Physics Midterm

Consider a small satellite moving in a circular orbit (radius r) about a spherical planet (mass M). Which expression gives this satellite's orbital velocity?

(GM/r)1/2

A 50-N object was lifted 2.0 m vertically and is being held there. How much work is being done in holding the box in this position?

0 J

A 3.0-kg object moves to the right at 4.0 m/s. It collides in a perfectly inelastic collision with a 6.0 kg object moving to the left at 2.0 m/s. What is the total kinetic energy after the collision?

0J

Two planets have the same surface gravity, but planet B has twice the mass of planet A. If planet A has radius r, what is the radius of planet B?

1.41r

A ball is thrown with a velocity of 20 m/s at an angle of 60° above the horizontal. What is the horizontal component of its instantaneous velocity at the exact top of its trajectory?

10m/s

A net force F accelerates a mass m with an acceleration a. If the same net force is applied to mass m/2, then the acceleration will be

2a

An Olympic athlete throws a javelin at four different angles above the horizontal, each with the same speed: 30°, 40°, 60°, and 80°. Which two throws cause the javelin to land the same distance away?

30° and 60°

Two objects, with masses m1 and m2, are originally a distance r apart. The gravitational force between them has magnitude F. The second object has its mass changed to 2m2, and the distance is changed to r/4. What is the magnitude of the new gravitational force?

32F

At what angle should a water-gun be aimed in order for the water to land with the greatest horizontal range?

45°

Two planets have the same surface gravity, but planet B has twice the radius of planet A. If planet A has mass m, what is the mass of planet B?

4m

Compared to yesterday, you did 3 times the work in one-third the time. To do so, your power output must have been

9 times yesterday's power output.

A small object collides with a large object and sticks. Which object experiences the larger magnitude of momentum change?

Both objects experience the same magnitude of momentum change.

Which of the following is an accurate statement?

If an object is acted on by a non-zero net external force, its momentum will not remain constant.

A horizontal force accelerates a box from rest across a horizontal surface (friction is present) at a constant rate. The experiment is repeated, and all conditions remain the same with the exception that the horizontal force is doubled. What happens to the box's acceleration?

It increases to more than double its original value.

When a football in a field goal attempt reaches its maximum height, how does its speed compare to its initial speed?

It is less than its intial speed

A spaceship is traveling to the Moon. At what point is it beyond the pull of Earth's gravity?

It is never beyond the pull of Earth's gravity.

An acorn falls from a tree. Compare its kinetic energy K, to its potential energy U.

K increases and U decreases.

A ball falls from the top of a building, through the air (air friction is present), to the ground below. How does the kinetic energy (K) just before striking the ground compare to the potential energy (U) at the top of the building?

K is less than U.

Who was the first person to realize that the planets move in elliptical paths around the Sun?

Kepler

In an inelastic collision, if the momentum is conserved, then which of the following statements is true about kinetic energy?

Kinetic energy is lost.

A Ping-Pong ball moving east at a speed of 4 m/s, collides with a stationary bowling ball. The Ping-Pong ball bounces back to the west, and the bowling ball moves very slowly to the east. Which object experiences the greater magnitude impulse during the collision?

Neither; both experienced the same magnitude impulse.

If you blow up a balloon, and then release it, the balloon will fly away. This is an illustration of

Newton's 3rd law

Two cars collide head-on. At every moment during the collision, the magnitude of the force the first car exerts on the second is exactly equal to the magnitude of the force the second car exerts on the first. This is an example of

Newton's 3rd law

Can work be done on a system if there is no motion?

No, because of the way work is defined.

What is the SI unit of momentum?

N∙s

A soldier throws a grenade horizontally from the top of a cliff. Which of the following curves best describes the path taken by the grenade?

Parabola

Let the average orbital radius of a planet be r. Let the orbital period be T. What quantity is constant for all planets orbiting the Sun?

T2/R3

An object of mass m sits on a flat table. The Earth pulls on this object with force mg, which we will call the action force. What is the reaction force?

The object pulling upward on the Earth with force mg.

Two equal mass balls (one red and the other blue) are dropped from the same height, and rebound off the floor. The red ball rebounds to a higher position. Which ball is subjected to the greater magnitude of impulse during its collision with the floor?

