2. Work
Occurs when force is applied to an object and the object moves
in the direction of the force
Work = force × distance
𝑊 = 𝐹𝑑
Force (N) × distance (m) = N×m (called a “Joule,” J)
4. Work
No work is being done
when holding a heavy
object, like a suitcase
However, you feel tired
because your muscle
cells are doing work
individually
5. Work at an Angle
Work is easily calculated when the force and displacement
are in the same direction, but how is work calculated when
the force is at an angle to the displacement?
6. Work at an Angle
Only the component of the force in the direction of the
displacement does work.
In the image, the component of force in the direction of
displacement is F cosθ
7. Work
Work can be positive, negative, or zero
Work is positive if the force has a
component in the direction of motion
(Figure a)
Work is zero if the force has no component
in the direction of motion (Figure b)
Work is negative if the force has a
component opposite the direction of
motion (Figure c)
8. Work
When more than
one force acts on
an object, the
total work is the
sum of the work
done by each
force separately
Wtotal = W1 + W2
+ W3 + …
9. Work and Energy
When work is done on an object,
the object’s energy changes
What kind of energy is increasing
in these examples?
Pushing a shopping cart
Climbing a mountain
10. Review of Energy Types
Kinetic energy: energy of
motion
Potential energy: stored energy
(position or condition)
Types of potential energy:
Gravitational potential
Elastic potential
Chemical potential
11. Work and Energy
How is work related to
kinetic energy
mathematically?
𝑎 =
𝐹
𝑚
𝑣𝑓
2 = 𝑣𝑖
2 + 2𝑎𝑑
𝑊 = 𝐹 × 𝑑
13. Practice Problem
Calculate the kinetic energy of a truck traveling at 6.0 m/s.
The truck has a mass of 3900 kg.
What would happen to the amount of kinetic energy if the
truck doubled its speed to 12 m/s?
14. Work and Energy
The total work done on an object equals the change in
that object’s kinetic energy.
𝑊𝑡𝑜𝑡𝑎𝑙 = ∆𝐾𝐸
𝑊𝑡𝑜𝑡𝑎𝑙 =
1
2
𝑚𝑣 𝑓
2 −
1
2
𝑚𝑣𝑖
2
Example Problem: How much work is required for a
74-kg sprinter to accelerate from rest to a speed of 2.2
m/s?
Is this work positive or negative?
15. Work and Energy
Work must be done to lift a bowling ball from the floor
onto a shelf.
Even though the ball has no kinetic energy once it's resting
on the shelf, the work done in lifting the ball is not lost—it
is stored as potential energy.
16. Work and Potential Energy
Potential energy (PE) is stored for later
Gravitational potential energy is stored in an object that is
at some height above the ground and has the potential to
fall
Lifting a mass (m) from the ground to a height (h) requires
a force (ma or mg). Thus, the work done and potential
energy acquired equals FORCE x DISTANCE, or…
𝑊 = 𝑚𝑔ℎ
17. Practice Problem
Find the gravitational potential energy of a 65-kg person
standing on a diving board that is 3.0 meters high.
18. Conservation of Energy
Law of Conservation of Energy states that energy can be
transformed from one form to another but never lost or
gained
This concept is essential for solving many physics problems
NOTE: objects moving downward through the same
vertical distance but following different paths will have the
same final speed
19. Practice Problem
A woman drops her keys from a height of 1.2 meters. If
the keys have a mass of 0.12 kg, what is their final speed
when they hit the ground?
20.
21. Power
Doing the same amount of work in a shorter amount of
time takes more effort. Scientists refer to this effort as
POWER.
𝑃 =
𝑊
𝑡
Units:
𝐽
𝑠
or watts (W)
22. Power
To be powerful, an engine must produce a substantial
amount of work in a relatively short time. Similarly, you
produce more power when running up a flight of stairs
than when walking up.
