1. DO NOW:
Work = _________________
In order for work to be done, an object must have a
__________________
Is work done when a person pushes against a wall?
Name a unit that goes with work__________

2. Work- Mechanical Energy -
‘To Do Work, Forces Must Cause Displacements’
frictionless
Mechanical Energy -

3. “ONLY THE COMPONENT OF THE FORCE IN THE DIRECTION OF THE MOTION IS USED TO DO WORK”
Ex: A box is pulled by a rope with a force of 10N at 30° for 2m. How much work was done?
10N Fx 2m
At 90° - there is no force component in the direction of the force and no work is done
As the component of the force in the direction of the motion decreases, less work is done

4. Positive (+) and Negative Work (-)
Since Work/Energy is a __________ quantity,
it has no ____________. Therefore (+) and (-) values simply means ME is being added to or taken away from system.

5. Note: FORCE IS IN SAME DIRECTION AS MOTION
Ex: How much work is done by the waiter who carries a tray horizontally across the room? v
Ex: How much work is done pushing a car up an incline 10m if the incline is set at 20°?
10m
F = 500N
Note: FORCE IS IN SAME DIRECTION AS MOTION

6. Gravitational Potential Energy –
m = 100kg
How much work is done lifting the barbell?
b) How much PE does the barbell have after being lifted?
c) How fast will the barbell strike the ground if dropped? 2m
d) If the barbell was lifted twice as high, how much work would be the work done in terms of (W), the potential energy gained in terms of (PE), and the kinetic energy when it hits the ground in terms of (KE)?

7. When work is done against an objects inertia, what type of energy does it obtain?
When work is done against Earth’s gravitational field, what type of energy does it obtain?
A 10kg object has 196.2Nm of work done on it lifting it against gravity? How high did it go?

8. How much work does the sun do holding the Earth in orbit?

9. In which situation is more work done on the ball
In which situation is more work done on the ball? In which situation does the ball have greater PE? h h h
A 155-newton box is pushed 5.6m up a ramp to a vertical height of 1.8m by force ‘F’.
What is the total amount of gravitational potential energy gained by the box?
What is the angle of the ramp?
If the ramp has rollers and can be considered frictionless,
what force is need to push the box up the ramp?

11. Kinetic Energy –
A force of 50N is applied to pull back a .1kg arrow back .4m…
How much work is done on the bow?
How much KE will the arrow have when released?
What will be the velocity of the arrow when released?
d) What would be the KE if the arrow left with 2x and 3x the velocity?
During Flight
BONUS: If the arrow was shot straight upward, how high would it go (neglecting friction)?
KE (J)
v(m/s)

12. Conservation of Energy (Ideal System) – Roller Coaster

13. If there is no friction, the speed of a roller coaster will depend only on its height compared to its starting height.

14. When work is done on a system, Energy is given to it…
Mechanical Energy
energy of motion stored energy
Kinetic Energy Potential Energy
Work-Energy Theorem:
Try to trace back to the origin of the arrows KE…

15. Conservation of Energy (Ideal System)

16. Conservation of Energy (Ideal System)
A 1kg pendulum swings back and forth…
What is the Total Mechanical Energy of the system?
If point A is .3m above point B, how fast does the pendulum move at point B?
If the string is cut at point B that is m above the floor, what will be the range of the projectile?

17. Conservation of Energy (Ideal System) – Roller Coaster
m=1000kg
75m
20m
10m 5m 5m
Work done to get to point ‘A’?
b) Force machine exerts to move the coaster 75m.
How much PE is lost at point ‘B’?
d) How much KE is gained at point ‘B’?
e) How fast is car going at points B,C,D and E?
What is the highest the coaster will make it up
the last hill?

18. Graphical Representations
The graph below shows the force required to compress a spring in a marble shooter.
How much force was required to compress the spring 4cm?
b) How much work was done on the spring compressing it 4cm?
If all of the stored energy in the spring went into the KE of
the 10g marble, what velocity would it have leaving the barrel?
If the marble landed 2.2m away from the base of the table, how high
is the table?
Assuming the average force on the marble from the spring was 4N,
how long does the marble acceleration in the barrel? What is the acceleration of the marble during that time?
Displacement (cm)

19. Work Done by External Forces: Wext
Remember that Work/Energy is a Scalar quantity and that positive (+) or negative (-) work can be done effectively changing the total amount of Mechanical Energy (ME) a system has…
Wext = ΔME
Therefore, the work done by some external force on a system must lead to a change in either PE or KE…
KE + PE ± Wext = KE +PE

20. If a force causes an increase in the ME energy of the system, we say the net work done or Wext = _______
If a force causes an decrease in the ME energy of the system, we say the net work done or Wext = _______

21. Conservation of Energy (Non–Ideal System)
A .06kg tennis ball is dropped from a height of 2m…
What is the PE at 2m?
What force was required to raise the tennis ball?
How much Mechanical Energy (ME) was lost between point ‘A’ and point ‘B’?
What percent of the ME was lost and where did it go?
A 10kg crate starts from rest atop a 2m high incline and slides to the bottom.
If the velocity of the crate at the bottom of the incline is 4m/s,
much work was done by friction?
b) If length ‘d’ of the incline is 5m, how much friction was present?

22. SPRINGS Revisited…
A 1kg mass hangs from a spring stretching the spring 20cm…
a) What is the restorative force of the spring?
b) What is the spring constant of the spring?
c) How much elastic PE is stored in the spring?
Which plot represents the spring with the greatest spring constant?
Given the values, what is the value for ‘k’ of spring B?
How much work was done on spring B?
If the slope of D is half of B, how much force would be required stretch spring D the same as B?

23. Applying Conservation Laws to Springs…
A 1kg mass falls for .45s before landing on a spring…
a) From what height did the mass fall from?
b) How much KE does the mass have when it lands on the spring?
c) If the spring has a spring constant of 50N/m, how far is the spring
compressed?
d) If the spring compressed twice as far, how much energy would
be stored in the spring?
e) If it fell from the same height, how much falling mass would
cause the amount of stored energy from your answer to ‘d’?

24. Power -
Power
60W
100W
time
2 different people run up a flight of stairs to a height of 4m. Determine the power of each person if…
a) A 50kg person makes it up the stairs in 4 seconds.
b) A 100 kg person makes it up in 8 seconds.

25. A horse pulls a plow with a force of 500N at a constant velocity of 2m/s…
How much power is generated by the horse?
How much energy does the horse use in 1s?
How far does the horse get in 5s, and 10s?
If 1hp = 746Watts, how many hp does the horse have?
Work
Power
time
velocity
A weightlifter lifts a 50kg mass using energy at a rate of 250J/s for 1.5s…
How much power is dissipated by the weightlifter?
How high does the mass rise in 1.5s?
With what velocity does the mass rise?