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

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Presentation transcript:

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

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

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.

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

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 = 41.67 m/s

Note: 150 km/h = 41.67 m/s Energy before + Work(-) = Energy after d = 400m Note: 150 km/h = 41.67 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

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

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) 55-57 (varying) WB pg 158 - 166

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