Exam 3 Flashcards by Sera Cook

A red ball with a velocity of +3.0 m/s collides head-on with a yellow ball of equal mass moving with a velocity of −2.0 m/s. What is the velocity of the two
balls after the elastic collision?

The velocity of the red ball is −2.0 m/s; the velocity of the yellow ball is +3.0 m/s

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A freight car moves along a frictionless level railroad track at constant speed. The freight car is open on the top. A large load of sand is suddenly dumped into the freight car. What happens to the speed of the freight car?

The speed of the freight car decreases

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Kinetic energy is conserved in what type of collision(s)?

Elastic

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A lump of putty and a rubber ball have equal mass. Both are thrown with equal speed against a wall. The putty sticks to the wall. The ball bounces back at nearly the same speed with which it hit the wall. Which object experiences the greater momentum change?

The ball

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In a lab environment, you are investigating the impulse of a force exerted on a brick when the brick’s speed is reduced from 2.5 m/s to a complete stop. First, you allow the brick to slam into a secured piece of wood, bringing the brick to a sudden stop. Second, you allow the brick to plow into a large slab of gelatin so that the brick comes to a gradual halt. In which situation is there a greater impulse of the force on the brick?

The impulse is the same in both situations

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A net force of 200 N acts on a 100-kg boulder, and a force of the same magnitude acts on a 100-g pebble. Is the change of the boulder’s momentum in one second greater than, less than, or equal to the change of the pebble’s momentum in the same time period?

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A net force of 200 N acts on a 100-kg boulder, and a force of the same magnitude acts on a 100-g pebble. Is the change in the boulder’s speed in one second greater than, less than, or equal to the change in speed of the pebble in the same time period?

Less

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A 22.6 kg dog is running northward at 2.51 m/s , while a 6.01 kg cat is running eastward at 3.26 m/s . Their 75.1 kg owner has the same momentum as the two pets taken together.

Find the magnitude of the owner’s velocity in m/s.

Find the direction of the owner’s velocity in degrees above the positive x axis.

.799

70.9

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Object 1 has a mass m1 and a velocity v1 = 2.30 m/s in the positive x direction.

Object 2 has a mass m2 and a velocity v2 = 3.30 m/s in the positive y direction.

The total momentum of these two objects has a magnitude of 18.0 kg ⋅ m/s
and points in a direction 66.0 ∘ above the positive x axis.

Find m1 in kg. And m2 in kg

4. 98

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Find the magnitude of the impulse delivered to a soccer ball when a player kicks it with a force of 1450 N . Assume that the player’s foot is in contact with the ball for 5.60×10−3 s . Give your answer in SI units.

8.12

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Two ice skaters stand at rest in the center of an ice rink. When they push off against one another the 66-kg skater acquires a speed of 0.66 m/s .

If the speed of the other skater is 0.86 m/s , what is this skater’s mass in kg?

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A car with a mass of 1010 kg and a speed of v1 = 18.0 m/s is driving east when it approaches an intersection. A 1310 kg minivan traveling at v2 is driving north and is heading for the same intersection.
The car and minivan collide and stick together. The direction of the
wreckage after the collision is θ = 43.0 ∘ above the x axis.

Find the initial speed in m/s of the minivan, assuming external forces can be ignored.

Find the final speed in m/s of the wreckage

10. 7

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A girl and a boy are riding on a merry-go-round that is turning at a constant rate. The girl is near the outer edge, and the boy is closer to the center. Who has greater linear speed?

The girl

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A girl and a boy are riding on a merry-go-round that is turning at a constant rate. The girl is near the outer edge, and the boy is closer to the center. Who has greater tangential acceleration?

The both have 0

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A girl and a boy are riding on a merry-go-round that is turning at a constant rate. The girl is near the outer edge, and the boy is closer to the center. Who has greater angular displacement?

Same nonzero

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When you stand on the observation deck of the Empire State Building in New York, is your linear speed due to the Earth’s rotation greater than, less than, or the same as when you were waiting for the elevators on the ground floor?

Greater

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Rank the following in order of decreasing angular speed: an automobile tire rotating at 2.00 × 103 deg/s, an electric drill rotating at 400.0 rev/min, and an airplane propeller rotating at 40.0 rad/s.

drill > propeller > tire

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Two children, Jason and Betsy, ride on the same merry-go-round. Jason is a distance R from the axis of rotation; Betsy is a distance 2R from the axis. Is the rotational period of Jason greater than, less than, or equal to the rotational period of Betsy?

It takes the same amount of time for the merry-go-round to complete a revolution for all points on the merry-go-round.

