1. Work and Energy
Tyburn Physics 2019

2. Work
When a force acts upon an object to cause a displacement of the object
W = F • d • cos Θ
F = force
d = displacement (distance)
Θ = the angle between the
Force and the Displacement
(Cos 0o = 1.0, so if force is in the direction of movement all of it
contributes to displacement)

3. Force causes displacement +W
Force opposite displacement -W
Force doesn’t cause displacement W=0

4. The Joule is the unit of Work
W = F • d • cos Θ = (N) • (m) = Joule (J)

5. Force at an angle to displacement
It is only the horizontal component of the tension force in the chain that causes Fido to be displaced to the right.
The horizontal component is found by multiplying the force F by the cosine of the angle between F and d.

6. Works for Lawn Mowing as well..
Most of his work is just pushing the mower DOWN into the grass!

7. Why is the work all the same?
The force is directed along the ramp. The displacement is along the ramp. The angle of the ramp to the ground is irrelevant!
Hmmmmm………
Why is the work all the same?

8. Question:
ONLY Fapp is doing work.
W = (10N)(5.0m)(cos 0)
W = 50 J
A 10-N force is applied to push a block across a friction free surface for a displacement of 5.0 m to the right.  Indicate which force(s) are doing work upon the object. Then calculate the work done by these forces.

9. Conceptual:
In the last question, if the force was friction, slowing the object instead, the work would be negative.
If both friction and an applied force are present, the friction does negative work and the applied force does positive work.
If the friction and applied forces cancel (no acceleration), the magnitude of the work done by each is equal but opposite in direction.

10. Ben carries a 200-N suitcase up three flights of stairs (a height of 10.0 m) and then pushes it with a horizontal force of 50.0 N at a constant speed of 0.5 m/s for a horizontal distance of 35.0 meters. How much work does Ben do on his suitcase during this entire motion?
vertical W = (200 N) * (10 m) * cos (0 deg) = J.
horizontal W = (50 N) * (35 m) * cos (0 deg) = 1750 J.
3750 J

11. Kinetic Energy
Energy of MOTION!

12. KE = ½ m v2 Kg (m/s)2 Kg m2/s2 Kgm/s2  m N  m Joule
Kinetic Energy – the energy of motion
KE = ½ m v2
Kg (m/s)2
Kg m2/s2
Kgm/s2  m
N  m
Joule

13. PE  KE  PE of a swing…