# CP4 Concept Test (javierLazare)

## 11 terms

### Many automobile passengers suffer neck injuries when struck by cars from behind.How does Newtons law of inertia apply here? How do headrests help to guard against this type of injury?

They keep the body and head moving together when the car is suddenly accelerated forward.

### Suppose you place a ball in the middle of a wagon that is at rest and then abruptly pull the wagon forward. describe the motion of the ball relative to (a) the ground and (b) the wagon.

Except for some change in motion due to friction, there will be no motion of the ball relative to the ground; but relative to the wagon, the ball will move toward the back

### When a junked car is crushed into a compact cube, does its mass change? its volume? its weight?

No; yes; no; it has the same amount of matter befor and after being crushed.

### When you compress a sponge, which quantity changes: mass, inertia, volume, or weight?

Only Volume; the mass and therefor the inertia are the same because the same amount of matter is present. the weight is the same because there is no difference in gravitational force upon it.

### A massive ball is suspended on a string and slowly pulled by another string attached to it from below, as shown in Figure A below a. Is the string tension greater in the upper or the lower string? which string is more likely to break? which property, mass or weight, is important here? b. If the string is instead snapped downward, which string is more likely to break? is mass or weight important this time?

A. Upper string because it supports the weight of the ball in addition to your pull. The lower string supports only your pull. The upper string is more likely to break. Weight is important here.
B. bottom string; the mass of the ball(its inertia) is more important then its weight in this case.

### The head of a hammer is loose and you wish to tighten it by banging it against the top of a workbench. why is it better to hold the hammer with the handle down, as shown in figure B below, rather than with the head down? Explain in terms of inertia.

The handle stops when it hits the bench, but the relatively massive head tends to keep moving. The head moves toward the handle and tightens

### The little girl in the figure hangs at rest from the end of the rope. how does the reading on the scale compare with her weight?

half; each strand supports half her weight

### Harry the painter swings years after year from his bosun's chair. His weight is 500 N ans the rope , unknown to him, has a breaking point of 300 N Why doesn't the rope break when he is supported as shown on the left? One day Harry is painting near a flagpole, and for a change, he ties the free end of the rope to the flagpole instead of to his chair (right) . Why did harry end up taking his vacation early?

the tension in each strand is only 250 N ; when harry is supported by one strand, his weight is greater than the breaking tension of the rope. The sudden acceleration when he hits the ground changes his vacation plans.

### As earth rotates about its axis, it takes three hours for the United States to pass beneath a point above Earth that is stationary relative to the sun. what is wrong with the following scheme? To travel from Washington D.C to San Francisco using very little fuel, simply ascend in a helicopter high over Washington D.C and wait three hours until San Francisco passes below.

The helicopter has the same horizontal speed as Earth's surface. when it ascends, it keeps this horizontal speed and moves with earth below.

A;C

### Why can't the strong man pull hard enough to make the chain straight?

The vertical components of the tension in the two sides of the chain must add up to the weight of the load. As the chain is straightened and the angle approaches zero, the tension in the chain must approach infinity. Most difficult!