Physics Exam 3 study guide

On what does the angular momentum of an object depend? (Select all that apply.)

- The shape of the object - The axis of rotation - The mass of the object - The rate at which that the object rotates

Considering the same stars as described in Part B, how does the acceleration of Procyon A compare to the acceleration of Procyon B as they orbit each other?

0.4aB

Calculate the force of gravity on a 1-kg mass if it were 1.3×107m above Earth's surface (that is, if it were three Earth radii from Earth's center).

1.1 N

An astronaut is on the surface of a new planet that has a radius of 6.1×106m (similar to Venus) and a mass of 6.4×1023kg (similar to Mars). If she dropped her cell phone, what would be the free-fall acceleration of the phone? Recall that G=6.67×10−11N⋅m2/kg2.

1.1 m/s^2

The rock and meterstick balance at the 25-cm mark, as shown in (Figure 1). The meterstick has a mass of 1.4 kg. What must be the mass of the rock?

1.4 kg

Ch 10 HW

101 - 149

Find the change in the force of gravity between two planets when the distance between them is decreased by 13.

169

Ch 8 HW

2 - 52

Calculate the force of gravity between Earth (mass 6.0×1024 kg) and the Moon (mass 7.4×1022 kg). The average Earth-Moon distance is 3.8×108 mm.

2.1 x 10^20 N

A small space telescope at the end of a tether line of length 𝐿 moves at linear speed 𝑣 about a central space station. If the initial linear speed of the telescope is 0.90 m/s, what is its speed when pulled in to one-third its initial distance from the space station?

2.7 m/s

To tighten a bolt, you push with a force of 85 N at the end of a wrench handle that is 0.25 m from the axis of the bolt. What torque are you exerting?

21 N⋅m

To tighten a bolt, you push with a force of 85 N at the end of a wrench handle that is 0.25 m from the axis of the bolt. If you move your hand inward to be only 0.10 m from the bolt, find the force that you should exert to achieve the same torque.

210 N

There are four ice skaters as shown in (Figure 1). Three skaters are spinning with equal rates of rotation, while the fourth skater is not spinning at all. Rank the skaters in terms of their angular momentum. Rank the skaters from largest to smallest angular momentum. To rank items as equivalent, overlap them.

3, 2,1,4

A small space telescope at the end of a tether line of length 𝐿 moves at linear speed 𝑣 about a central space station. What will be the linear speed of the telescope if the length of the line is reduced to 0.33 𝐿?

3.03v

Calculate the force of gravity a newborn baby (mass 2.7 kg) and the planet Mars (mass 6.4×10^23 kg) exert on each other when Mars is at its closest to Earth (distance 5.6×10^10m).

3.7 x 10 ^-8 N

A circular race track has two well-defined lanes, the outer one longer than the inner one. The radius of the inner lane is 20.0 m; the radius of the outer lane is 21.0 m. The circumference of a circle is C=2πr. For a 5-lap race, how much head start should a runner on the outer lane have to compensate for the differences of track distances?

31 m

Calculate the torque produced by a 60-N perpendicular force at the end of a pipe extending the length of a wrench to 0.56 m

34 N⋅m

Pretend you stood atop a ladder so tall that you were 2 times as far from Earth's center as you presently are. Find the ratio of your weight on the Earth surface to the weight at the top of the ladder.

4

Calculate the force of gravity between the Moon (mass 7.4×1022kg) and the Sun (mass 2.0×1030kg). The average Sun-Moon distance is 1.5×1011m.

4.4 x 10^20 N

80-kg person sits 1.5 m from the center of a horizontally rotating platform with a tangential speed of 2.0 m/s. If the person's speed doubles and all else remains the same, what will be the person's angular momentum?

480 kg⋅m^2/s

Calculate the tension in a horizontal string that whirls a 2.4-kg toy in a circle of radius 2.4 m when it moves at 2.4 m/s on an icy surface.

5.8 N

Ch 09 HW

54 - 99

A projectile is launched at 30° above ground level. What other angle at the same speed will result in the same range?

60°

Many people mistakenly believe that the astronauts that orbit Earth are "above gravity." Earth's mass is 6.0×1024kg, and its radius is 6.38×106m (6380 km). Calculate 𝑔 for space shuttle territory, 900 km above Earth's surface.

7.6 m/s^2

Many people mistakenly believe that the astronauts that orbit Earth are "above gravity." Earth's mass is 6.0×1024kg, and its radius is 6.38×106m (6380 km). Your answer from part A is what percentage of 9.8 m/s2?

77 %

What is the relationship between the center of gravity and support base for an object in stable equilibrium?

A vertical line through the center of gravity must pass inside the support base of the object.

Why doesn't the Leaning Tower of Pisa topple over?

A vertical line through the center of gravity passes inside its support base.

