WORK AND POWER
WHEN OBJECTS MOVE
October Sky Quiz 1
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
How did those living in Homer’s hometown make a living?
What was Homer’s daily job before school?
What happened to Homer’s grandfather in the mine?
When Homer wasn’t playing in the mountains, what did he enjoy doing
most?
What was Homer’s nickname?
What was Sputnik?
Which country was responsible for Sputnik?
Give one detail of the fight between Homer and his brother Jim
explained in chapter 2.
In what sport did Homer’s brother Jim excel?
Why did Homer’s mom want him to be successful at building a rocket?
Changes in Motion
 Any change in an object’s motion is
caused by a FORCE.
 When we apply a force on an object to
cause this motion, we do WORK on the
object
 Work done is a result of a force over a
DISTANCE
Work
 Work is done only if the force
causes an object to move in
the SAME direction as the
force
Concept Check
State whether work is done in each of the following situations:
1. A teacher applies a force to a wall and becomes
exhausted.
2. A book falls off a table and free falls to the ground.
3. A waiter carries a tray full of meals above his head by one
arm straight across the room at a constant speed.
(Careful! This is a difficult question.)
4. A rocket accelerates through space.
Doing Work
When a weight lifter
raises a heavy barbell,
he does work on the
barbell.
Doing Work
When a weight lifter
simply holds a barbell
overhead, he does NO
work on the barbell.
Calculating Work
Work = F x d
Force applied(N) X Distance (m)
Example Problem 1
 Bud, a very large man of mass 130
kg, stands on a pogo stick. How
much work is done as Bud
compresses the spring of the pogo
stick 0.50 m?
Work done at an ANGLE
 Remember that a force is
a vector quantity
 The work done is parallel
to the direction of the
objects motion.
 This is the horizontal
component of the force
Calculating Work at an Angle

X: direction of motion
Equation
W  F cos d
The angle is always between the
force and the displacement
The Waiter
 To do work the force
must cause the
displacement
 A vertical force does
not cause a
horizontal
displacement
W  F cos d
No work is done
because the cosine of
90 degrees is ZERO.
UNIT of Work
 The unit for work is the Newton-meter
 We call the newton-meter the joule (J)
for short.
 One joule of work is done when a force
of one Newton is exerted over a distance
of one meter.
1 J = 1 Nm
Example Problem 2
 A force of 50 N acts on the block at
the angle shown in the diagram. The
block moves a horizontal distance of
3.0 m. How much work is done by the
applied force?
W = F (cosɵ)d
W = (50 N)(cos 30)(3 m)
W = 129.9 Joules
Example 3
 Renatta Gass is out with her friends. Misfortune
occurs and Renatta and her friends find themselves
getting a workout. They apply a cumulative force of
1080 N to push the car 218 m to the nearest fuel
station. Determine the work done on the car.
W=F xd
W = (1080 N)(218 m) W
= 2.35 x 105 Joules
Example 4
A student with a mass of 80.0 kg runs up three flights of
stairs in 12.0 sec. The student has gone a vertical
distance of 8.0 m. Determine the amount of work done
by the student to elevate his body to this height. Assume
that her speed is constant.
W=F xd
W = (80kg x 9.8m/s2) (8 m)
W = 6272 Joules
Example 5
Hans Full is pulling on a rope to drag his backpack to
school across the ice. He pulls upwards and rightwards
with a force of 22.9 Newtons at an angle of 35 degrees
above the horizontal to drag his backpack a horizontal
distance of 129 meters to the right. Determine the work
(in Joules) done upon the backpack.
W = F (cosɵ)d
W = (22.9 N)(cos 35)(129 m)
W = 2420 Joules
Example 6
Ben Travlun carries a 200-N suitcase up three flights of
stairs (a height of 10.0 m) and then pushes it with a
horizontal force of 50.0 N at a constant speed of 0.5 m/s
for a horizontal distance of 35.0 meters. How much work
does Ben do on his suitcase during this entire motion?
W=F xd
W = (200 N)(10 m)
W = 2000 Joules
W=F xd
W = (50 N)(35.0 m)
W = 1750 Joules
3750 J