Physics exam 2

If you push a crate horizontally with 100 N across a 10-m factory floor and friction between the crate and the floor is a steady 70 N, how much kinetic energy is gained by the crate?

(100-70)*10 30*10 =300J

Show that the gravitational potential energy of a 1000-kg boulder raised 5.0 m above ground level is 50,000 J.

1,000*10*5=50,000

A 2-kg blob of play dough moving at 3 m/s slams into a 2-kg blob of play dough at rest. Show how the speed of the two stuck-together blobs immediately after colliding is 1.5 m/s.

1.5m/s

Show that 24 J of work is done when a 3.0-kg block of ice is moved from rest to 4.0 m/s.

1/2(3)(4)^2=24J

Show that the kinetic energy of a 1.0-kg hamster running at 3.0 m/s is 4.5 J.

1/2*(1)(3)^2=4.5J

An astronaut lands on a planet that has the same mass as Earth but three times the diameter. How does the astronaut's weight differ from that on Earth?

1/9 of the weight

Lil drives her car with a mass of 1000 kg at a speed of 20 m/s. Show that to bring her car to a halt in 10 s road friction must exert a force of 2000 N on the car.

1000kgx20/10

Calculate the torque produced by a 50-N perpendicular force at the end of a 0.2-m-long wrench.

10Nm

How many watts are expended when a force of 12 N moves a book 2 m in 3 s?

12*2/3 = 8W

What is the momentum of a 40-kg carton that slides at 4 m/s across an icy surface?

160kg*m/s

What is the impulse on a 9-kg ball rolling at 2 m/s when it bumps into a basket and stops?

18kg*m/s

What is the momentum of a 9-kg bowling ball rolling at 2 m/s?

18kg*m/s

Show that the tension in a horizontal string that whirls a 2.0-kg toy in a circle of radius 1.5 m when it moves at 2.0 m/s on an icy surface is 5.3 N.

2*2^2/1.5

Show that a 2,500,000-J change in kinetic energy occurs for an airplane that is moved 500 m in takeoff by a sustained force of 5000 N.

2,500*1000=2,500,000

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 m.

2.05x10^20 N

What impulse occurs when the same force of 10 N acts on the cart for twice the time?

200N*S

How much impulse stops a 50-kg carton sliding at 4 m/s when it meets a rough surface?

200kg*m/s

Calculate the torque produced by the same 50-N force when a pipe extends the length of the wrench to 0.5 m.

25Nm

What impulse occurs when an average force of 10 N is exerted on a cart for 3 s?

30N*S

Calculate the work done when a force of 4.0 N moves a book 1.2 m.

4*1.2=4.8J

39. Show that when a 4.0-kg book is lifted 2.0 m its increase in gravitational potential energy is 80 J. (Don't forget g, which can be expressed in units N/kg, equivalent to m/s2.)

4*10*2 =80J

Show that a machine that has an input of 100 J and an output of 40 J is 40% efficient.

40/100=0.40 40%

Calculate the force of gravity between Earth and the Sun (Sun's mass is 2.0 * 1030 kg; average Earth-Sun distance is 1.5 * 1011 m).

5.33x10^32 N

Calculate the work done in lifting a 500-N barbell 2.0 m above the floor. (What is the gain of potential energy of the barbell when it is lifted to this height?)

500*2.0=1000J

Show that 1000 W of power is expended when a 500-N crate is lifted 4.0 m above the floor in 2.0 s.

500*4/2=1000W

Calculate the kinetic energy of a 100-kg scooter moving at 10 m/s.

500J 10^2*100/2

If an input of 100 J in a pulley system increases the potential energy of a load by 60 J, what is the efficiency of the system?

60%

Bowling Barry asks how much impulse is needed to stop a 10-kg bowling ball moving at 6 m/s. Show that the answer is 60 N *s

60N*S

A car carrying a 75-kg crash-test dummy drives into a wall at 25 m/s and is brought to rest in 0.1 s. Show that the average force exerted by the seat belt on the dummy is 18,750 N.

75kgx25/0.1

4Show that the angular momentum of the person in the previous problem is 480 kg #m2>s.

80*2*3

Show that 360 N of friction keeps an 80.0-kg person sitting 2.0 m from the center of a horizontally rotating platform with a tangential speed of 3.0 m/s.

