# Physics Chapter 4 Homework

Poke or kick the boxes. The one that more greatly resists a change in motion is the one with the greater mass–the one filled w sand.
In the orbiting space shuttle, you are handed two identical closed boxes, one filled w sand and the other filled w feathers. How can you tell which is which without opening the boxes?
Mainly the first law, for the bag in motion tends to continue in motion, which results in a squashed hand.
Your empty hand is not hurt when it bangs lightly against a wall. Why does your hand hurt if it is carrying a heavy load? Which of Newton’s laws is most applicable here?
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Newton’s first law again–when the stone is released it is already moving as fast as the ship, and this horizontal motion continues as the stone falls. Much more about this in chapter 6
Before the time of Galileo and Newon, it was thought by many learned scholars that a stone dropped from the top of a tall mast on a moving ship would fall vertically and hit the deck behind the mast by a distance equal to how far the ship had moved forward while the stone was falling. In light of your understanding of Newton’s laws, what do you think about this idea?
You exert a force to overcome the force of friction. This makes the net force zero, which is why the wagon moves without acceleration. If you pul harder, then net force will be greater than zero and acceleration will occur.
To pull a wagon across a lawn at a constant velocity, you must exert a steady force. Reconcile this fact with Newton’s first law, which states that motion with a constant velocity indicates no force.
Like the wagon of the preceeding exercise, you run your engine to provide a force large enough to overcome friction. A net force of zero requires you provide this force.
When your car moves along the highway at a constant velocity, the net force on it is zero. Why, then, do you continue running your engine?
The force that you exert on the ground is greater than your weight, for you momentarily accelerate upward. Then the ground simultaneously pushes upward on you with the same amount of force.
As you are leaping upward from the ground, how does the force that you exert on the ground compare with your weight?
Only on hill B does the acceleration along the path decrease with time, for the hill becomes less steep as motion progresses. When the hill levels off, acceleration will be zero. On hill A, acceleration is constant. On hill C, acceleration increases as the hill becomes steeper. In all three cases, speed increases.
On which of these hills does the ball roll down with increasing speed and decreasing acceleration along the path? (Use this example if you wish to explain to someone the difference between speed and acceleration (page 74)
100N, the same reading it would have if one of the ends were tied to a wall instead of tied to the 100N hanging weight. Although the net force on the system is zero, the tension in the rope within the system is 100N, as shown on the scale reading.
Two 100-N weights are attached to a spring scale as shown. (picture on page 74) Does the scale read 0N, 100N, or 200N, or does it show some other reading? (Hint: Would it read any differently if one of the ropes were tied to the wall instead of to the hanging 100N weight?)
The acceleration of any object is a=Fnet/m, and Fnet in free fall =mg. So a=mg/g=g. The greater the weight, the greater the mass.
Aristotle claimed that the speed of a falling object depends on its weight. We now know that objects in free fall, whatever their weights, undergo the same gain in speed. Why does weight not affect acceleration?
When the barbell is accelerated upward, the force exerted by the athlete is greater than the weight of the barbell. (The barbell, simultaneously, pushes with greater force against the athlete.) When acceleration is downward, the force supplied by the athlete is less.
When the athlete holds the barbell over head, the reaction force is the weight of the barbell on his hand. How does this force vary for the case in which the barbell is accelerated upward? Accelerated downward? (picture on page 74)
The strong man can exert only equal forces on both cars, just as your push against a wall equals the push of the wall on you. Likewise for two walls, or two freight cars. Since their masses are equal, they will undergo equal accelerations and move equally.
The strong man will push apart the two initiall stationary freight cars of equal mass before he himself drops straight to the ground. Is it possible for him to give either of the cars a greater speed than the other? Why or why not? (picture on page 75)
The friction on the crate is 200N, which cancels your 200N push on the crate to yield the zero net force that accounts for the constant velocity (zero acceleration). Although the friction force is equal and oppositely directed to the applied force, the two do not make an action-reaction pair of forces. That’s because both forces do act on the same object–the crate. The reaction to your push on the crate is the crate’s push back on you. The reaction to the frictional force of the floor on the crate is the opposite friction force of the crate on the floor.
If you exert a horizontal force of 200N to slide a crate across a factory floor at a constant velocity, how much friction is exerted by the floor on the crate? Is the force of friction equal and oppositely directed to your 200N push? Does the force of friction make up the reaction force to your push? Why not?
The person with twice the mass slides half as far as the twice-as-massive person. That means the lighter one slides 4 feet and the heavier one slides 8 feet (for a total of 12 feet).
Suppose that one person in the preceding exercise has twice the mass of the other. Ho far does each person slide before they meet?

(preceding exercise) Two people of equal mass attempt a tug-of-war with a 12-m rope while standing on frictionless ice. When they pul on the rope, each person slides toward the other. How do their acceleration compare, and how far does each person slide before they meet?

In accord w Newton’s third law, Steve and Gretchen are touching each other. One may initiate the touch but the physical interaction can’t occur without contact between both Steve and Gretchen. Indeed, you cannot touch without being touched!
The photo shows Steve Hewitt and his daughter Gretchen. Is gretchen touching her dad, or is he touching her? Explain.
a) A skydiver encountering NO air esistance is in free fall. One at terminal velocity does encounter air resistance and is not in free fall.

b) The only force acting on a satelite is that due to gravity. So a satelite is in free fall.

Free fall is motion in which gravity is the only force acting.

a. Is the skydiver who has reached terminal speed in free fall?

b. Is a satelite Circling Earth above the atmosphere in free fall?

Air resistance is not really negligible for so high a drop, so the heavier ball does strike the ground first. (This idea is shown is figure 4:15) But although a twice-as-heavy ball sgtrikes first, it falls only a little faster, and not twice as fast, lwhich is what folowers of Anistotle believed. Galileo recognized that the small difference is due to friction and would not be present if there were no friction.
If and when Galileo dropped two balls from the top of the Leaning Tower of Pisa, air resistance was not really negligible. Assuming that both balls were the same size yet one was much heavier than the other, which ball struck the ground first? Why?
The tension indeed increases, and more than the weight of the bird depending on the amount of sag. To exaggerate, if the wire drooped so as to be almost vertical, the added tension would be almost half the bird’s weight. If the wires make an angle of 30 degrees with the horizontal, the added tension would equal the bird’s weight. Angles less than 30 degrees produce added tension greater than the bird’s weight.
When a bird alights upon a stretched power line wire, does the tension in the wire change? If so, is the increase more than, less than, or about equal to the bird’s weight?
a) The other vector is upward as shown. (see homework solution page)

b) It is called the normal force

A stone is shown at rest on the ground.

a. The vector shows the weight of the stone. Complete the vector diagram showing another vector that results in zero net force on the stone.

b. What is the conventional name of the vector you have drawn?

see homework solution sheet for chapter 4
See #44, 45, 49 on page 75 for more drawings and questions
The given pair of forces produces a net force of 200N forward, which accelerates the cart. To make the net force zero, a force of 200N backward must be exerted on the cart.
When two horizonta forces are exerted on a cart, 600N forward and 400N backward, the cart undergoes acceleration. What additional force is needed to produce nonaccelerated motion?
Acceleration a= Fnet/m = (20N-12N)2kg= 8 N/2 kg = 4 m/s squared.
You push with a 20N horizontal force on a 2kg box of cookies resing on a horizontal surface against a horizontal friction force of 12N. Show that the acceleration of the box will be 4m/s squared.

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