1.Using F cos()d, how can the work done on an object be equal to zero? 2. A comm
ID: 1413018 • Letter: 1
Question
1.Using F cos()d, how can the work done on an object be equal to zero? 2. A common classroom demonstration involves holding a bowling ball attached by a rope to the ceiling close to your face and releasing it from rest. In theory, you should not have to worry about being hit by the ball as it returns to its starting point. However, some of the most "exciting" demonstrations have involved the professor being hit by the ball! A) Why should you expect not to be hit by the ball? B) Why might you be hit if you actually perform the demonstration? 3.Can a change in kinetic energy ever have a negative value? Explain your answer. 4.Define the concept of "nonconservative force" in your own words. Give three examples. 5.A rubber dart can be launched over and over again by the spring in a toy gun. Where is the energy generated to launch the dart? 6.A box is dragged across a floor by a force F which makes an angle with the horizontal (see figure below). If the magnitude of the force is held constant but the angle is increased up to 90°, the work done by the force does which of the following? 7.A 306-kg box is pulled 8 m up a 30° inclined plane by an external force of 4000 N that acts parallel to the frictionless plane. Calculate the work done by the following. A) the external force B) gravity C) the normal force 8.A 15-kg block rests on a level floor; the coefficient of kinetic friction between the block and the floor is 0.42. A force of 400 N acts for 8 m. How fast is the block moving after being pushed 8 m? Assume it starts from rest. 9.An ice cube slides without friction down a track as shown in the figure below. Calculate the speed of the cube at points B, C, D, and E. The ice cube has no speed at point A. Ya=5m Yb=3m yc=2m yd=3m ye=4m 10.A force of 1200 N pushes a man on a bicycle forward. Air resistance pushes against him with a force of 800 N. If he starts from rest and is on a level road, how fast will he be moving after 30 m? The mass of the bicyclist and his bicycle is 95 kg. 11.A 20-kg suitcase falls from a hot-air balloon that is floating at a height of 311 m above the surface of Earth. The suitcase has an initial speed of 0 m/s but it reaches a speed of 28 m/s just before it hits the ground. Calculate the percentage of the initial energy that is "lost" to air resistance. 12.The coefficient of restitution, e, of a ball hitting the floor is defined as the ratio of the speed of the ball right after it rebounds from the impact to the speed of the ball right before it hits the floor, that is, e = vafter/vbefore. (a) Derive a formula for the coefficient of restitution when a ball is released from an initial height H and rebounds to a final height h. (Use the following as necessary: H and h.) erestitution = (b) Calculate the coefficient of restitution for a golf ball that bounces to a height of 62 cm after having been released from a height of 72 cm.
Explanation / Answer
1.
W = F d Costheta
the work done is 0 when force acting on the object is 0
the work done is zero when the net displacement "d" is 0
the work done is zero when the angle between force F and displacement "d" is 90
3.
yes change in kinetic energy can be negative when the final kinetic energy is smaller compared to initial Kinetic energy. this is when the object is slowing down
5.
the energy is stored in thespring as spring potential energy and this energy is converted to kinetic energy of rubber dart.
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