What did Newton discover about gravity that was not known at the time?

All of the above

How do linear momentum and angular momentum differ?

Angular momentum depends on the distribution of mass, whereas linear momentum depends on the total mass.

If a spinning skater pulls her arms in so as to reduce her rotational inertia by half, by how much will her rate of spin increase?

Angular momentum remains the same, while her rate of spin doubles.

Why are all tides greatest at the time of a full Moon or new Moon?

At full Moon and new Moon, the tides from the Moon and the Sun add because they are in line with Earth.

A baseball bat can be rotated around many different axes of rotation. Three such possibilities are shown in (Figure 1). Rank the baseball bat's moment of inertia about each of these three axes of rotation. Rank the moment of inertia from largest to smallest and overlap axes labels if the same.

B, A, C

You hold a meterstick horizontally with the same mass suspended at the end as shown in (Figure 1). Rank the torque needed to keep the stick steady, from largest to smallest.

B, C, A

Why it is easier to remove an old bolt if we add an extension to the wrench handle?

Because a higher torque is applied

Given the same baseball bat and possible axes of rotation shown in (Figure 1), for which axis of rotation would it be the easiest to rotate the bat from rest?

C

Consider three axes of rotation for a pencil as shown (Figure 1): (A) along the lead; (B) at right angles to the lead at the middle; (C) at right angles to the lead at the eraser at one end. Rate the rotational inertias about each axis from large to small.

C, B, A

When a rock tied to a string whirled in a horizontal circle somehow doubles in mass but keeps the same speed, the string tension _______.

Doubles

When the rotational speed of a rotating system doubles, its angular momentum _______.

Doubles

Earth's gravitational field is strongest at _______.

Earth's surface

What is required to change the angular momentum of a system?

External torque

The constellation Canis Minor has a binary star system consisting of Procyon A and Procyon B. Procyon A, at 3×1030kg, has 2.5 times the mass of Procyon B; and they are roughly 2×1012m apart. How does the force on Procyon A from Procyon B compare to the force on Procyon B from Procyon A?

FBonA

Show two formulas: Newton's law of gravity as a proportion and as an exact equation.

F𝐹 ~ (m1m2)/r2(𝑚1𝑚2)/𝑟2, F=(Gm1m2)/r2

The planet and its moon gravitationally attract each other. Rank the force of attraction between each pair from greatest to least.

Greatest - 2m d m, and m d 2m Middle - mdm Least - 3M 2d m

Rank the microtidal forces on your own body, from greatest to least, produced by the following:

Greatest - Earth Middle - Moon Least - Sun

Under what condition is the angular momentum of an object conserved?

If there is no net torque acting on it.

How does the Moon "fall" without getting closer to us?

It falls away from the straight path that it would follow if there were no forces acting on it.

As distance increases between most of the mass of an object and its center of rotation, how does rotational inertia change?

It increases.

What is a gravitational field, and how can its strength be measured?

It is a force field on any body with mass. Its strength is the force per unit mass on a test mass.

What is meant by the "lever arm" of a torque?

It is the perpendicular distance from the rotational axis to the line along which the force acts.

The light of multiple candle flames has the same brightness. Rank the light that enters your eye from brightest to dimmest for the following situations.

Largest - 1 candle, 1m Middle - 2 candle, 2m Least - 3 candle, 3m

When you are standing on Earth, orbiting the Sun, and looking at a broken cell phone on the ground, there are gravitational pulls on the cell phone from you, the Earth, and the Sun. Rank the gravitational forces on the phone from largest to smallest. Assume the Sun is roughly 109109 times further away from the phone than you are, and 10281028 times more massive than you. Rank the following choices in order from largest gravitational pull on the phone to smallest. To rank items as equivalent, overlap them.

Largest - Pull on phone from Earth Middle - Pull on phone from the Sun Smallest - Pull on phone from you

Pretend you fall into a hole bored completely through the Earth. Ideally with no obstructions, rank positions A, B, C, and D from most to least for your speed.

Most - B MIddle - C, A Least - D

Pretend you fall into a hole bored completely through the Earth. Ideally with no obstructions, rank positions A, B, C, and D from most to least for your acceleration toward Earth's center.

Most - D Middle - C,A Least - B

What great discovery followed finding perturbations in the planet Uranus?

Neptune was discovered.

How is rotational inertia similar to inertia as studied in previous chapters?

Rotational inertia is the resistance to a change in rotational motion, which is similar to inertia which is a resistance to a change in velocity.

Which varies with radial distance on a rotating turntable, tangential speed or rotational speed?

Tangential speed increases with distance. Rotational speed is constant.

What kind of force acts on a ladybug inside a can that is whirled in a circular path?

The can exerts on the ladybug an inward centripetal force.

