Physics Exam 2

Astronauts on the International Space Station feel weightless because

Their apparent weight is zero.

A passenger on a carnival ride rides in a car that spins in a horizontal circle, with its acceleration vector pointing in towards the center of the circle, and the velocity vector point perpendicular from that straight in front of the car. the instant shown, which arrow gives the direction of the net force on one of the riders? (A-up, B-right, C-down, D-left)

D, in towards center of circle

The centers of a 13 kg lead ball and a 60 g lead ball are separated by 11cm. What gravitational force does each exert on the other?

F = 4.3×10^−9 N

Box A, with m 20kg, F 30N, and coefficient of kinetic friction 0.4, and Box B, with m 10, F 30, and coefficient of kinetic friction 0.5 are both sliding. Select the correct explanation of f(A) > f(B).

Select the correct explanation of that.

Two blocks with masses M1 and M2 hang one under the other. For this problem, take the positive direction to be upward, and use g for the magnitude of the acceleration due to gravity. Find T2, the tension in the lower rope.

T(2) = M(2)g

Each of 138 identical blocks sitting on a frictionless surface is connected to the next block by a massless string. The first block is pulled with a force of 138 N. What is the tension in the string connecting block 69 to block 70?

T(69 to 70) = 69 N

A 17 g audio compact disk has a diameter of 12 cm. The disk spins under a laser that reads encoded data. The first track to be read is 2.3 cm from the axis; as the disk plays, the laser scans tracks farther and farther from the center. The part of the disk directly under the read head moves at a constant 1.2 m/s. As the disk plays, how does the angular speed change?

The angular speed decreases.

When the dog shakes his body, his fur and the droplets of water clinging to it move in circular arcs. If the droplets leave the fur, what will be the subsequent motion? (For the short times in the video, motion due to gravity is negligible.)

The droplets will move in straight-line paths.

When two objects are in contact, moving together, which of the following statements must be true? Choose all that apply.

The objects must have the same acceleration. The objects must exert the same magnitude force on each other.

One mass is sitting on a frictionless surface, a rope attached to a pulley is attached it its left side. A second mass sits on top of it with a rope attached to the pulley also attached to its left side. Velocity vector points from the first mass to the right. In which direction is the kinetic friction force on block 2 in the figure?

To the left

One block, m = 2kg sits on a surface, it's right side attached to a pulley by a rope. A 1 kg mass hangs off the surface from a rope attached to the other side of the pulley. the 2 kg mass has a velocity vector pointing right (towards the pulley). What is the upper block's acceleration if the coefficient of kinetic friction between the block and the table is 0.17?

a = 2.2 m/s^2

On a snowy day, when the coefficient of friction μs between a car's tires and the road is 0.50, the maximum speed that the car can go around a curve is 24 mph. What is the maximum speed at which the car can take the same curve on a sunny day when μs=1.0?

34 mph

A 1000-kg car is moving around a circular turn with a radius of 18.5 meters and decreasing in speed at a rate of 35.2 m/s^2. At the instant the car is moving at 16.8 m/s, what is the car's total acceleration?

38.4 m/s^2

An earth satellite moves in a circular orbit at a speed of 5500 m/s. What is its orbital period?

4.2 hr

Two boxes are suspended from a rope over a pulley. Each box has weight 50 N. What is the tension in the rope?

50 N

A 1700 kg car drives around a flat 230-m-diameter circular track at 25 m/s. What is the magnitude of the net force on the car?

9200 N

Which of these objects is in equilibrium?

A block sitting at rest on a table. A car driving down the road at a constant speed. A skydiver falling at a constant speed.

The droplets don't leave the dog's fur until the speed of the motion exceeds some minimum value. How can we explain this?

As the speed increases, so does the force necessary to provide the centripetal acceleration. Above a certain speed, the adhesion force isn't large enough to keep the droplets moving in a circle.

An old-fashioned LP record rotates at 33⅓ rpm. What is its period, in seconds?

T = 1.80 s

A 17 g audio compact disk has a diameter of 12 cm. The disk spins under a laser that reads encoded data. The first track to be read is 2.3 cm from the axis; as the disk plays, the laser scans tracks farther and farther from the center. The part of the disk directly under the read head moves at a constant 1.2 m/s. When a disk is inserted, it takes 2.4 s to spin up from rest. What is the torque of the motor?

τ = 6.7×10^−4 N⋅m

A bar 100 cm long has a pivot 25 cm from the left, a force of 8 N upwards on the left side and a Force of F upwards on the right. What is the net torque on the bar shown in (Figure 1), about the axis indicated by the dot? Suppose that F = 13 N .

τ = 7.8 N⋅m

Earth is 149.6 billion meters from the Sun and takes 365 days to make one complete revolution around the Sun. Mars is 227.9 billion meters from the Sun and has an orbital period of 687 days. What is the ratio of Earth's centripetal acceleration to Mars's centripetal acceleration?

