Work, Power, Machines

Chapter 8 Part One

Work

• In physics, work is done when a forcemoves an object through a distance.

• When you push a box up an incline, you “do work” on the box.

dFW ⋅=

Work = Force x Distance

Units

• Force is measured in Newtons.

• Distance is measured in meters.

• N x m = Nm = Joules

• Joules and Nm are the same things.

Force and Distance must be in the same direction!!

• If you pick an object up, the force must be the upward force.

• If you slide an object sideways on a table, the force must be the sideways force.

• If you measure the distance up an incline, the force must be the one exerted in a direction up the incline.

Is work vector or scalar.• Force and displacement are both vector.

• But, due to complicated math

(vector dot products), work is scalar.

• Power and energy are scalar as well.

• It does make the math easier.

– Use arithmetic instead of vector addition.

– Joules and watts don’t have direction.

How much work do you do to lift a 15 N object upward 4 meters?

• When you pick an

object up, you have to

apply a force equal to

its weight (15 N). (why?, CVPM? UAPM?)

W = F x d

W = 15 N x 4 m = 60 J

How much work do you do to lift a 5-kg object upward 4 meters?

• When you pick an

object up, you have to

apply a force equal to

its weight

(5kgx10m/s2=50 N).

W = F x d

W = 50 N x 4 m = 200 J

How much work do you do to slide a 15 N object 4

meters sideways if the force of friction is 3 N?

• When you slide an

object sideways, you

have to apply a force

equal to friction (3 N). (why? CVPM? UAPM?)

W = F x d

W = 3 N x 4 m = 12 J

How much work do you do to slide a 15 N object

up an incline 4 meters long if the force the person

must push with is 7 N?

• When you slide an

object up an incline,

you have to use the

force up the incline

(7N).

W = F x d

W = 7 N x 4 m = 28 J

How much work do you do to slide a 5-kg object

sideways 4 meters if the acceleration is 6 m/s2?

F=ma

F = 5-kg x 6 m/s2

F = 30-N

W = F x d

W = 30 N x 4 m = 120 J

Sometimes work can be negative!

• If the force and displacement are

exactly opposite, then work is negative.

• When you lift a box, you pull upward and do positive work.

• But gravity pulls downward and does negative work!

How much work is done by…

• Fa

(10)(15)= 150J

• FF

(-10)(15)= -150J

• FN

(100)(0)= 0J

• Fg

(100)(0)= 0J

dFW ⋅=

CVPM tells you what about the forces?

WHY?!

How much work is done by…

• FT

(30)(6)= 180J

• Fg

(-20)(6)= -120J

WHY?!

Special Case – No Work Done!

• The force (weight) is

downward. There is

virtually no sideways

force.

• The distance is

sideways. There is no

downward motion.

W = 14-N x 0-m = 0J

W = 0-N x 10-m = 0J

Assignment 8a

• Due Tomorrow

• Start the Pulley Lab Today