Work

Work is a scalar quantity and has the following conditions:• a force must be exerted on an object• the object must be displaced by the force• at least part of the force must be in the same direction as the displacement

for positive work

holding a still box = no work lifting a box = work done

• work is the product of the magnitude of the applied force and the displacement of the object in the direction of the force

• work is measured in joules (J)

W = F∆d

Ex. 1

A worker pulls a cart with a force of 40N through a displacement of 5.0 m. Calculate the work done.

F = 40 Nd = 5 mW = ?

W = F ∆d = (40)(5) = 200 J

\the work done is 200 J

4.2 Prac. #1, 3-6

Positive and Negative Work

• work is positive when it is in the same direction as the displacement• work can be negative when the acting force is in the opposite direction of the

displacement • i.e. friction

Ex. 2

A 20 kg boy slides on the grass which has a coefficient of kinetic friction of 0.25. Calculate the work done by kinetic friction if the boy slides for 5m.

m = 20 kgg = 9.81 m/s2

FN = ?

FN = mg

= (20)(9.81) = 196.2 N

FN = 196.2 Nµ = 0.25Ff = ?

Ff = µ FN

= (0.25)(196.2) = 49.05 N W = F ∆d = (49.05)(5) = 245.25 J

4.2 Prac. #8

Work Done Against Gravity

• if an object is lifted against gravity with no acceleration, the force used must overcome the force of gravity (fnet = Fa + Fg a=0 fnet = 0 Fa = Fg)

• the force of gravity is the product of the mass of the object and the acceleration due to gravity (Fg=mg)

Ex. 3

A man lifts a 10 kg box 95 cm from the floor at a constant speed.Calculate the work done

F = mg = (10)(9.81) = 98.1 N

W = Fd = (98.1)(0.95) = 93.2 J

4.2 Prac. #9-11

Zero Work• no work is accomplished if there is no displacement, no force, or

both don’t happen• therefore, there still may be a force applied but no displacement

therefore no work is accomplished

4.2 Prac. #12-14 UC #1-6