2. should understand the definition of work, including when
it is positive, negative, or zero,
Calculate the work done by a specified constant force
on an object that undergoes a specified displacement.
Describe relationship between work, energy, and force
Students will explore the concept of work by taking
notes and solving practice
LEARNING OBJECTIVES
3. • Work is done when a force is
applied to move an object.
As far as Physics is concerned…
When a force is applied to an
object, over a distance, then work
is done on that object.
4. Work-Energy Relationship
• The net work done in a body is equivalent to
Kinetic energy (Work-Kinetic Energy
Theorem).
• This allows us to think of kinetic energy as the
work an object can do while it changes speed.
5. Is Work Being Done?
• A boy pushes a lawn mower. –
• Yes The mower is being pushed by the boy.
Energy is transferred in the process. The
displacement of the object and the force are
in the same direction.
6. Is work being done?
A waiter serving his customers.
Yes and No. As he lifts the dishes he is doing work
on them. As he holds them –no work is done
Even if he is moving the dishes horizontally at
constant speed, no work is done. More about that
later…AND when he places them back down,
negative work is done. More about that later…
7. Work is a scalar quantity and has
positive or negative displacement.
• Work is positive when the component of the
force is the same direction as the displacement.
• For example, when you lift a box, the work done
by the force you exert on the box is positive
because the force is upward, in the same
direction as the displacement.
8. • Work is negative when the force is in the
direction opposite the displacement.
• For example, the force of kinetic friction
between a sliding box and the floor is
opposite the displacement of the box, so the
work done by the force of friction on the box
is negative.
9. • In order for positive work to
occur, the force and displacement
must be in the same direction.
10. • Assume you forgot to set the parking break and your car
starts rolling down a hill.
• You try in vain to stop it by pulling as hard as you can on the
bumper, but the car keeps on moving forward.
• You exert a force on the car opposite to the direction of
travel. The distance traveled in the direction of the force is
negative, you do negative work on the car.
• But the car is pulling you in the direction of travel with a
force of equal magnitude (Newton's third law). The car is
doing positive work on you.
12. For each of the following, indicate whether the
work done on the second object will have a
positive or negative value.
• The road exerts a friction force on a
speeding car skidding to a stop.
• Work has a negative value because
the work caused the moving car to
slow down.
13. Work Sign tips:
• Force is in direction of motion + work
• Force opposes motion - work
• Force is 90o to motion no work
• Object not in motion no work
14. Is Work done while carrying an object
at constant speed?
• NO – The force acting on the object must
be in the same, or opposite direction to
the object’s displacement or at some
angle less that 90o (perpendicular to the
object).
15.
16. In Summary…
• When a force moves a body on
which it acts in the direction of
the force, we say it has done
work.
17. Calculating Work
• The work done by a force is found by
multiplying the force by the distance it
has moved the body in the direction of
the force.
• Equations:
W = Fd F = W/d d = W/F
Units of Work: Joules 1J = 1 Newton-Meters
Also Foot-Pound
Joule = 0.737562149 foot pounds
18. • A student’s backpack weighs 30N. She lifts it
from the floor to a shelf 1.5m high. How
much work is done on the pack full of books.
• Since the backpack weighs 30N, the force needed to
lift it off the ground is equivalent to the 30N it
weighs. (Lifting it requires a force equivalent to the
force of gravity attracting it to the Earth.)
F = 30N d = 1.5m W = F x d
= (30N)(1.5m)
= 45N.m
= 45 J
19. A person pulls a block 2 m along a horizontal surface by a
constant force F = 20 N. Determine the work done by force F
acting on the block.
Force (F) = 20 N
Displacement (s) = 2 m
Solution :
W = F d cos θ = (20)(2)(cos 0) = (20)(2)(1) = 40 Joule
20. A body falls freely from rest, from a height of 2 m.
If acceleration due to gravity is 10 m/s2, determine the work
done by the force of gravity!
Known :
Object’s mass (m) = 1 kg
Height (h) = 2 m
Acceleration due to gravity (g) = 10 m/s2
Solution :
W = F d = w h = m g h
W = (1)(10)(2) = 20 Joule
21. A force F = 10 N accelerates a box over a displacement 2 m.
The floor is rough and exerts a friction force Fk = 2 N.
Determine the net work done on the box.
Known :
Force (F) = 10 N
Force of kinetic friction (Fk) = 2 N
Displacement (d) = 2 m
Solution :
Work done by force F :
W1 = F d cos 0 = (10)(2)(1) = 20 Joule
Work done by force of kinetic friction (Fk) :
W2 = Fk d = (2)(2)(cos 180) = (2)(2)(-1) = -4 Joule
Net work :
Wnet = W1 – W2
Wnet = 20 – 4
Wnet = 16 Joule
22. Activity 2.3
1. How much work is done when a 50N crate is pushed along a
floor a distance of 10m?
2. A carpenter lifts a 45 kg beam 1.2m. How much work is done
on the beam?
3. Work done by Tom and Jerry so the car can move as far as 4
meters. Forces exerted by Tom and Jerry are 50 N and 70 N.