2.33

In the situation shown in the figure, a person is pulling with a constant, nonzero force F⃗ on string 1, which is attached to block A. Block A is also attached to block B via string 2, as shown. For this problem, assume that neither string stretches and that friction is negligible. Both blocks have finite (nonzero) mass. Which one of the following statements correctly descibes the relationship between the accelerations of blocks A and B?

Both blocks have the same acceleration.

Two identical blocks 3.0 kg are stacked on top of each other. The bottom block is free to slide on a frictionless surface. The coefficient of static friction between the blocks is 0.35. What is the maximum horizontal force that can be applied to the lower block without the upper block slipping?

F = 21 N

Blocks with masses of 3.0 kg, 4.0 kg, and 5.0 kg are lined up in a row on a frictionless table. All three are pushed forward by a 11 N force applied to the 3.0 kg block. How much force does the 4.0 kg block exert on the 3.0 kg block?

F(4.0 on 3.0) = 8.3 N

Blocks with masses of 3.0 kg, 4.0 kg, and 5.0 kg are lined up in a row on a frictionless table. All three are pushed forward by a 11 N force applied to the 3.0 kg block. How much force does the 4.0 kg block exert on the 5.0 kg block?

F(4.0 on 5.0) = 4.6 N

This problem concerns the concept of tension in a rope. Consider a rope subjected to a pulling force on its two ends as shown in (Figure 1). The rope is stationary. An arbitrary point P divides the rope into a left-hand segment L and a right-hand segment R. Assume that segment R exerts a force of magnitude T on segment L. What is the magnitude FLR of the force exerted on segment R by segment L?

F(LR) = T

A mass hangs down from a rope. Determine the tension in the rope at the point indicated with a dot in (Figure 1). The object is at rest. The string is massless. Suppose that m = 6 kg .

T = 60 N

Two blocks with masses M1 and M2 hang one under the other. For this problem, take the positive direction to be upward, and use g for the magnitude of the acceleration due to gravity. Find T1, the tension in the upper rope.

T(1) = (M(1) + M(2))(g+a)

You are walking up an icy slope. Suddenly your feet slip, and you start to slide backward. Will you slide at a constant speed, or will you accelerate?

You will accelerate because the kinetic friction is less than the maximum static friction.

At what height above the earth is the free-fall acceleration 25 % of its value at the surface? Assume R(earth) = 6.37 × 10^6 m.

h = 6.4×10^6 m

For uniform circular motion, the acceleration __________.

is directed toward the center of the circle

You are driving your car through a roundabout that has a radius of 11 m. Your physics textbook is lying on the seat next to you. What is the fastest speed at which you can go around the curve without the book sliding? The coefficient of static friction between the book and the seat is 0.35.

v(max) = 6.1 m/s

The crankshaft in a race car goes from rest to 3120 rpm in 2.2 s. What is the angular acceleration of the crankshaft?

α = 150 rad/s^2

Two workers are sliding 300 kg crate across the floor. One worker pushes forward on the crate with a force of 380 N while the other pulls in the same direction with a force of 300 N using a rope connected to the crate. Both forces are horizontal, and the crate slides with a constant speed. What is the crate's coefficient of kinetic friction on the floor?

μ(k) = 0.23

A pulley with a diameter of 4 cm has a downward force of 20 N on the left side and a downward force of 30 N on the right side. What is the net torque about the axle on the pulley?

τ = -0.20 N⋅m

A 230 g , 21-cm-diameter plastic disk is spun on an axle through its center by an electric motor. What torque must the motor supply to take the disk from 0 to 2000 rpm in 4.4 s ?

τ = 6.0×10^−2 N⋅m

A small grinding wheel has a moment of inertia of 4.0×10^−5 kg⋅m2 . What net torque must be applied to the wheel for its angular acceleration to be 150 rad/s2 ?

τ = 6.0×10^−3 N⋅m

Specifications require the oil filter for your car to be tightened with a torque of 30 N⋅m. You are using a 15-cm-long oil filter wrench, and you apply a force at the very end of the wrench in the direction that produces maximum torque. How much force should you apply?

200 N

A ball on a string moves around a complete circle, once a second, on a frictionless, horizontal table. The tension in the string is measured to be 10 N . What would the tension be if the ball went around in only half a second?

40 N

A 5.0 kg dog sits on the floor of an elevator that is accelerating downward at 1.20 m/s2. What is the magnitude of the force of the dog on the elevator floor?

43 N

A 5.0 kg dog sits on the floor of an elevator that is accelerating downward at 1.20 m/s2. What is the magnitude of the normal force of the elevator floor on the dog?

43 N

Eric has a mass of 60 kg. He is standing on a scale in an elevator that is accelerating downward at 1.7 m/s2. What is the approximate reading on the scale?

