physics quiz 1

Three parachutists, A, B, and C, each have reached terminal velocity at the same distance above the ground below. a) From fastest to slowest, rank their terminal velocities. b) From longest to shortest, rank their times to reach the round.

(a) C, A, B (b) B, A, C

You are driving north on a highway. Then, without changing speed, you round a curve and drive east. (a) Does your velocity change? (b) Do you accelerate? Explain.

(a) Yes, because of the change of direction (b) Yes, because velocity changes

the moon

1.6 m/s^2

the earth

10 m/s^2 (9.8 m/s^2)

For straight-line motion, explain to your classmates how a speedometer indicates whether or not acceleration is occurring.

Acceleration occurs when the speedometer reading changes. No change, no acceleration.

The auto in the sketch moves forward as the brakes are applied. A bystander says that during the interval of braking, the auto's velocity and acceleration are in opposite directions. Do you agree or disagree?

Agree. Acceleration (slowing the car) is opposite velocity (the direction the car is moving).

A ball tossed upward will return to the same point with the same initial speed when air resistance is negligible. When air resistance is not negligible, how does the return speed compare with its initial speed?

Air drag decreases speed. So a tossed ball will return with less speed than it possessed initially

Here we see a top view of an airplane being blown off course by winds in three different directions. Use a pencil and the parallelogram rule to sketch the vectors that show the resulting velocities for each case. Rank the speeds of the airplane across the ground from fastest to slowest.

B,A,C

A parachutist, after opening her parachute, finds herself gently floating downward, no longer gaining speed. She feels the upward pull of the harness, while gravity pulls her down. Which of these two forces is greater? Or are they equal in magnitude?

Both forces have the same magnitude. This is easier to understand if you visualize the parachutist at rest in a strong updraft—static equilibrium. Whether equilibrium is static or dynamic, the net force is zero.

If gold were sold by weight, would you rather buy it in Denver or in Death Valley? If it were sold by mass, which of these locations makes the best buy? Defend your answers.

Buy by weight in Denver because the acceleration of gravity is less in Denver than in Death Valley. Buying by mass would be the same in both locations.

In all three cases, A, B, and C, the crate is in equilibrium (no acceleration). Rank them by the amounts of friction between the crate and the floor, from greatest to least.

C, B, A

The weights of Burl, Paul, and the scaffold produce tensions in the supporting ropes. Rank the tensions in the left rope, from most to least, in the three situations, A, B, and C.

C,B,A

Light travels in a straight line at a constant speed of 300,000 km/s. What is the acceleration of light?

Constant velocity means no acceleration, so the acceleration of light is zero.

If you drop an object, its acceleration toward the ground is 10 m/s2. If you throw it down instead, would its acceleration after throwing be greater than 10 m/s2? Why or why not?

If air drag is not a factor, an object's acceleration is the same 10 m/s2 regardless of its initial velocity. If it is thrown downward, its velocity will be greater, but not its acceleration.

If there were no air resistance, why would it be dangerous to go outdoors on rainy days?

If it were not for the slowing effect of the air, raindrops would strike the ground with the speed of high-speed bullets

Suppose that a freely falling object were somehow equipped with a speedometer. By how much would its reading in speed increase with each second of fall?

Its speed reading would increase by 10 m/s each second

When blocking in football, a defending lineman often attempts to get his body under the body of his opponent and push upward. What effect does this have on the friction force between the opposing lineman's feet and the ground?

Lifting the opponent decreases the force with which the ground supports him, and correspondingly decreases the force of friction he can muster. The reduced friction limits the opponent's effectiveness.

Can an object be in mechanical equilibrium when only a single nonzero force acts on it? Explain.

No, if only a single nonzero force acts on an object, its motion will change and it will not be in mechanical equilibrium. There would have to be other forces to result in a zero net force for equilibrium

Is it possible to move in a curved path in the absence of a force? Discuss why.

No. An object can move in a curve only when a force acts. With no force, its path would be a straight line.

Asteroids have been moving through space for billions of years. What keeps them moving?

Nothing keeps asteroids moving. The suns force deflects their paths but is not needed to keep them moving

When skydiver Nellie opens her parachute, the air drag pushing the chute upward is stronger than Earth's force of gravity pulling her downward. A friend says this means she should start moving upward. Discuss with your friend why this isn't so, and what does happen.

Nowhere is her velocity upward. The upward net force on Nellie during the short time that air drag exceeds the force of gravity produces a momentary upward net force and upward acceleration. This produces a decrease in her downward speed, which is nevertheless still downward.

For a freely falling object dropped from rest, what is the acceleration at the end of the fifth second of fall? Tenth second of fall? Defend your answers.

The acceleration of free fall at the end of the 5th, 10th, or any number of seconds is g. Its velocity has different values at different times, but since it is free from the effects of air resistance, its acceleration remains a constant g

A regular tennis ball and another filled with lead pellets are dropped at the same time from the top of a building. Which one hits the ground first? Which one experiences greater air resistance? Defend your answers

The heavier tennis ball will strike the ground first for the same reason the heavier parachutist in Figure 4.15 strikes the ground first. Note that although the air resistance on the heavier ball is smaller relative to the ball's weight, it is actually greater than the air resistance that acts on the other ball. Why? Because the heavier ball falls faster, and air resistance is greater at higher speed.

Suppose that the three balls shown in Exercise 88 start simultaneously from the tops of the hills. Which one reaches the bottom first? Explain.

The one in the middle. That ball gains speed more quickly at the beginning where the slope is steeper, so its average speed is greater even though it has less acceleration in the last part of its trip.

