1. Reading Quiz Ch
You push a loaded cart with 20 N and move 15 meters. Did you do work?
You carry a penguin at level height at constant velocity. Did you do work?
You push on a cabinet really hard, with lots of force, but can’t get it to move. Did you do work?
What has more kinetic energy, a heavy ball or a light ball, both moving together at 5 m/s?
What has more Potential energy, a penguin at ground level, or a penguin sitting on the desk?
Write the equtions for: Kinetic Energy, Work, Gravitational Potential Energy.

2. A way of transferring Energy into or out of a system.
Work:
W = F•d•cos
W = work (J)
F = Force (N)
d= distance (m)
= angle between F and d
For Work , a FORCE must be exerted and the DISPLACEMENT must be in the direction of the force.

3. SI Units of Work
W = F•x•cos
Joule = newton•meter
J = N•m = kg m2/s2
One calorie (1 cal) =energy required to warm 1 g of water by 1°C
1 cal = J
1 Cal (food calorie) = 1k cal (1000 cal) = 4184 J

4. Where does the cos  come from?
Work =
F cos , is the component of the force parallel to d F 
Fcos x

5. Work Work = F x Work = 0 When exerted Force is // to Displacement
When F = 0
When x = 0

6. When is work Done? yes no no yes yes W=F x cos W=0 W=0 W=F x cos

7. Practice Problem 1:
A net force of 60N pushes a crate across a floor. How much work is required to push the crate 15 m?
15 m
W = F•x•cos 0
W = (60 N)(15 m)
60 N
W = 900 N•m
W = 900 J

8. W = F∙x∙cos 0 W = mg(x) Practice Problem 2:
If you lift a 2 kg ball up a distance of 3m, much work is done to lift the ball against gravity?
W = F∙x∙cos 0
What minimum force (F) must you apply to lift the ball?
The ball’s weight
Weight = -Fg = -mg = -(2kg)(-9.8m/s2)
W = mg(x)
W = (2 kg)(9.8 m/s2)(3 m)
W = 58.8kgm2/s2 = 60 J

9. Energy due to an object’s motion.
Kinetic Energy
Energy due to an object’s motion.
KE = Kinetic Energy (J)
m = mass (kg)
v = velocity (m/s)
KE = ½ mv2
If an object is not moving, it has zero kinetic energy

10. A 0.5 kg ball is thrown at a speed of 20 m/s.
Practice Problem 6:
A 0.5 kg ball is thrown at a speed of 20 m/s.
(a) Find its kinetic energy.
KE = ½ mv2
KE = ½ (0.5 kg)(20 m/s)2
KE = 100 J
All energy is measured in units of Joules (J)

11. Work-Energy Theorem When Applying A Force to an Object Causes Motion Work Done = Energy of Motion
Let the applied force be in same direction as the displacement…
W = F•x
W = ma x
but F = ma, so
from Kinematics, acceleration x displacement gives us velocity: vf2= v02 + 2a(x)
rearranging vf2 - v02 2
= a(x)
W = ½ m(vf2-v02)

12. Gravitational Potential Energy
Energy that results from an object’s position above the ground. This object has the potential to do work.
PEgrav = mgh
m = mass (kg)
g = 9.8 m/s2
h = height (m)

13. Gravitational Potential Energy
Energy from a FIELD
PEgrav = mgh = (mass)(9.8)(height)
Where m = mass (kg)
g = 9.8 m/s2 (on Earth at sea level)
h = height of object above ground
The assumption is that PE=0 at ground level
The 0 of PE is a like the 0 of time – we set it where convenient.
What you ACTUALLY measure is PE and t

14. Practice Problem 7:
An 880. kg piano is raised to 4.00 m above the ground. How much PE does it have after being raised?
PE = mgh
PE = (880kg)(9.8m/s2)(4 m)
=34496
kgm2 s2
PE= J

15. Conservation of Energy
Energy cannot be created or destroyed.
Energy can be transformed from one form to another, but the total energy never changes.

16. Examples of Energy :
Kinetic Energy
Thermal Energy
Sound
Light
Electrical
Chemical Energy (energy of molecular bonds)
Nuclear Energy (energy of nuclear bonds)
mass (E=mc2, where c=3x108m/s)
Energy Transfer:
Work
Heating
Radiation