Lecture18%20Friction%20and%20Drag%20Forces-handout

Lecture18%20Friction%20and%20Drag%20Forces-handout - Today...

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1 Today Newton’s 3 rd Law continued Friction and air drag

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Newton’s Third Law Weight and normal force are not an action/reaction pair. Action/reaction partners have same type, act on two different objects. Two surfaces in contact exert a normal force on each other. Reaction partner to weight of the TV is the gravitational attraction of Earth by the TV. Weight and normal force, same magnitude here. Why not consider the normal force the reaction partner to the weight, since the weight causes the normal force. Because they are not always equal in magnitude and opposite in direction: w n
W12 Q3 A 1200 kg car pushes a 2800 kg truck that has a dead battery. When the driver steps on the accelerator, the drive wheels of the car push against the ground with a force of 1500 N. What is the magnitude of the force of the truck on the car? Hint: Identify the forces and the correct action/reaction pair.

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Solution The blue arrows show the correct action/reaction pair: Force of the car on the truck (F ConT ) and the Force of the truck on the car (F TonC ). The force exerted by the ground on the car F GonC is shown in red . We know it’s 1500 N. For the car, 2 nd law yields: F GonC – F TonC = m C a For the truck we obtain: F ConT = m T a Substitute for acceleration a and solve for F TonC (Ans.: 1050 N)
Tension Forces A student is pulling on a rope that is fixed to a tree. His force is 100 N. 1) What is the tension in the rope? 2) What is the force of the tree on the rope? A. 0 N B. 100 N C. 200 N D. More than 200 N

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Tension Forces Two students are pulling on a rope. Each students pulls with a force of 100 N.
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