CTN-1A glider is gliding along an air track at constant speed. There is no friction (assume that the air resistance is small enough to ignore).What can you say about the net force (total force) on the glider?A) The net force is zero.B) The net force is non-zero and is in the direction of motion.C) The net force is non-zero and is in the direction opposite the motion.D) The net force is non-zero and is perpendicular to the motion.Answer: The net force on the glider must be zero. Since its velocity is constant, its acceleration is zero. Since its acceleration is zero, the net force on the glider must be zero, according to NII: netF ma=rrCTN-2 An astronaut in intergalactic space is twirling a rock on a string. Suddenly the string breaks when the rock is at the point shown)Which path (A, B, C, or D) does the rock follow after the string breaks?Answer: C: Before the string breaks, the rock is always moving tangential to the circle. After the string breaks, there are NO forces acting on the rock, so according to Newton's first law, it must move with constant velocity. v = constantSnap!pinkgreenyellowpurpleABCDCTN-3 An object is being lowered on a cord at a constant speed. How does the tension T in the cord compare to the weight mg of the object?A) T = mgB) T > mgC) T < mgAnswer: T=mg Since velocity = constant, acceleration a = 0. By Newton's 2nd law, since a = 0, it must be that Fnet = 0. Since Fnet = 0, the upward force must exactly cancel the downward force. mg T Free-body diagram velocity = constantCTN-4 An object is being lowered on a cord at a speed which is decreasing. There are two forces on the object, the weight, magnitude mg, and the tension, magnitude T, in the cord.What is the direction of the acceleration?A) up B) down C) a=0Which equation is true:A) T = mgB) T > mgC) T < mgAnswers: The acceleration is upward (even though the velocity is downward).T > mg . Since the acceleration is upward, the net force must be upward . This is becauseof Newton's 2nd Law: netF ma=rr. If the net force is upward, then the upward force T must be greater magnitude than the downward force mg.T mg a Free-body diagram speed decreasingCTN-5A sailboat is being blown across the sea at a constant velocity. What is the direction of the net force on the boat?A) Left  B) Right  C) Net force is zeroD) Down  E) Up Answer: net force is zero. The boat is moving with constant velocity, therefore its acceleration is zero. Therefore, the net force on it must be zero, according to NII:netF ma=rrCTN-6A glider is on a tilted air track and is sliding downhill.What is the direction of the net force on the glider?D) None of these. Some other direction.Answer: The net force is always in the same direction as the acceleration. The acceleration is along the track (since the velocity is increasing in the direction along the track), so the net force must be along the track. A free body diagram: There are only two forces acting on the glider: the force of gravity and the normal force of the track on the glider. Would the answer be different if the glider was moving uphill because it had recently been pushed uphill? ABCA) Yes, the net force would now be in a different direction than before.B) No, the net force would be the same as before.Answer: The answer is the same as before; the net force is still in the downhill direction, parallel to the track. This is because the acceleration is the same as it was before. Note carefully that the glider is not being pushed by the hand of the pusher. The glider was pushed at some time in the past, but it is not being pushed now. CTN-7Consider a person standing in an elevator that is moving upward at constant speed. The magnitude of the upward normal force, N, exerted by the elevator floor on the person's feet is (larger than/same as/ smaller than)the magnitude of the downward weight, W, of the person.A) N > W B) N = W C) N < WAnswer: The upward normal force has the same magnitude as the downward force of gravity. Since velocity v = constant, then acceleration a = 0, and so Fnet = 0. Even though the person is moving upward, the net force is zero. speed vCTN-8A glider on a level air track is coasting along at constant velocity. Which of the followingfree-body diagrams correctly indicates all the forces on the glider? Assume that there is no air resistance or friction.Answer: A. The only forces on the glider (while it is coasting) are the force of gravity down and the normal force up.CTN-9 A moving van collides with a sports car in a high-speed head-on collision. Crash!During the impact, the truck exerts a force Ftruck on the car and the car exerts a force Fcar on the truck. Which of the following statements about these forces is true)A) The force exerted by the truck on the car is the same size as the force exerted by the car on the truck) Ftruck = FcarB) Ftruck > FcarC) Ftruck < FcarAnswer: By Newton's third law, Ftruck = Fcar. The forces on the two colliding objects (car and truck) are the same size. What is different is the accelerations of car and truck. For both car and truck, it is true that Fnet = ma. The truck has a BIG mass M; the car has a v = constantABCDE) None of these A B E)small mass m. Since each objects experiences the same sized force F during the collision,each must have the same size mass acceleration. If mass is big, acceleration must be small; if mass is small, acceleration must be big: M a = m A. Because of its small mass, the car experiences a much greater acceleration during the collision, and it is this sudden change in velocity which injures its occupant. CTN-10 In the 1600's, Otto Van Güricke, a physicist in Magdeburg, fitted two hollow bronze hemispheres together and removed the air from the resulting sphere with a pump. Two eight-horse teams could not pull the spheres apart, even though the hemispheres fell apartwhen air was re-admitted. Suppose von Güricke had tied both teams of horses to one sideand bolted the other side to a heavy tree trunk. In this case the tension in the rope would be...A) twice B) exactly the same as C) half what it was before.Answer: The tension in the rope is twice what is was before. Situation I (8-horse teams on either side): Situation II (both 8-horse teams on same side): Note that situation I is the same as if one side was tied to a tree trunk and the other side toa single 8-horse team. After all, the horses on the right have no idea whether they are pulling against