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Exam 3: Mechanical Objects, Part 1

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Question 31

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Your apparent weight is equal to your weight

A) In an elevator accelerating upwards.
B) In an elevator accelerating downwards.
C) In an elevator in free fall.
D) In an elevator not accelerating.

E) A) and D)
F) All of the above

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Question 32

Multiple Choice

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Like a baseball bat, a tennis racket has a sweet spot at its center of percussion. If a tennis ball hits this center of percussion, the racket's handle does not accelerate. This is because

A) an impact at the center of percussion exerts no torque about the racket's center of mass and doesn't cause the racket to undergo angular acceleration.
B) the racket's center of mass accelerates backward while its handle rotates forward about its center of mass and the two motions cancel one another at the handle.
C) an impact at the center of percussion transfers no momentum to the racket and doesn't cause the racket to accelerate.
D) the racket's velocity doesn't change when the ball hits its center of percussion.

E) A) and B)
F) All of the above

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Question 33

Multiple Choice

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When you drop a rubber ball on the floor and it bounces, the direction of its velocity reverses because

You are swinging a tin can around your head in a perfectly horizontal circle of radius 4 m. (This is really not possible but we're saying it almost happens.) If you exert a force of 9 N and the mass of the can is 1 kg, with what speed is the can moving around the circle?

The spin cycle in your washing machine partially dries clothes by spinning them in a perforated metal basket. Such a process works because

Suppose a soap box car is coasting on a track (no motor; no brake), moving from left to right, and part of the track is shown below. Friction /air resistance is present and the car comes to a stop at point E. -Please identify points of maximum and minimum kinetic energy, if any.

Suppose that you are riding a Ferris wheel which rotate at any constant speed you wish. The chairs swivel so you are always sitting upright. Please explain why it is possible for you to feel heaviest at only one spot on the Ferris wheel. What spot is this? Here, "heaviness" is defined to mean that the normal force of contact from the chair on you (the rider) is maximum.

You are at a coffee shop with a close friend and the talk turns towards masses bouncing vertically on springs. Your friend makes the statement that "There is one unique point in time when the speed of the object is maximum, and that is when the acceleration is maximum too. Please comment on the scientific accuracy of your friend's statement.

Trace the energy transfer in a perfectly inelastic ball that is dropped from rest.

You have heard the tried and true phrase "it is like running in to a brick wall". Now it is time to dig a little deeper and modify the phrase. Assume you are in a car driving and come in contact with this proverbial brick wall. Your car can do three things after the strike: it can go through, come to stop or bounce back. Select the option which will be the most dangerous to you from an impulse point of view and briefly discuss why the assumption of equal times has to be made.

You have always wondered how much one of your friends weighs and devise a scheme to measure his weight secretly. You have him sit in a tubular steel chair. This popular style of chair consists of a single steel tube that is bent into a frame and that supports a seat bottom and a back. The empty chair weighs 10 pounds and is 30 inches tall. The frame acts as a spring and bends downward slightly when the chair is occupied. When you sit properly in the chair yourself, it bends downward 1 inch. When your friend sits properly in the chair, it bends downward 2 inches. From that observation, you know that your friend weighs about

Suppose that you are riding a Ferris wheel which rotate at any constant speed you wish. The chairs swivel so you are always sitting upright. Please explain why it is possible for you to feel weightless at only one spot on the Ferris wheel. What spot is this? Here, "weightless" is defined to mean that the normal force of contact from the chair on you (the rider) is zero.

From an energy point of view, explain why car collisions are much less dangerous when the bumpers lock.

Trace the energy transfer in an elastic ball that is dropped from rest, hits the floor and rebounds, being caught.

A person is swinging a tin can around in a vertical circle at a constant speed. Please identify the points, if any, of -maximum/ minimum kinetic energy

Skateboarders often can skate on the inner surface of cylindrical or bowl - shaped tracks. As a result, the normal (support) force is able to point inwards towards the center of the circle wherever the person is. That center-directed or centripetal force on the skate wheels is important because it

Suppose a soap box car is coasting on a track (no motor; no brake), moving from left to right, and part of the track is shown below. Friction /air resistance is present and the car comes to a stop at point E. -What must be true of the car's motion at point A?

A classroom demo that is popular with students and unpopular with custodians is to fill a Styrofoam cup with water, poke a hole in the bottom and then drop the cup into a garbage can waiting below. On the way down, the leak will

The coefficient of restitution for a particular ball is 0.45. If he ball hits a surface traveling at 100 m/s its rebound speed will be

Suppose a soap box car is coasting on a track (no motor; no brake), moving from left to right, and part of the track is shown below. Friction /air resistance is not present and the car is almost stopped, barely moving at point E. -Please identify points of maximum and minimum total energy, if any.