1. Work Who does the most work? Definition of work in physics:
A. the one who lifted the weight to the table
B. the one who moved the weight to the far end of the table
C. same work done.
Definition of work in physics:
Work (done by the force F): the component of force along the displacement times the magnitude of the displacement.
Unit: 1 Joule =  

2. Work

3. Positive or negative work
If acting by itself the force would speed up the object along its path, the work by that force on the object is _____
If acting by itself the force would slow down the object along its path, the work by that force on the object is _____
If the force is perpendicular to the path , the work by that force on the object is _____
demo: ball on string

4. Decide if the work is …
a) positive
b) negative
c) zero
The girl’s force on the sled
P1. Friction force on the sled
P2. The force of gravity on the sled
P3. The sled’s force on the girl
P4. The normal force on the sled

5. Net work and kinetic energy changes
 Connection to kinematic equation
If the object sped up, the net work done was ______.
If it slowed down, the net work done was _______.
If its speed didn’t change, the net work done was ___.

6. Gravitational potential energy
 PEgrav = mgy keeps track of the work done against (and energy stored by) gravity
What is change of the box’s PE when we push it up the ramp 3 meters?

7. Change in PE for different paths? y y
Conservative forces
e.g. gravity, perfect springs
Nonconservative forces
friction, muscles, motors

8. Change in PE depends only on the initial and final points, not path
Nonconservative work depends on the path

9. Conservation of energy
(only KE and PE for now…no friction, muscles, motors) or
Pendulum speed at bottom of swing
Energy vs time

10. Energy is easier!  y
Some problems that are complicated using kinematics and Newton’s 2nd law can be worked easily with energy ideas, if you don’t need to know _______!

11. Energy is easier!  y
Some problems that are many steps using kinematics and Newton’s 2nd law can be worked easily with energy ideas, if you don’t need to know ____!
time
A block going 8 m/s at the bottom will be going 2 m/s after it moves a distance ______ m along the ramp

12. Energy is easier!  y
From a cliff of height h you throw balls straight up, straight down and horizontally, all with the same initial speed. Ignore air friction
P5. Which ball (all thrown at the same speed) has the highest speed just before it hits the ground? Think energy conservation!
thrown straight up
thrown straight down
thrown horizontally
all the same speed

13. Energy is easier!  y
In a new Olympic sport, skiers are sped up by a falling mass, and then are launched off a ramp. When M has fallen a distance D from rest, how fast will the skier be going? Ignore friction.
(Can choose y=0 differently for each object)

14. Energy is easier!  y
P6. A 500 kg car starts from rest on a track 100 m above the ground. It does a loop-de-loop that is 25 m from the ground at the top. There is no friction. How fast is it going at the top of the loop?_____ m/s
h=100m
h=0