490 N

Newton's law of gravity describes the gravitational force between __________.

Earth and the moon. the sun and the planets. Earth and the sun. a person and the earth.

In short-track speed skating, the track has straight sections and semicircles 16 m in diameter. Assume that a 67 kg skater goes around the turn at a constant 11 m/s . What is the horizontal force on the skater?

F = 1000 N

In the situation shown in the figure, a person is pulling with a constant, nonzero force F⃗ on string 1, which is attached to block A. Block A is also attached to block B via string 2, as shown. For this problem, assume that neither string stretches and that friction is negligible. Both blocks have finite (nonzero) mass.

How does the magnitude of the tension in string 1, T1, compare with the tension in string 2, T2?

If a planet has the same mass as the earth, but has twice the radius, how does the surface gravity, g, compare to g on the surface of the earth?

It is four times smaller.

A car with 68-cm -diameter tires accelerates uniformly from rest to 20 m/s in 18 s . How many times does each tire rotate?

N = 84 times

This problem concerns the concept of tension in a rope. Consider a rope subjected to a pulling force on its two ends as shown in (Figure 1). The rope is stationary. An arbitrary point P divides the rope into a left-hand segment L and a right-hand segment R. Which of the following phrases, if they appear in a problem, allow you to assume that T2=T1 in a horizontally oriented rope?

The rope is massless. The rope is moving at constant speed.

For an object in uniform circular motion, what can you say about the directions of the velocity, acceleration, and net force vectors?

The velocity vector is perpendicular to the acceleration vector; the acceleration vector is parallel to the net force vector.

In the video, a car rounding a corner and a car going over the crest of a hill are both presented as examples of

Uniform circular motion.

A net torque applied to an object causes __________.

the angular velocity of the object to change

In uniform circular motion, which of the following quantities are constant?

the magnitude of the net force speed

One mass is sitting on a frictionless surface, a rope attached to a pulley is attached it its left side. A second mass sits on top of it with a rope attached to the pulley also attached to its left side. Velocity vector points from the first mass to the right. In which direction is the kinetic friction force on block 1 in the figure?

To the right.

You are riding in an elevator that is accelerating upward. Suppose you stand on a scale. The reading on the scale is __________.

greater than your true weight

The crankshaft in a race car goes from rest to 3120 rpm in 2.2 s. How many revolutions does it make while reaching 3120 rpm?

n = 57 revolutions

If an object is rolling without slipping, how does its linear speed compare to its rotational speed?

v = Rω

What is the speed of a satellite orbiting at that height? Assume M(earth) = 5.98 × 10^24 kg.

v(s) = 5600 m/s

A 100 cm bar with a pivot 25 cm from the left has a 8 N force 30∘ from horizontal on the left and a 10 N from 80∘ from horizontal on the right. What is the net torque about on the bar shown in (Figure 1) about the axis indicated by the dot?

τ = 6.4 N⋅m

An object's moment of inertia is 2.5 kg⋅m^2 . Its angular velocity is increasing at the rate of 3.2 rad/s per second. What is the net torque on the object?

τ = 8.0 N⋅m

What is the angular speed of the tip of the minute hand on a clock, in rad/s?

ω = 1.75×10^−3 rad/s

The drag force pushes opposite your motion as you ride a bicycle. If you double your speed, what happens to the magnitude of the drag force?

The drag force goes up by a factor of 4

A 1000-kg car is moving at a constant speed around a circular turn with a radius of 18.5 meters. How fast must the car move to have an acceleration of 25.2 m/s2?

21.6 m/s

Wings on race cars push them into the track. The increased normal force makes large friction forces possible. At one Formula One racetrack, cars turn around a half-circle with diameter 190 m at 68 m/s . For a 610 kg vehicle, the approximate minimum static friction force to complete this turn is

30000 N

For an extended object, the weight force can be considered to act at which point?

At the center of gravity of the object

A 60 kg man hangs from a rope on the left side of a pulley, while a 100 kg mass rests on the floor attached to the rope on the other side of the pulley. What is the tension in the rope of the figure?

T = 590 N

Two blocks with masses M1 and M2 hang one under the other. For this problem, take the positive direction to be upward, and use g for the magnitude of the acceleration due to gravity. Find T1, the tension in the upper rope.

T(1) = (M(1) + M(2))g

Two blocks are at rest on a frictionless incline, m(1) hangs from a rope from m(2), which is attached by a rope to a wall. The incline is 20 degrees. What is the tension in the string number 1 if m1 = 3.5 kg and m2 = 4.0 kg ?

