When work is done on something, that something undergoes a change in either position (increase in potential energy)
or in motion (increase in kinetic energy = increase of speed)

The effect of exerting a force on something depends on how long the force acts, how long was meant as time.
 ( impulse = F t = change in momentum  -    increasing the time = increase the change in motion).
Now, time, is meant as distance when we speak about work. (Like a long barrel will produce a high speed bullet
because the force acts a long time /  because the force a long distance). (increasing the distance = increasing the energy).

For example:
1) If you lift a book, you are doing work on the book.
Your book gains energy (its height relative to the ground  increases) and,
 you are losing energy (you are burning some calories = chemical potential energy is used ).

A)What kind of energy the  book is gaining:
kinetic energy ? thermal energy ? gravitational potential energy ? nuclear energy ?  

B) If the mass of the book is 2kg and the change is height is 50cm, what is the work done of the book?
hint: i) find the force applied. the earth exerts a force on the book downward (mg) so you need to exert a force upward (mg) to balance it.
your force F  = weight = mg
ii) the displacement is the height. convert to meters
iii) use Work = force x displacement. Units are in joule.
C) In this situation, we say: you are doing work against gravity. Why?

2) To do work on an object, the object needs to move in the same direction as the force.
Let's say you are pushing really hard on a wall and you are all sweaty.
A) Are you doing work on the wall? (of course the wall won't move), why ?
B) When you are pushing, your muscles move. So you are losing energy. Where did the energy go ?
C) If you lift an object, the object is gaining _________________ potential energy. This energy is stored and ir due to the new position of the object.
This energy is ready to be used. A stone falling from 10 yards will do more damage than a stoine falling from 1 yard.
The potential energy is propotional to the ________ and to the _________.
 (PE = mgh, h is the height, m the mass and g = 9.8m/s/s on Earth)

D) If  the speed of an object increases, its ________________ energy increases.
The kinetic energy in proportional to the speed _________. (KE = 0.5 m V²)

3) You are pushing a small red wagon with a constant force F = 5N.
A) According to Newton's second Law   F = m ( ∆V/∆T ) :  
the velocity of the wagon _____________________ .
B) Are you doing work on the wagon ?
C) What kind of energy is transfered to the wagon ?
D) If the displacement = 5 m, find the work done on the wagon.
E)Fill the blanks.
 Let's say now you are pushing the wagon but it is moving at a constant speed. There is no acceleration because
the net force on the wagon is zero.  Indeed, your push is balanced by ____________ force. along the horizontal.
Along the vertical the weight is balanced by the _____ force. Draw all the forces acting on the wagon.

Because there is no acceleration, the speed is ___________. So is there an increase of energy in the system wagon ? _________.
(kinetic ?  or gravitational ? )
That means the net work done on the wagon is _______. (your work is being canceled by the negative work of friction).


4) Lee pushes a 20kg mass 100 m across a floor with a horizontal force of 80N.
A) CAlculate the amount of work Lee does.
B) what kind of energy is transfered to the mass ?

5) THe third floor of a house is 8 m above street level.
A) How much work is needed to move a 150kg fridge to the third floor?   (first find the weight)

B) what kind of energy is transfered to the fridge.
C) you are doing work against _________________________

6) Stan does 176 J of work lifting himself 0.300m . What is Stan's mass? (find the weight, then the mass)

7) Suppose you are trying to move a ball from A to B. (the ball moves along tracks).  A) Are you doing any work
if your push is perpendicular to the displacement (AB is the vector displacement) ? (figure)

So work done = ________ if force is ________ to the displacement AB.

B)What if you apply your push parallel to the displacement ?

The force is parallel to the displacement, you are doing work.
work =_____ x ________  (most efficient way to transfer ________to the ball).

C) If the force is neither perpendicular not parallel, only the ___________component  the force is doing work

So work done = ________ x AB   (is it Fx or Fy ? )


D) if you carry a suitcase around (moving at a constant speed) , are you doing work on the suitcase ? draw the situation.
(ask yourself if the suitcase gets energy: is it moving faster ? is it getting higher ? )

E) A student librarian picks up a 2.2kg book from the floor to a height of 1.25m.
He carries the book 8.0m to the stacks and places the book on a shelf that is 0.35m higher .
How much work does he do on the book? (draw a cartoon).