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Convert 35 degrees to radians.

.61

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After fixing a flat tire on a bicycle you give the wheel a spin. Its initial angular speed was 6.40 rad/s and it rotated 14.3 revolutions before coming to rest. What was its average angular acceleration in SI units?

For what length of time in seconds did the wheel rotate?

0.228

28. 1

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The drill used by most dentists today is powered by a small air-turbine that can operate at angular speeds of 350000 rpm. These drills, along with ultrasonic dental drills, are the fastest turbines in the world-far exceeding the angular speeds of jet engines. Suppose a drill starts at rest and comes up to operating speed in 2.1 s .

Find the angular acceleration produced by the drill, assuming it to be constant. Express your answer in SI units.

How many revolutions does the drill bit make as it comes up to speed?

17,000

6100

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As you drive down the road at 16 m/s , you press on the gas pedal and speed up with a uniform acceleration of 1.17 m/s2 for 0.65 s.

If the tires on your car have a radius of 33 cm, what is their angular displacement during this period of acceleration? Express your answer in SI units.

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A merry-go-round is rotating at constant angular speed. Two children are riding the merry-go-round: Ana is riding at a radius R and Bobby is riding at a radius 2R.

Which child moves with greater magnitude of linear velocity

Who moves with greater magnitude of angular velocity?

Who moves with greater magnitude of tangential acceleration?

Who has the greater magnitude of centripetal acceleration.

Who moves with greater magnitude of angular acceleration?

Bobby

Same

Bobby

Same

The minute and hour hands of a clock have a common axis of rotation and equal mass. The minute hand is long, thin, and uniform; the hour hand is short, thick, and uniform. Is the moment of inertia of the minute hand greater than, less than, or equal to the moment of inertia of the hour hand?

Greater than, because having mass farther from the axis of rotation results in a greater moment of inertia.

A heavy boy and a lightweight girl are balanced on a massless seesaw. The boy moves backward, increasing his distance from the pivot point. What happens to the seesaw?

The side the boy is on will tilt downward

A constant net torque is applied to a rotating object. Which of the following best describes the object's motion?

The object will rotate with constant angular acceleration

Two equal forces are applied perpendicular to a door. The first force is applied at the midpoint of the door; the second force is applied at the doorknob. Which force exerts the greater torque?

Doorknob

Two spheres of equal mass and radius are rolling across the floor with the same speed. Sphere 1 is a uniform solid; sphere 2 is hollow. Is the work required to stop sphere 1 greater than, less than, or equal to the work required to stop sphere 2?

Less than, because Sphere 2 has the greater moment of inertia and hence the greater rotational kinetic energy.

A torque of 0.10 N ⋅ m is applied to an egg beater. If the egg beater starts at rest, what is its angular momentum after 0.60 s ?

.06

A torque of 0.10 N ⋅ m is applied to an egg beater. If the moment of inertia of the egg beater is 2.4×10−3 kg ⋅ m2 , what is its angular speed after 0.60 s ?

A windmill has an initial angular momentum of 8600 kg ⋅ m2 /s . The wind picks up, and 5.96 s later the windmill's angular momentum is 9750 kg ⋅ m2 /s . What was the torque acting on the windmill, assuming it was constant during this time?

190

An electric fan spinning with an angular speed of 11 rad/s has a kinetic energy of 4.9 J . What is the moment of inertia of the fan?

.081

Two objects attract each other gravitationally. If the distance between their centers decreases by a factor of 2, how does the gravitational force between them change?

The gravitational force increases by a factor of 4

Two objects attract each other gravitationally. If the mass of each object doubles, how does the gravitational force between them change?

The gravitational force increases by a factor of 4.

A hypothetical planet has a mass one-third of and a radius three times that of Earth. What is the acceleration due to gravity on the planet in terms of g, the acceleration due to gravity on Earth?

g/27

Two planets, planet A and planet B, have the same surface gravity. However, planet B has twice the radius of planet A. How does the mass of planet B compare to the mass of planet A?

The mass of planet B is four times the mass of planet A

Earth's orbit around the Sun is slightly elliptical. Thus, Earth actually gets closer to the Sun during part of the year. What happens to Earth's orbital speed when it is closer to the Sun?

Earth's orbital speed is greater when it is closer to the Sun than when it is farther from the Sun

Why does a satellite in a circular orbit travel at a constant speed?

There is no component of force acting along the direction of motion of the satellite.

The gravitational field on the surface of the earth is stronger than that on the surface of the moon. If a rock is transported from the moon to the earth, which properties of the rock change?

