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Forces. Three forces act on an object. In which direction does the object accelerate?

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Presentation on theme: "Forces. Three forces act on an object. In which direction does the object accelerate?"— Presentation transcript:

1 Forces

2 Three forces act on an object. In which direction does the object accelerate?

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4 An elevator suspended by a cable is moving upward and slowing to a stop. Which free-body diagram is correct?

5 The acceleration must be down, so the net force must be down.

6 A Martian lander is approaching the surface. It is slowing its descent by firing its rocket motor. Which is the correct free-body diagram for the lander?

7 To slow down while descending, the acceleration must be up. To accelerate up, the net force must be up. Choice a is the only free body diagram consistent with that determination.

8 Youve just kicked a rock, and it is now sliding across the ground about 2 meters in front of you. Which of these forces act on the ball? Check all that apply. A.Gravity, acting downward. B. The normal force, acting upward. C.The force of the kick, acting in the direction of motion. D.Friction, acting opposite the direction of motion.

9 Youve just kicked a rock, and it is now sliding across the ground about 2 meters in front of you. Which of these forces act on the ball? A.Gravity, acting downward. B. The normal force, acting upward. C.The force of the kick, acting in the direction of motion. D.Friction, acting opposite the direction of motion. Once the foot is no longer in contact with the rock, it no longer exerts a force on the rock

10 The box and the floor are made of the same materials in all situations. Rank order, from largest to smallest, the size of the friction forces toin these 5 different situations. A. f c > f d > f e > f b > f a. B. f b > f c > f d > f e > f a. C. f a > f c = f d = f e > f b. D. f a = f b > f c = f d = f e. E. f b > f c = f d = f e > f a.

11 The box and the floor are made of the same materials in all situations. Rank order, from largest to smallest, the size of the friction forces toin these 5 different situations. A. f c > f d > f e > f b > f a. B. f b > f c > f d > f e > f a. C. f a > f c = f d = f e > f b. D. f a = f b > f c = f d = f e. E. f b > f c = f d = f e > f a. The is no friction in case a. Case b is near maximum static friction which is larger than kinetic friction. Kinetic friction does not depend on speed.

12 A. n > w. B. n = w. C. n < w. D.We cant tell about n without knowing v. A car is rolling over the top of a hill at speed v. At this instant,

13 A. n > w. B. n = w. C. n < w. D.We cant tell about n without knowing v. A car is rolling over the top of a hill at speed v. At this instant, The acceleration is towards the center of the circle, in this case down. The net force must be down by Newtons second law.

14 A ball on a string is swung in a vertical circle. The string happens to break when it is parallel to the ground and the ball is moving up. Which trajectory does the ball follow?

15 The velocity when the string breaks is straight up. The only force acting on the ball is gravity which has no x component, so there is no subsequent motion in the x direction.

16 Car B is stopped for a red light. Car A, which has the same mass as car B, doesnt see the red light and runs into the back of B. Which of the following statements is true? A.B exerts a force on A but A doesnt exert a force on B. B.B exerts a larger force on A than A exerts on B. C.B exerts the same amount of force on A as A exerts on B. D.A exerts a larger force on B than B exerts on A. E.A exerts a force on B but B doesnt exert a force on A.

17 Car B is stopped for a red light. Car A, which has the same mass as car B, doesnt see the red light and runs into the back of B. Which of the following statements is true? A.B exerts a force on A but A doesnt exert a force on B. B.B exerts a larger force on A than A exerts on B. C.B exerts the same amount of force on A as A exerts on B. D.A exerts a larger force on B than B exerts on A. E.A exerts a force on B but B doesnt exert a force on A. This is true by Newtons third law.

18 Boxes A and B are sliding to the right across a frictionless table. The hand H is slowing them down. The mass of A is larger than the mass of B. Rank in order, from largest to smallest, the horizontal forces on A, B, and H. A. F B on H = F H on B = F A on B = F B on A B. F B on H = F H on B > F A on B = F B on A C. F B on H = F H on B < F A on B = F B on A D. F H on B = F H on A > F A on B

19 Boxes A and B are sliding to the right across a frictionless table. The hand H is slowing them down. The mass of A is larger than the mass of B. Rank in order, from largest to smallest, the horizontal forces on A, B, and H. A. F B on H = F H on B = F A on B = F B on A B. F B on H = F H on B > F A on B = F B on A C. F B on H = F H on B < F A on B = F B on A D. F H on B = F H on A > F A on B The equalities are true by Newtons third law. Block B must be accelerating to the left, so the net force is to the left. This makes F HB > F AB.

20 A.greater than B.less than C.equal to All three 50 kg blocks are at rest. Is the tension in rope 2 greater than, less than or equal to the tension in rope 1?

21 A.greater than B.less than C.equal to All three 50 kg blocks are at rest. Is the tension in rope 2 greater than, less than or equal to the tension in rope 1? Newtons first law for the block on the left proves that the tension equals the weight. Newtons first law for either of the individual blocks on the right proves that the tension equals the weight for an individual.

22 A.greater than B.less than C.equal to The block on the far right is moving up with constant speed. Is the tension in rope 2 greater than, less than or equal to the tension in rope 1?

23 A.greater than B.less than C.equal to The block on the far right is moving up with constant speed. Is the tension in rope 2 greater than, less than or equal to the tension in rope 1? This is still a Newtons first law situation!

24 In the figure to the right is the tension in the string greater than, less than, or equal to the weight of block B? A.Greater than B.Less than C.Equal to

25 In the figure to the right is the tension in the string greater than, less than, or equal to the weight of block B? A.Greater than B.Less than C.Equal to This is a Newtons second law situation for each of the blocks. Block A will accelerate to the right and block B will accelerate down. The net force on B must be down by Newtons second law. The tension force exerted by the rope on block B must be less than the weight force exerted by the earth on block B.

26 A small car is pushing a larger truck that has a dead battery. The mass of the truck is larger than the mass of the car. Which of the following statements is true? A.The car exerts a force on the truck but the truck doesnt exert a force on the car. B.The car exerts a larger force on the truck than the truck exerts on the car. C.The car exerts the same amount of force on the truck as the truck exerts on the car. D.The truck exerts a larger force on the car than the car exerts on the truck. E.The truck exerts a force on the car but the car doesnt exert a force on the truck.

27 A small car is pushing a larger truck that has a dead battery. The mass of the truck is larger than the mass of the car. Which of the following statements is true? A.The car exerts a force on the truck but the truck doesnt exert a force on the car. B.The car exerts a larger force on the truck than the truck exerts on the car. C.The car exerts the same amount of force on the truck as the truck exerts on the car. D.The truck exerts a larger force on the car than the car exerts on the truck. E.The truck exerts a force on the car but the car doesnt exert a force on the truck. This is true by Newtons third law.


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