T(1) = 12 N

This problem concerns the concept of tension in a rope. Consider a rope subjected to a pulling force on its two ends as shown in (Figure 1). The rope is stationary. An arbitrary point P divides the rope into a left-hand segment L and a right-hand segment R. Now imagine two points, Q and P, that divide the rope into segments L, M ,and R. (Figure 2)The rope remains stationary. Assume that segment L exerts a force of magnitude FLM on segment M. What is the magnitude FRM of the force exerted by segment R on segment M?

F(RM) = F(LM)

This problem concerns the concept of tension in a rope. Consider a rope subjected to a pulling force on its two ends as shown in (Figure 1). The rope is stationary. An arbitrary point P divides the rope into a left-hand segment L and a right-hand segment R. Now consider a rope that, unlike those usually studied in mechanics problems, actually has a significant inertia m. The tension at the right end of this rope is T2 and that at the left end is T1. (Figure 3)The rope has an acceleration arope to the right. Complete the following equation for the force on the section of the rope of inertia m, taking the positive direction to be to the right.

F(rope) = ma(rope) = T(2) -T(1)

The centers of a 13 kg lead ball and a 60 g lead ball are separated by 11cm. What is the ratio of this gravitational force to the weight of the 60 g ball?

F(w) = 7.3×10^−9

In short-track speed skating, the track has straight sections and semicircles 16 m in diameter. Assume that a 67 kg skater goes around the turn at a constant 11 m/s . What is the ratio of this force to the skater's weight?

F/w = 1.5

This problem concerns the concept of tension in a rope. Consider a rope subjected to a pulling force on its two ends as shown in (Figure 1). The rope is stationary. An arbitrary point P divides the rope into a left-hand segment L and a right-hand segment R. For segment R and segment L to hold together, they must exert forces on each other. What is the direction of the force exerted on segment R by segment L?

Left

You are going sledding with your friends, sliding down a snowy hill. Friction can't be ignored. Riding solo on your sled, you have a certain acceleration. Would the acceleration change if you let a friend ride with you, increasing the mass?

No, increasing the mass does increase the net force on the system, but it also increases the inertia. a=F(net)/m. Since both the net force and mass are increased they still cancel, leaving the acceleration the same.

A 600 kg piano is being lowered into position by a crane while two people steady it with ropes pulling to the sides. Bob's rope pulls to the left, 11 ∘ below the horizontal, with 310 N of tension. Ellen's rope pulls toward the right, 28 ∘ below the horizontal. What tension must Ellen maintain in her rope to keep the piano descending vertically at a constant speed?

T(1) = 340 N

Each of 138 identical blocks sitting on a frictionless surface is connected to the next block by a massless string. The first block is pulled with a force of 138 N. What is the tension in the string connecting block 138 to block 137?

T(138 to 137) = 1.0 N

Two blocks are at rest on a frictionless incline, m(1) hangs from a rope from m(2), which is attached by a rope to a wall. The incline is 20 degrees. What is the tension in the string number 2 if m1 = 3.5 kg and m2 = 4.0 kg ?

T(2) = 25 N

A 600 kg piano is being lowered into position by a crane while two people steady it with ropes pulling to the sides. Bob's rope pulls to the left, 11 ∘ below the horizontal, with 310 N of tension. Ellen's rope pulls toward the right, 28 ∘ below the horizontal. What is the tension in the vertical main cable supporting the piano?

T(2) = 6100 N

Two blocks with masses M1 and M2 hang one under the other. For this problem, take the positive direction to be upward, and use g for the magnitude of the acceleration due to gravity Find T2, the tension in the lower rope.

T(2) = M(2)(g+a)

A road race is taking place along the track shown in the figure (Figure 1). All of the cars are moving at constant speeds. The car at point F is traveling along a straight section of the track, whereas all the other cars are moving along curved segments of the track. et velocity vector(A) be the velocity of the car at point A. What can you say about the acceleration of the car at that point?

The acceleration is perpendicular to velocity vector(A) and directed toward the inside of the track.

A typical running track is an oval with 74-m-diameter half circles at each end. A runner going once around the track covers a distance of 400 m . Suppose a runner, moving at a constant speed, goes once around the track in 1 min 40 s. What is her centripetal acceleration during the turn at each end of the track?

a(r) = 0.43 m/s^2

An old-fashioned LP record rotates at 33⅓ rpm. What is its frequency in rev/s?

f = 0.556 rev/s

Box A, with m 20kg, F 30N, and coefficient of kinetic friction 0.4, and Box B, with m 10, F 30, and coefficient of kinetic friction 0.5 are both sliding. Is the friction force on A larger than, smaller than, or equal to the friction force on B? Assume the boxes are made of the same material and are on the same surface.

f(A) > f(B)

A 4000 kg truck is parked on a 7.0∘ slope. How big is the friction force on the truck?

f(s) = 4800 N

When a car turns a corner on a level road, which force provides the necessary centripetal acceleration?

friction

In general, the coefficient of static friction is __________.

greater than the coefficient of kinetic friction