Weight only

An object is lifted from the surface of a spherical planet to an altitude equal to the radius of the planet. As a result, what happens to the mass and weight of the object?

mass remains the same; weight decreases

Is the amount of energy required to get a spacecraft from the Earth to the Moon greater than, less than, or equal to the energy required to get the same spacecraft from the Moon to the Earth?

Greater than, because The escape speed of the Moon is less than that of the Earth; therefore, less energy is required to leave the Moon.

The orbital speed of the Earth is greatest around January 4 and least around July 4. Is the distance from the Earth to the Sun on January 4 greater than, less than, or equal to its distance from the Sun on July 4?

Less than, because the Earth sweeps out equal area in equal time, thus it must be closer to the Sun when it is moving faster.

Three identical very dense masses of 4500 kg each are placed on the x axis. One mass is at x1 = -130 cm , one is at the origin, and one is at x2 = 400cm. What is the magnitude and direction (+ or -) of the net gravitational force Fgrav on the mass at the origin due to the other two masses? What is the magnitude and direction (+ or -) of the gravitational force due to the mass at x2 = 400 cm ? Take the gravitational constant to be G = 6.67×10−11 N ⋅ m2 /kg2 .

- 0.0008 | 0. 0007

To tighten a spark plug, it is recommended that a torque of 25 m.N be applied. If a mechanic tightens the spark plug with a wrench that is 19 cm long, what is the minimum force, in Newtons, necessary to create the desired torque?

130

What is an inertial reference frame

a frame of reference in which a body remains at rest or moves with constant linear velocity unless acted upon by forces: any frame of reference that moves with constant velocity relative to an inertial system is itself an inertial system

What does special relativity reveal about the speed of light relative to its source?

Light moves at the same speed independent of the motion of the source.

An object is moving with a velocity that approaches the speed of light. How does the length of the moving object appear to a stationary observer relative to its rest length?

The length of the moving object appears less than its rest length.

A clock is moving relative to an observer with a velocity that approaches the speed of light. How does the passage of time measured by the moving clock compare to the passage of time measured by a stationary clock?

The moving clock appears to run more slowly.

Suppose one twin takes a ride in a spaceship traveling at a very high speed to a distant star and back again, while the other twin remains on Earth. When the traveling twin returns, how do the ages of the twins compare?

The traveling twin is younger than the twin who remained on Earth.

You are in a spaceship, traveling directly away from the Moon with a speed of 0.9c. A light signal is sent in your direction from the surface of the Moon. As the signal passes your ship, do you measure its speed to be greater than, less than, or equal to 0.1c?

Greater than, because the speed you measure will be greater than 0.1c; in fact, it will be c, since all observers in inertial frames measure the same speed of light.

A clock in a moving rocket is observed to run slow. If the rocket reverses direction, does the clock run slow, fast, or at its normal rate?

Slow. The clock will run slow, just as before. The rate of the clock depends only on relative speed, not on direction of motion.

Suppose you are a traveling salesman for SSC, the Spacely Sprockets Company. You travel on a spaceship that reaches speeds near the speed of light, and you are paid by the hour. When you return to Earth after a sales trip, would you prefer to be paid according to the clock at Spacely Sprockets universal headquarters on Earth, according to the clock on the spaceship in which you travel, or would your pay be the same in either case?

According to the clock at Spacely Sprockets universal headquarters on Earth. You want to be paid according to the clock on Earth, because the clock on the spaceship runs slow when it approaches the speed of light.

You are at the top of the Empire State Building on the 102nd floor, which is located 373 m above the ground, when your favorite superhero flies over the building parallel to the ground at 70.0 % the speed of light. You have never seen your favorite superhero in real life. Out of curiosity you calculate her height to be 1.50 m . If the superhero landed next to you, how tall would she be when standing in meters? What is the height in meters of the 102nd floor of the Empire State Building as measured by the superhero while flying above it?

2.10 L=L0sqrt(1-(v^2/c^2)) 1.5=L0sqrt(1-((.7c)^2/c^2)) 373

Suppose you fly on a rocket with a speed v = 0.99 c for 1 y, according to the ship's clocks and calendars. How much time elapses in years on Earth during your 1-y trip? If you were 25 y old when you left home and your sister was 20, what is your age in years when you return? What is your sister's age in years when you return?

7.1 or 7 26 27

Suppose the speed of light in a vacuum were only 26.0 mi/h . Find the length in meters of a bicycle being ridden at a speed of 21.0 mi/h as measured by an observer sitting on a park bench, given that its proper length is 1.90 m .

1.12