Transcript Work Powerpoint

Work
Physics 11
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 – The physics definition
 Work is the transfer of mechanical energy (total
kinetic and potential energy).
 Work is done only if an object moves.
 When the work is done upon the object, that object
gains energy
 Work is only done on an object when the force and
displacement are in the same direction.
Work
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The symbol is 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 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?
 If you swing a mass on a string, is
there work being done? Why or why
not?
 If you are pushing a grocery cart
through the store.
 If you are pushing a car and it is
going forward? Backward? Not
moving?
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.
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?
Comprehension Check
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.
 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?
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)
Using Graphs to Calculate Work
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
Example:
 Imagine a weight lifter. She must lift
the bar-bell up and then lower it
down. 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 in
Practice
 Page 235
 14, 15, 18
 Page 235
 Section Review