8) watch the movie in class and answers the questions
work and energy video
 Physics: A World In Motion: Work and Energy  (30mn)
on streaming.discovery
quiz:
i) An escalator lift you to a eight of 3m, your mass is 45kg. What is the work done by the escalator ?
(not taking in account the friction)
ii) If the elevator lift you to the second floor, what kind of energy do you gain?
iv) potential energy is the energy due to the __________ or the _____________ of an object ?
v) In the ride free-falll why the potential energy at the top is not equal to the kinetic energy at the bottom of the track ?
What are other form of energy (rather than Kinetic and potential )


9) A force of 8.00 10² N is needed to push a car across a lot. Two students push the car 40.0m. How much work is done?

10) A worker carried cement blocks , weighting 1.50  10² N (weight = force) each, up a ladder onto a scaffold 8.00 m high.
The worker carries them at a rate of 2 blocks per minute. How much work is done by the worker in:
 A) 10 minutes ? B) 1.00 hr?
(first find the work done to lift 1 block, then the work done to lift 2 blocks in 1 minute )

11) The hammer of a pile driver has a mass for 1.00 10² kg. THe machine's engine lifts it to a height of 5.00m every 10.0s.
A) How much work must the machine do to lift the hammer ? (first find the weight of the hammer then the work done in 10s)
B) How much work does the machine do in 1.00 min?

11) If you apply a constant force  F on an object over a distance,  the work done W = force x distance.
If you plot  force vs distance you will get a horizontal line if the force is constant:



Along the x-axis, the distance increases by 1m. Along the yaxis, the force increases by 1N.
A) What is the force applied ? F = ___________N
B) what is the total work done ? W= ___________ J
C) the area under the line is ____________ N.m
D) So the work done is the ____________under the line

12) Using what you learnt from 11):
observe the following graphs and compute the work done:



WORK DONE = __________________


WORK DONE = _______________

13) If the graph force vs displacement is not made of lines but is a curve.
(see the first image of this web page).
How could you find the work done? (see the first image, above the title)
any idea ?

14) You do work if you lift a book 1 meter above the ground.
A) How does the amount of work change if you lift the book twice as high

B) You lift 2 identical books one meter above the ground.

15) REVIEW
The unit for work is ____________
work = ________ (N)x ____________(m)
Suppose that you apply a 50N horizontal force to a 25kg box pushing the box 6 meters across the floor. How much work do you do on the
box ?
WHy isn't work being done on a barbell when a weight lifter is holding the barbell over his head ?
The amount of work done when 1 newton force moves an object 1 meter is _____________

16)

A rock has a mass of 10kg and is standing at the top of a cliff.
A)Find the potential energy of the rock. PE = ________ J
(hint: PE = mgh)
B) If you were to lift (first figure) this rock, vertically, at a constant speed, from the ground to the height of 2m,
the work done, by you, on the rock = ____________ J
(hint: work done on rock = increase of energy of the rock)
In that case, you need to work against gravity and the force you need to exert on the rock = _________N
(work = force x height = weight x height,)
C) Because you took Physics 101, you know about machine like an inclined plane. Instead of lifting the rock vertically,
you push it along an inclined plane, 4 meters long. (see figure 2). At a constant speed. In that case, what is the force you exert on the rock ?
hint: same change in energy (or work) (see A ) but now  work = F (unknown) x distance (4m)   solve for F
F= _________ N is it easier ? _______ your force has been multiplied by ________ because the distance is multiplied by _______.
So the price to pay is to push a __________ distance. (smaller ? greater ? )
(hint: increase of energy of the rock is still the same = same work done = force x distance)
D) Now (figure 3) you are using a stair to lift the rock. Each stair is ________ m high .
What is the work done to lift the rock ? _______ J . Are you doing work when rolling the rock along the horizontal ?
E) So no matter the trajectory from the ground to the hill, the change in potential energy is the ___________.
Only the force and the __________ change. 






machines video:
Work, Energy, and the Simple Machine: Compound Machines
on streaming.discovery

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