:

(1) something has to move

(2) the motion must be in the direction of the applied force.

Work can be calculated by using the following formula:

Work=force x distance

Work is done on the books when they are being lifted, but no work is done on them when they are being held or carried horizontally.

• Work is done only when components of a force are parallel to a displacement

• Components of the force perpendicular to a displacement do not do work

Examples of work being done

• You car runs out of gas and you have to push it down the road to a gas station (must push car with constant force)

• You pick up your book back to leave class

• You pick up something off the ground and lift it straight up.

When is work NOT done?• You hold a heavy book bag at arm’s length for several minutes

• You carry the book bag over to a friend (along a horizontal path) while walking at constant velocity

• WORK IS NOT DONE ON AN OBJECT UNLESS THE OBJECT IS MOVED BECAUSE OF THE ACTION OF FORCE.

• Imagine pushing a box across a floor. If the force you exert is horizontal, all of your effort moves the box. If your force isn’t horizontal, only the horizontal component of applied force causes a displacement and does work

Displacement

Force

W = Fd(cos )

= 0o then cos 0o = 1; W = Fd = 90o then cos 90o = 0; W = 0

• Wnet = Fnetd (cos )

• Unit: newton-meters (Nm) or joules (J)

Example 1• How much work is done on a vacuum cleaner pulled 3.0 m by a force of 50.0 N at an angle of 30.0o above the horizontal?

F = 50.0 N = 30.0o d = 3.0 m

W = (50 N)(3.0 m) (cos 30.0o)

W = 130 J

The sign of work is important

• Positive - component of force is in same direction as displacement

• Negative - force is in the opposite direction of displacement

• If net work is positive - object speeds up and net force does work on object

• Net work is negative - object slows down and work is done by the object on another object

Problems for you to do1. A tugboat pulls a ship with a

constant net horizontal force of 5000 N and causes the ship to move. How much work is done on the ship if it moves 3.00 km?

2. A weight lifter lifts a set of weights a vertical distance of 2.00 m. If a constant force of 350 N is exerted on the weights, what is the net work done on the weight?

3. If 2.0 J of work is done in raising a 180 g apple, how far is the apple lifted?

Equations to use• W = Fd• W = Fd cos • W = (mg)(d)• W = Fapplied d(cos )• W = Fkd(cos ) = -Fgkd (cos )• Wnet = W1 + W2 +…• Wnet = Fnetd (cos ) = (Fg - Fk) d(cos )

• Note: Fg= mg where g = 9.8 m/s2

Homework

• Section review pg 171• # 2, 3, 4, 5, 6

REVIEW