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Aim: More friction Do Now: Why do objects at rest have more friction than objects in motion?

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Presentation on theme: "Aim: More friction Do Now: Why do objects at rest have more friction than objects in motion?"— Presentation transcript:

1 Aim: More friction Do Now: Why do objects at rest have more friction than objects in motion?

2 A piece of steel whose mass is 100 kg is placed on a steel beam as a force is applied to it causing it to move with a constant velocity. F FgFg FNFN F Draw a free-body diagram labeling all the forces

3 What is the weight? F g = mg F g = (100 kg)(9.8 m/s 2 ) F g = 980 N What is the normal force? F N = F g F N = 980 N What is the net force? 0 N (velocity is constant) F FgFg FNFN F = 980 N

4 A new force of 700 N is applied. Does steel move? Solve for the maximum static force of friction F F = µ s F N F F = (0.74)(980N) F F = 725.2 N The steel does not move. To get it to move, you must have a force greater than 725.2 N

5 A car whose mass is 1,000 kg is accelerating at a rate of 5 m/s 2 with a force of 10,684 N on a surface containing friction. Draw and label a free-body diagram F = 10,684 N FgFg FNFN F

6 What is the weight? F g = mg F g = (1,000 kg)(9.8 m/s 2 ) F g = 9,800 N What is the normal force? F N = F g F N = 9,800 N What is the net force? F Net = ma F Net = (1,000 kg)(5 m/s 2 ) F Net = 5,000 N F = 10,684 N FgFg FNFN F = 9,800 N

7 What is the force of friction? F = 10,684 N F g = 9,800 N F N = 9,800 N F F Net = F – F F 5,000 N = 10,684 N – F F F F = 5,684 N = 5,684 N

8 What is the coefficient of friction? F F = µ k F N 5,684 N = µ k (9,800 N) µ k = 0.58 What two objects are in contact with one another? Rubber on wet concrete

9 An 80-kilogram skier slides on waxed skis along a horizontal surface of snow at constant velocity while pushing with his poles. What is the horizontal component of the force pushing him forward? F = F f (constant velocity) F = μ k F N F = μ k F g (F N = F g ) F = μ k mg F = (0.05)(80 kg)(9.8 m/s 2 ) F = 39.2 N

10 A car’s performance is tested on various horizontal road surfaces. The brakes are applied, causing the rubber tires of the car to slide along the road without rolling. The tires encounter the greatest force of friction to stop the car on (1)dry concrete (2)wet concrete (3)dry asphalt (4)wet asphalt


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