80*3^2/2

Show that 40 W of power is required to impart 80 J of energy to something in 2.0 s.

80/20=40W

If the person's speed doubles and all else remains the same, what will be the person's angular momentum?

960kg*m^2/s

Jogging Jake runs along a train flatcar that moves at the velocities shown. In each case, Jake's velocity is given relative to the car. Call direction to the right positive. Rank the following from greatest to least.

A. b=d,a=c b. d,c,a=b

Your friend says that the kinetic energy of an object depends on the reference frame of the observer. Explain why you agree or disagree.

Agree, because speed is relative to the frame of refrencfe

What is the advantage of launching space vehicles from high-flying aircraft instead of from the ground?

Air drag is much less when high above the ground

Why is it important that a satellite be above Earth's atmosphere?

Air resistance will slow it down

What is the role of airbags in vehicles in terms of impulse and momentum?

Airbags increase the time the force is applied and decreases the force, so the result is a lesser force for a longer duration

Which requires more work—lifting a 50-kg sack vertically 2 m or lifting a 25-kg sack vertically 4 m?

Both are the same amount of work

To increase the momentum of an object, should you exert a large force, or extend that force for as long a time as possible—or both? Explain.

Both; To increase momentum, increase impulse by using a large force over a long time.

Where is the center of mass of Earth's atmosphere?

Center of Earth

Does the KE of a car undergo a greater change when it goes from 10 to 20 km/h or from 20 to 30 km/h?

Changes more when it goes from 20-30km/h

What is the shape of the orbit when the velocity of the satellite is everywhere perpendicular to the force of gravity?

Circle

Where is its center of mass for a baseball bat—closer to the grip or the thick end?

Closer to the thicker end

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

Difference in strength

Which is easier to get swinging, a baseball bat held at the narrow end or one held closer to the massive end (choked up)?

Easier to swing bat holding closer to the big end

You can't throw a raw egg against a wall without breaking it. But Peter Hopkinson can throw an egg at the same speed into a sagging sheet without breakage. Explain, using concepts from this chapter.

For the egg to break, it requires a high impulse, or change in momentum when another force acts upon it over time

Why do gymnasts use floor mats that are very thick?

Gymnast use floor mats to increase the time taken by them to come to a full stop after impact with the floor

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?

Her rate of spin will double

What are two ways to increase impulse?

Increasing force or increasing time

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

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

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

It is has tengential velocity

Why is kinetic energy a constant for a satellite in a circular orbit but not for a satellite in an elliptical orbit?

KE is constant in circular orbit

How do linear momentum and angular momentum differ?

Linear momentum does not depend on radius but angular momentum does

For orbits of greater altitude, is the period longer or shorter?

Longer

How does momentum relate to inertia?

Momentum is defined as the product of mass and velocity. Momentum is partially determined by its inertia

When you roll a ball off a tabletop will the time it takes to hit the floor depend on the speed of the ball? (Will a fast ball take a longer time to hit the floor?) Defend your answer.

No, it will depend on height

Which planets have a more-than-one-Earth-year period, planets nearer than Earth to the Sun, or planets farther from the Sun than Earth?

Planets farther from sun than earth

Give an example in which a force is exerted on an object without doing work on the object.

Pushing on a brick wall

As distance increases between most of the mass of an object and its center of rotation, does rotational inertia increase or decrease?

Rotational inertia increases with increased distance

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

Rotational intertia is the resistance to change in rotational motion, which is similar to plain inertia

Why doesn't the force of gravity change the speed of a satellite in circular orbit?

Speed doesn't change when theres no component of gravitational force

Dan and Sue cycle at the same speed. The tires on Dan's bike are larger in diameter than those on Sue's bike. Which wheels, if either, have the greater rotational speed?

Sues tires have a greater rotational speed

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

Tangential speed increases with distance

What are the units of measurement for tangential speed and rotational speed?

Tangential speed= m/s Rotational Speed= RPM

A block is raised a certain distance by pushing it up an incline. How much potential energy does the block have compared to being raised vertically to the same height?

The block is equal to the work done against gravity lifting it

Why is it easier to carry the same amount of water in two buckets, one in each hand, than in a single bucket?

The center of gravity remains in the center as equal masses are on different sides

When you whirl a can overhead by a string in a circular path, what is the direction of the force exerted on the can?

