Class Notes
07 November
• Turn in HW / Tonights…
 Use physics concepts to explain
 All:Use Wnet=KE, not kin or F=ma
• Today:
 More E1 + WNC = E2
 Power
Example
• A 1.00-kg ball is thrown from a
5.20-m-tall platform with a speed
of 15.0 m/s at an angle of 40.0°.
What is the speed of the ball just
before it hits the ground?
B
Solution
EA + WNC = ED
A
KEA+PEg,A+WNC=KE
C D+PEg,D
vD = 18.1
?
m/s
D
If rather than throwing the ball up I
threw it down, at same speed, with
what speed would it hit the ground?
a.
b.
c.
d.
Greater than
previous answer
Less than previous
answer
Equal to previous
answer
Depends on angle
4
3
1
0
a.
b.
c.
d.
Example
• A 1.00-kg ball is thrown from a
5.20-m-tall platform with a speed
of 15.0 m/s at an angle of 40.0°.
What is the speed of the ball just
before it hits the ground?
• Repeat if air resistance saps
20.0 J of energy.
vD = 16.9 m/s
Using Conservation of M. E.
What forces are acting on the system?
Nonconservative
or unsure
Conservative
Do they do work?
Use E1 = E2
YES
NO
Use E1 + WNC = E2
Example
• A 500.-g block starts from rest at
the top of a 83.3-cm-long hill. Find
the speed of the ball at the bottom
of the hill and the max. distance
the spring is compressed, if the
incline is 36.9º and k = 300. N/m.
q
Solution
Given:
m = 0.500 kg
l = 0.833 m
q = 36.9°
vB = 3.13 m/s
xCD = 12.8 cm
A
q
k = 300. N/m
m=0
vB=? xCD=?
B
C D
Homework
• Finish reading Chapter 6
• Do problems on Worksheet 30
 More E1 + WNC = E2 problems