1. Unit 3, Lesson 3: Conservative and Non-Conservative Forces

2. Conservative Forces:
The work done in moving something from A to B is not dependant on the path.
E.g.
Gravity
Electrostatic Force
Elastic Force

3. Non-Conservative Forces:
The work done moving something from A to B depends on the path taken.
E.g.
Friction
Air resistance
Work done against a non-conservative force is converted to other forms of energy, such as heat and sound.
Work done against a conservative force is stored as potential energy.

4. Conservation of Energy:
If there are no non-conservative forces:
PE1 + KE1 = PE2 + KE2
With non-conservative forces:
W’ = work done by the non-conservative force
W’ = ΔE = ΔPE + ΔKE
PE1 + KE1 + W’ = PE2 +KE2

5. Example: Bri skis down a 30°slope starting from rest
Example: Bri skis down a 30°slope starting from rest. After she has lost 200 m in elevation, her speed is 150 km/h. If her mass is 60 kg, find:
The work done by friction and air resistance (W1)
The average resistance force.
d = 400m
200 m
30°
Note: 150 km/h = m/s

6. Note: 150 km/h = 41.67 m/s Energy before + Work(-) = Energy after
d = 400m
Note: 150 km/h = m/s
200 m
30°
Energy before + Work(-) = Energy after
mgh + W’ = ½mv2
a) W’ = ½mv2 – mgh
= ½ (60)(41.67)2 – (60)(9.8)(200)
= 52,100 – 117,600
= -65,500 J
b) W’ = F • d
-65,500 = - F (400)
F = 160 N

7. Varying Forces F d
W = F • d =
area under F vs. d graph F d

8. AP: pg. 146 # 35-41(non cons) 55-57 (varying)
Example: Loyally defending his friend Hannah, Logan fires a 10 g bullet through a 60 cm long gun barrel.
F(N)
d(cm)
Using the graph, find:
a) The work done on the bullet.
b) The velocity of the bullet as it exits the gun.
1500 N 20 40 60
W = area = 0.4 m • 1500 N = 600 J
Note: distance must be in meters
b) W = ΔKE
600 = ½ (0.01)v2
v = 350 m/s
AP: pg. 146 # 35-41(non cons) (varying)
WB pg

9. Recommended Questions:
Energy Practice Problems #1, 3, 8-10, 14-17, and 19-20
15 min