The force is towards the center of the circle

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

The lever arm is the shortest distance between applied force and rotational axis

Why do 8-ounce boxing gloves hit harder than 16-ounce gloves?

The lighter gloves have less padding, and less ability to extend the time of impact

Why does a long cannon impart more speed to a cannonball than a small cannon for the same force?

The long cannon has a greater impulse because of the period of time the force is applied

Since the Moon is gravitationally attracted to Earth, why doesn't it simply crash into Earth?

The moons tangential velocity keeps the moon casting around earth

1. Which has a greater momentum, a heavy truck at rest or a mouse running along the street?

The mouse

On a playground slide, a child has potential energy that decreases by 1000 J while her kinetic energy increases by 800 J. What other form of energy is involved, and how much?

The other 200J of energy turns to thermal energy caused by friction.

A softball and a basketball start from rest and roll down an incline. Which ball reaches the bottom first? Defend your answer.

The softball

Explain how a swarm of bees can have a net momentum of zero.

The swarm will have a net zero if they stay in the same location

Explain why the first hill of a roller coaster is always the highest.

The top hill designates the energy the roller coaster has in total, which spreads through shorter hills.

When a ball is thrown upward at an angle, what happens to the vertical component of its velocity as it rises? As it falls?

The vertical component of velocity decreases as the stone rise and increases as it falls

How do clockwise and counterclockwise torques compare when a system is balanced?

They are equal in magnitude and opposite direction

T or F: One watt is the unit of power equivalent to 1 Joule per second

True

If two sacks, one twice as heavy as the other, are lifted the same vertical distances in the same time, how does the power required for each compare?

Twice the power for the heavier sack

If the equally massive railroad cars A and B stick together after an inelastic collision, how does their speed after the collision compare with the initial speed of car A?

Two cars stuck together after a collision will have a lower combined speed after comparing to the speed of the moving care before

Why does the vertical component of velocity for a projectile change with time, whereas the horizontal component of velocity doesn't?

Vertical motion is influenced by gravitational force. There is no gravitational force acting horizontally

Two people who weigh the same climb a flight of stairs. The first person climbs the stairs in 30 s, and the second person climbs them in 40 s. Which person does more work? Which person uses more power?

Work done by each is the same. The on who climbs in 30s uses more power.

Inertia depends on mass. Does rotational inertia depend on mass also?

Yes

Compared to slowly rolling a ball off a tabletop, will a faster-moving ball hit the floor with a higher speed? Defend your answer.

Yes, both balls have the same amount of speed

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

Your weight is greater than mg when accelerating upward, your weight is zero when in free fall

(a) How much work is done when you push a crate horizontally with 100 N across a 10-m factory floor? (b) If the force of friction on the crate is a steady 70 N, show that the KE gained by the crate is 300 J. (c) What becomes of the "missing energy?"

a) 100*10=1000J b) (100-70) (10) =300J c) The missing energy becomes heat

The mass and speed of the three vehicles, A, B, and C, are shown. Rank from greatest to least their (a) momentum. (b) kinetic energy c) work needed to bring them up to respective speeds from rest.

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

The roller coaster ride starts from rest at point A. Rank from greatest to least at each point: (a) Speed (b) KE (c) PE

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

The balls have different masses and speeds. Rank the following from greatest to least. A. momentum B. impulse need to stop the ball

a. a,c,b,d b. b,d,c,a

Jake pushes crates starting from rest across his classroom floor for 3 s with a net force as shown. For each crate, rank the following from greatest to least.

a. abc b. abc c.cba d. abc

A hungry fish is about to have lunch at the speeds shown. Assume the hungry fish has a mass 5 times that of the small fish. Immediately after lunch, rank the speed of the formerly hungry fish for each case from greatest to least.

cab

Distinguish between force and impulse.

force: any push or pull impulse: force X time Impulse is the change of the momentum depending on force and time. Force is any influence that tends to accelerate an object

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

it 4 times as big

When the speed of a moving car is doubled, how much more kinetic energy does it have?

it has four times the kinetic energy

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

m=1kg

If there is an attractive force between all objects, why do we not feel ourselves gravitating toward massive buildings in our vicinity?

the gravitational force between us and earth is more than us and the building

At any location on Earth we say the direction of gravitational force is "straight down." Define the direction "down" with respect to gravity.

towards center of mass