Your empty hand is not hurt when it bangs lightly against a wall. Why does it hurt if you're carrying a heavy load? Which of Newton's laws is most applicable here?

When you carry a heavy load, there is more mass involved and a greater tendency to remain moving. If a load in your hand moves toward a wall, its tendency is to remain moving when contact is made. This tends to squash your hand if it's between the load and the wall—an unfortunate example of Newton's first law in action.

A friend says that, as long as a car is at rest, no forces act on it. What do you say if you're in the mood to correct the statement of your friend?

You explain the distinction between an applied force and a net force. It would be correct to say no net force acts on a car at rest.

If you're in a car at rest that gets hit from behind, you can suffer a serious neck injury called whiplash. What does whiplash have to do with Newton's first law?

Your body tends to stay at rest. The back of the seat pushes you forward. Without support at the back of your head your head us not pushed forward with your body, which likely injures your head

Is a fine for speeding based on one's average speed or instantaneous speed? Explain.

Your fine for speeding is based on your instantaneous speed—the speed registered on a speedometer or a radar gun.

Because Earth rotates once every 24 hours, the west wall in your room moves in a direction toward you at a linear speed that is probably more than 1000 kilometers per hour (the exact speed depends on your latitude). When you stand facing the wall, you are carried along at the same speed, so you don't notice it. But when you jump upward, with your feet no longer in contact with the floor, why doesn't the high-speed wall slam into you?

a body in motion tend to remain in motion, so you move with the moving earth whether or not your feet are in contact with it. when you jump, your horizontal motion matches that of the earth and you travel with it

A ball is released at the left end of three different tracks. The tracks are bent from equal-length pieces of channel iron. a) From fastest to slowest, rank the speeds of the balls at the right ends of the tracks. b) From longest to shortest, rank the tracks in terms of the times for the balls to reach the ends. c) From greatest to least, rank the tracks in terms of the average speeds of the balls. Or do all the balls have the same average speed on all three tracks?

a) B, A=C b)A,B,C c)C,B,A

Different materials, A, B, C, and D, rest on a table. Rank how much they resist being set into motion, from greatest to least. Rank the support (normal) forces the table exerts on them, from greatest to least.

a) B,A,C,D b) B,A,C,D

Free fall is motion in which gravity is the only force acting. (a) Is a skydiver who has reached terminal speed in free fall? (b) Is a satellite above the atmosphere that circles Earth in free fall?

a) No. Air resistance is also acting. Free fall means free of all forces other than that due to gravity. A falling object may experience air resistance; a freely falling object experiences only the force due to gravity (b) Yes. Although getting no closer to the Earth, the satellite is falling

Newton's Second Law

acceleration= net force/mass, direction of net force is the same as acceleration

non-free fall

air resistance needs force, effected by shape and speed

mass

amount of matter in an object

Newton's First Law

an object in motion stays in that motion unless acted on by a net force

the equilibrium rule and force vectors

an object is in mechanical equilibrium when the net force in the object is zero

free fall

an object that is only under the influence of gravity

inertia

an object's resistance to a change in motion

average velocity formula

change in position/time to make change, needs a direction

average acceleration formula

change in velocity/change in time

speed formula

distance/time

terminal speed

fastest an object will fall through the air

friction

force that normally opposed the motion of an object

vector

has magnitude and direction

scalar

has only magnitude

speed units

m/s, mi/h

acceleration units

m/s^2

Start a ball rolling down a bowling alley and you'll find that it moves slightly slower with time. Does this violate Newton's law of inertia? Discuss and defend your answer.

no. if there were for force action on the ball, it would continue in motion with our slowing. but air drag does act, along with slight friction with the lane, and the ball slows. this doesn't violate the law of inertia because external force indeed act

net force

sum of all the forces acting on an object

static friction

surfaces are at rest relative to each other

kinetic friction

surfaces slide over one another

weight

the force due to gravity on an object

If you pull horizontally on a crate with a force of 200 N, it slides across the floor in dynamic equilibrium. How much friction is acting on the crate?

the friction on the crate has to be 200N, opposite to your 200-N pull

Why do you seem to lurch forward in a bus that suddenly slows? Why do you seem to lurch backward when the bus picks up speed? What law applies here?

the law of inertia applies in both cases. when the bus slows you tend to keep moving at the previous speed and lurch forward. when the bus picks up speed, you tend to keep moving at the previous (lower) speed and lurch backwards

Nellie Newton hangs at rest from the ends of the rope as shown. How does the reading on the scale compare with her weight?

the scale will read half her weight. in this way the net force (upward pull of left rope + upward pull of right rope - weight)= 0

Consider a crate at rest on a factory floor. As a pair of workmen begin lifting it, does the support force on the crate provided by the floor increase, decrease, or remain unchanged? What happens to the support force on the workmen's feet?

the support force on the crate decreases as the load against the floor decreases

A heavy bird sits on a clothesline. Will the tension in the clothesline be greater if the line sags a lot or if it sags a little?

the tension will be greater for a small sag because large vectors in each side of the rope supporting the bird are needed for a resultant that is equal and opposite to the birds weight

For the pulley system shown, what is the upper limit of weight the strong man can lift?

the upper limit he can lift is a load equal to his weight. beyond that he leaves the ground

Two forces act on a parachutist falling in air: the force of gravity and air resistance. If the fall is steady, with no gain or loss of speed, then the parachutist is in dynamic equilibrium.

two forces must be equal and opposite so the net force=0. the the parachutist is in dynamic equilibrium

What can you correctly say about a pair of vectors that add together to equal zero?

you can correctly say the vectors are equal in magnitude and opposite in direction