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Work
Physics 11
Comprehension Check
1. What is the impulse given to a golf ball of
mass 45.9g if it starts at rest and attains a
final velocity of 35m/s?
2. If the golf ball in problem 1 was in contact
with the golf club for 0.027s, what force
acted on the golf ball?
3. If there is no acceleration is there
momentum? Is there impulse?
4. Suppose that a 75.0kg goalkeeper catches
a 0.40 kg ball moving at 32m/s. With
what forward velocity must the goalkeeper
jump when she catches the ball so that she
and the ball have zero horizontal velocity?
Comprehension Check
1. 1.6Ns
2. 6.0x101N
3. Only momentum can have 0
acceleration. A force is needed for
impulse so there must be
acceleration.
4. V = + 0.17 m/s
Work
 In common language, work can mean
a variety of different things, however
when we describe work from a
scientific standpoint, work has a very
precise definition
 This means we must be careful not to
confuse work as used in the English
language and the work we will
determine in physics
Work
 Work is the transfer of mechanical energy
(total kinetic and potential energy).
 We will learn more about energy soon!
 In English, we can do work if we sit at our
desks and mark papers or write an essay.
In physics, we can only do work if we
move/are displaced! Sitting at your desk
does not count as physics work.
Work and Energy
 We haven’t talked about energy yet
but we will connect these two
concepts in the very near future.
 Work causes an object to gain energy.
 Example: If you lift a dumbbell above
your head, you move the dumbbell. You
also give the dumbbell POTENTIAL
ENERGY (as now it has the potential to
fall).
Work
 If you use force on an object but it
moves in a different direction than
the force, no work is done.
Work – Summary Slide
 Work is the transfer of mechanical energy
(total kinetic and potential energy).
 Work is done only if an object moves.
 When work is on an object, that object
gains energy
 Work is only done on an object when the
force and displacement are in the same
direction.
Work (copy)




Symbol = W
Units: Joule (J)
1 J = 1 Nm
Work is a
scalar***
 
W  F  d
W  Fd cos 
Zero Work Conditions
 How can we make work equal zero?
 1) Apply a force that does not cause
motion
 Example: Holding an object at the same
height for hours is not doing any work you may get tired but you are doing no
work on the object.
Zero Work Conditions
 2) Uniform motion in the absence of
a net force
 Example: If an object is already in
motion, it will continue in that same
motion (Newton’s First Law). If a
hockey puck is sliding across the ice
at a constant speed, no work is being
done.
Zero Work Conditions
 3) Applying a force that is perpendicular
to the motion
 Example: You are carrying a book down
the hallway. You are lifting the book (force
is upwards) but your motion is forwards
(perpendicular). Therefore there is no work
being done on the textbook by the person
once you are moving.
What about this situation… work or no
work? Complete with partner. Be
prepared to share!
 A) If you are pushing a grocery cart
through the store.
 B) If you are pushing a car and it is going
forward?
 C) “” going backward?
 D) “” not moving?
 E) If you are driving forward on cruise
control
 F) Swinging a ball on a rope in a circle
a) Yes – moving the cart, force is pushing the
cart in the same direction
• Yes – force is in same direction
• Yes since the car is moving in a direction
opposite this is called “negative work”
(discuss more later)
• No work done as car is not moving
• No work done as car is moving at constant
speed (so no net force)
• No work as the string’s tension force acts
in a direction perpendicular to the ball
Zero Work Conditions Summary
Slide
 3 cases:
 1) A force is used but causes no
displacement
 Example: Doing a wall squat for 2 minutes
 2) No acceleration
 Therefore no force
 Example: Something that is displaced at a
constant velocity
 3) Force is perpendicular to the
displacement
 Example: Walking with a book in your hands,
the book has no work done on it as the force is
upward and the displacement is direction of
walking.
Example 1: (page 220)
 A student is rearranging her room.
She decides to move her desk across
the room a distance of 3.00m. She
moves the desk at a constant velocity
by exerting a horizontal force of
200N. Calculate the amount of work
done on the desk by the student.
 W = Fd
 W = 200 x 3
 W = 600 J
Comprehension Check
1. How much work is done if you push on a wall
with 3500N but the wall does not move?
2. How much work is done by you on the book if
you are carrying the book down the hall at
constant velocity?
3. How much work is done by you if you push a
box that has a mass of 50kg down the hallway
45m with a force of 25N?
 Page 221, questions 1, 2, 3
Comprehension Check
1. 0
2. 0
3. W = Fd = 25 x 45 =1125 N = 1200N
Practice Problems
 Page 221, questions 1, 2, 3
Example 2:
 An applied force of 20. N accelerates
a block across a level, frictionless
surface from rest to a velocity of
8.0 m/s in a time of 2.5 s.
 Calculate the work done by this force.
Practice Problems
 Page 225
 4-10
Work at an Angle
 Remember, work is done if a force is
exerted in the direction of motion.
 If you are pushing or pulling
something at an angle, only the
component that acts in the direction
of motion is doing work.
 W = F d cos Θ
Example 3:
 A person is doing work on the lawn mower
by pushing with 105N it at an angle of 40°
to the horizontal. If the person pushes the
mower for 5.00m, how much work is being
done on the mower? WHAT EQUATION DO
WE USE? WHY?
 W = F d cos Θ
 We use this equation because the handle
on the lawn mower is at an angle so we
only need the Fx value (cos).
 W = (105)(5)cos40 =
Practice Problems
1. If you pull a crate with a force of 550N
at an angle of 35° to the horizontal and
it moves 25m horizontally, how much
work was done?
(1.1x104J)
 Page 235, questions 16, 17
Using Graphs to Calculate Work
(copy)
Estimating Work from a Graph
Positive and Negative Work
 When we consider work it is a scalar
so lacks direction
 How is it possible to have positive and
negative work?
Positive and Negative Work
 Positive work
occurs when the
angle between the
force and
displacement is 0°90°
 “Negative work”
occurs when the
angle between the
force and
displacement is
90°-180°
F
d
F
d
The Meaning of Negative Work
 On occasion, a force acts upon a moving
object to hinder a displacement.
 Examples might include a car skidding to a
stop on a roadway surface or a baseball
runner sliding to a stop on the infield dirt.
 In such instances, the force acts in the
direction opposite the objects motion
in order to slow it down. The force
doesn't cause the displacement but rather
hinders it.
Negative and Positive Work
Summary
 Positive work occurs when the angle
between the force and displacement is 0°90°
 “Negative work” occurs when the angle
between the force and displacement is 90°180°. Force acts in a direction opposite to
the displacement.
 Negative work means that the force is
hindering the displacement (like frictional
forces)
Example:
 Imagine a weight lifter. She must lift the barbell up and then lower it down.
 Overall how much work has she done when she
lifts the bar and then lowers to the same spot?
 How much work does she do on the lifting
part?
 How much work does she do on the lowering
part?
 Overall, she has done NO WORK but she has
done positive work when she lifts the bar-bell
and negative work (the same amount) to lower
it.
Practice Problems
 Page 229
 Questions 11 (a, b, c only), 12, 13
Page 235
 14, 15, 18
 Page 235
 Section Review q 2, 3, 4, 5
 Quiz… Tuesday after lunch