The red ball

A small car meshes with a large truck in a head-on collision. Which of the following statements concerning the magnitude of the average collision force is correct?

The small car and the truck experience the same average force.

Satellite A has twice the mass of satellite B, and rotates in the same orbit. Compare the two satellite's speeds.

The speed of B is equal to the speed of A.

A 3.0-kg object moves to the right at 4.0 m/s. It collides head-on with a 6.0-kg object moving to the left at 2.0 m/s. Which statement is correct?

The total momentum both before and after the collision is zero.

An object moves in a circular path at a constant speed. Compare the direction of the object's velocity and acceleration vectors.

The vectors are perpendicular.

You are standing in a moving bus, facing forward, and you suddenly fall forward as the bus comes to an immediate stop. What force caused you to fall forward?

There is not a force leading to your fall.

A lightweight object and a very heavy object are sliding with equal speeds along a level frictionless surface. They both slide up the same frictionless hill. Which rises to a greater height?

They both slide to the same height.

Is it possible for an object moving with a constant speed to accelerate? Explain.

Yes, although the speed is constant, the direction of the velocity can be changing.

Is it possible for a system to have negative potential energy?

Yes, since the choice of the zero of potential energy is arbitrary.

A rocket moves through empty space in a straight line with constant speed. It is far from the gravitational effect of any star or planet. Under these conditions, the force that must be applied to the rocket in order to sustain its motion is

Zero

A packing crate slides down an inclined ramp at constant velocity. Thus we can deduce that

a frictional force is acting on it.

A net force F acts on a mass m and produces an acceleration a. What acceleration results if a net force F/2 acts on mass 4m?

a/8

Action-reaction forces

always act on different objects.

If you exert a force F on an object, the force which the object exerts on you will

always be F

A pilot drops a bomb from a plane flying horizontally at a constant speed. Neglecting air resistance, when the bomb hits the ground the horizontal location of the plane will

be over the bomb

A constant net force acts on an object. Describe the motion of the object.

constant acceleration

If the net work done on an object is negative, then the object's kinetic energy

decreases

A ball drops some distance and gains 30 J of kinetic energy. Do not ignore air resistance. How much gravitational potential energy did the ball lose?

elastic

Action-reaction forces are

equal in magnitude but point in opposite directions.

Suppose a satellite were orbiting the Earth just above the surface. What is its centripetal acceleration?

equal to g

An object of mass m is hanging by a string from the ceiling of an elevator. The elevator is moving up at constant speed. What is the tension in the string?

exactly mg

Which of Newton's laws best explains why motorists should buckle-up?

first

An object is released from rest a height h above the ground. A second object with four times the mass of the first if released from the same height. The potential energy of the second object compared to the first is

four times as much

You are throwing a ball for the second time. If the ball leaves your hand with twice the velocity it had on your first throw, its horizontal range R (compared to your first serve) would be

four times as much

A roller coaster car is on a track that forms a circular loop in the vertical plane. If the car is to just maintain contact with track at the top of the loop, what is the minimum value for its centripetal acceleration at this point?

g downward

A hypothetical planet has a mass of half that of the Earth and a radius of twice that of the Earth. What is the acceleration due to gravity on the planet in terms of g, the acceleration due to gravity at the Earth?

g/8

The area under the curve on a Force versus time (F vs. t) graph represents

impulse

When is kinetic energy conserved?

in elastic collisions

A package of supplies is dropped from a plane, and one second later a second package is dropped. Neglecting air resistance, the distance between the falling packages will

increase

If the net work done on an object is positive, then the object's kinetic energy

increases

The speed of Halley's Comet, while traveling in its elliptical orbit around the Sun,

increases as it nears the sun

Glaciers push rocks in front of them. Before a stationary rock starts moving as a result of the force Fglacier exerted on it, the static friction between the rock and the ground

increases until it is equal to the product of µs and the normal force on the rock.

The total mechanical energy of a system

is constant, only if conservative forces act.

Two objects collide and stick together. Kinetic energy

is definitely not conserved.

When an object experiences uniform circular motion, the direction of the net force is

is directed toward the center of the circular path.