How does the angular momentum of a cat change in falling from a tree?

The cat rotates its body through an angle, but the angular momentum of the cat remains constant.

How does Earth's reduction of angular momentum affect the Moon's distance from Earth?

The decrease in Earth's angular momentum is accompanied by an equal increase in the angular momentum of the Moon, which results in the Moon's increasing distance from Earth.

Both the Moon and the Sun produce our ocean tides, but the tides are primarily due to the Moon. Why?

The difference in lunar pulls is greater than the difference in solar pulls.

What happens to the force of attraction between two planets when the distance between them is doubled?

The force decreases to one quarter.

What happens to the force of attraction between two planets when the masses of both are doubled?

The force quadruples

How does the force of gravity between two bodies change when the distance between them is halved?

The force quadruples.

What happens to the force of attraction between two planets if their masses are doubled and the distance between them is also doubled?

The force would remain the same.

For a rotating object experiencing no net external torque, what happens to the rate of rotation if the moment of inertia of the object decreases by a factor of 2?

The rate of rotation increases by a factor of 2.

Do seat belts provide a centripetal or centrifugal force on a passenger in a car rounding a curve?

The seat belts provide a centripetal force on the passenger.

Is it an inward force or an outward force that is exerted on the clothes during the spin cycle of a washing machine?

The tub of the machine exerts centripetal inward force on clothes.

Of the four fundamental forces presently known, the force of gravity is ________________.

The weakest

Why it is not possible to directly observe or photograph a black hole?

Their gravity is so strong that nothing, not even light, can escape a black hole.

When you weigh yourself on a bathroom scale in an elevator moving upward at constant velocity, are the springs in the scale more, or less, compressed? Moving downward at constant velocity?

There would be no more compression and no more expansion for both upward and downward motion.

Do tides depend more on the strength of gravitational pull or on the difference in strengths? Explain.

Tides depend only on the difference.

When G was first measured by Henry Cavendish, newspapers of the time hailed his experiment as the "weighing Earth experiment." Why?

With a known mass, a scale, and the radius of Earth, you can calculate the unknown mass of Earth if you know G.

How could your weight be greater than mg? How could your weight be zero? Give examples.

Your weight is greater than mg when you are accelerating upward. Your weight is zero when you are in free fall.

Consider someone in a rotating space habitat. Rotational motion provides _______.

a support force

Black holes are essentially _______.

any object with an escape velocity greater than the speed of light

The center of gravity of a basketball is located _______.

as its geometrical center

To double the rotational speed the centripetal force must _______.

be four times larger

When traveling twice as fast your kinetic energy is increased _______.

by four times

Einstein's theory of gravitation, compared with Newton's, _______.

eliminates the role of force

The torque exerted by a crowbar on an object increases with increased _______.

force and leverage distance

For a planet of uniform density, how would the magnitude of the gravitational field halfway to the center compare with the gravitational field at the surface?

g/2 m/s^2

The tangential speed on the outer edge of a rotating carousel is _______.

greater than toward the center Submit

Consider someone in a rotating space habitat. The outward force felt by the person _______.

has no reaction counterpart

Rotational inertia of an object rotating about an axis becomes greater with _______.

increased mass and increased distance to mass concentration

The horizontal and vertical components of velocity for a projectile are __________.

independent of each other

The rotational speed of every point of a rotating rigid object _______.

is the same

What happens to the strength of the gravitational field at the surface of a star that shrinks?

it increases

How does the brightness of light change when the source is held twice as far away?

it is 1/4 as bright

A girl spins around in a circle trying to make herself dizzy. Without changing her position, she starts spinning twice as fast. By how much did her rotational kinetic energy change?

it quadrupled

To improve the performance of a racing bicycle, the mass of the tires and rims is reduced so there is ____________.

less rotational inertia

The constant G in Newton's equation _______.

makes the units of measurement consistent

A ruler is in equilibrium on the tip of your finger only if __________.

net torque and net force are zero

One's weightlessness in space has most to do with _______.

no support force

When a rock tied to a string is whirled in a horizontal circle, doubling the speed _______.

quadruples the tension in the string

Isaac Newton synthesized _______.

terrestrial and cosmic laws

When an object moves away from the surface of Earth, the gravitational force decreases because __________.

the distance between Earth and the object increases

Ocean tides on Earth are mainly affected by ___________.

the moon and the sun

A solid sphere and a hollow sphere (spherical shell) of the same mass and same radius rotate with the same amount of kinetic energy. Which one is rotating faster?

the solid sphere

What Newton discovered most about gravity is that it is _______.

universal

To tighten a bolt, you push with a force of 85 N at the end of a wrench handle that is 0.25 m from the axis of the bolt. Do your answers depend on the direction of your push relative to the direction of the wrench handle?

yes