You are pushing a rock along level ground and making the rock speed up. How does the size of the friction force exerted by the ground on the rock compare with the size of the force the rock exerts on you? The force of friction

is smaller than the force the rock exerts on you.

Ignoring air resistance, the horizontal component of a projectile's acceleration

is zero.

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

it decreases

Consider a particle moving with constant speed such that its acceleration of constant magnitude is always perpendicular to its velocity.

it is moving in a circle

A child falls sideways off a sled while sledding on frictionless ice. What happens to the velocity of the sled?

it remains the same

The acceleration of an object is inversely proportional to

its mass

What is the correct unit of work expressed in SI units?

kg m2/s2

What is the correct unit of power expressed in SI units?

kg m2/s3

A ball drops some distance and loses 30 J of gravitational potential energy. Do not ignore air resistance. How much kinetic energy did the ball gain?

less than 30 J

An object of mass m is hanging by a string from the ceiling of an elevator. The elevator is moving upward, but slowing down. What is the tension in the string?

less than mg

When a cannon fires a cannonball, the cannon will recoil backward because the

momentum of the cannonball and cannon is conserved.

A ball drops some distance and gains 30 J of kinetic energy. Do not ignore air resistance. How much gravitational potential energy did the ball lose?

more than 30 J

A pilot executes a vertical dive then follows a semi-circular arc until it is going straight up. Just as the plane is at its lowest point, the force on him is

more than mg, and pointing up.

When the rocket engines on the starship NO-PAIN-NO-GAIN are suddenly turned off, while traveling in empty space, the starship will

move with constant speed

In the absence of an external force, a moving object will

move with constant velocity

A very light object moving with speed v collides head-on with a very heavy object at rest, in a frictionless environment. The collision is almost perfectly elastic. The speed of the heavy object after the collision is

much less than v

A very heavy object moving with velocity v collides head-on with a very light object moving with velocity -v. The collision is elastic, and there is no friction. The heavy object barely slows down. What is the speed of the light object after the collision?

nearly 3v

Two objects attract each other gravitationally. If the distance between their centers is halved, the gravitational force

quadruples

Ignoring air resistance, the horizontal component of a projectile's velocity

remains constant

If you push twice as hard against a stationary brick wall, the amount of work you do

remains constant at zero

A golf ball traveling 3.0 m/s to the right collides in a head-on collision with a stationary bowling ball in a friction-free environment. If the collision is almost perfectly elastic, the speed of the golf ball immediately after the collision is

slightly less than 3.0 m/s.

The force that keeps you from sliding on an icy sidewalk is

static friction

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

the ball

A brick and a feather fall to the earth at their respective terminal velocities. Which object experiences the greater force of air friction?

the brick

A 20-ton truck collides with a 1500-lb car and causes a lot of damage to the car. Since a lot of damage is done on the car

the force on the truck is equal to the force on the car.

The quantity 1/2 mv2 is

the kinetic energy of the object

The quantity Fd/t is

the power supplied to the object by the force

The gravitational force between two objects is proportional to

the product of the masses of two objects.

Compared to its mass on the Earth, the mass of an object on the Moon is

the same

The gravitational force between two objects is inversely proportional to

the square of the distance between the two objects.

A stone is thrown horizontally from the top of a tower at the same instant a ball is dropped vertically. Which object is traveling faster when it hits the level ground below?

the stone

Consider two masses m1 and m2 at the top of two frictionless inclined planes. Both masses start from rest at the same height. However, the plane on which m1 sits is at an angle of 30° with the horizontal, while the plane on which m2 sits is at 60°. If the masses are released, which is going faster at the bottom of its plane?

there both going the same speed

A bullet is fired horizontally, and at the same instant a second bullet is dropped from the same height. Ignore air resistance. Compare the times of fall of the two bullets.

they hit at the same time

The path of a projectile through space is called its:

trajectory

You are standing in a moving bus, facing forward, and you suddenly fall forward. You can imply from this that the bus's

velocity decreased.

Of the following, which is not a unit of power?

watt/second

If you walk 5.0 m horizontally forward at a constant velocity carrying a 10-N object, the amount of work you do is

zero

When you sit on a chair, the resultant force on you is

zero

It's more difficult to start moving a heavy carton from rest than it is to keep pushing it with constant velocity, because

µk < µs