Physics 104 Final Multiple Choice

You have an irregularly shaped flat object. To find its center of mass you can do which of the following? Assume that the object has constant density.

-Find a point where you can put a fulcrum to balance it. -Push the object in different directions and find the point of intersection of the lines of action of the forces that do not rotate the object. -Separate the object into several regularly shaped objects whose center of masses you know, and use the mathematical definition of the center of mass to find it.

For an INELASTIC collision, which of the following statements are true for a system that includes two colliding objects?

-Kinetic energy is lost. -Momentum is constant.

For an elastic collision, which of the following statements are true for a system that includes two colliding objects?

-Momentum is constant -Kinetic energy is constant.

Choose an example in which the momentum of a system is not constant.

A freely falling metal ball, with the ball as the system.

What is energy?

A physical quantity

Why do you tilt your body forward when hiking with a heavy backpack?

Bending forward moves the center of mass of you and the backpack above your feet.

When a bow string is stretched all the way back and held in this position, it has the following type of energy.

Elastic potential

Which device could best help you to successfully walk on a tight rope?

Long, heavy balancing pole that curves down at the ends and additional weights attached to long strings to the end of the pole.

In an inelastic collision of two objects, which of the following quantities is constant for the system of these two objects?

Momentum

Using a crowbar, a person can remove a nail by exerting little force, whereas pulling directly on the nail requires a large force to remove it (you probably can't). Why?

The applied force is at a large distance from the pivot, giving rise to a large torque

What happens to the position of the center of mass of a small boat - boatman system when the boatman moves to the right of the small boat?

The center of mass of the boat-boatman system moves towards the right.

What represents the impulse of the force in a graph of force versus time?

The impulse is equal to the area under the curve.

Two identical balls are thrown vertically upward. The second ball is thrown with an initial speed that is twice that of the first ball. How does the maximum height of the two balls compare?

The maximum height of the second ball is four times that of the first ball.

Which is a safer car bumper in a collision: one that is flexible and retracts or one that is rigid? Why?

The retractable bumper, because it extends the time interval of the collision, thus reducing the force exerted on the car.

A wagon full of medicine balls is rolling along a street. Suddenly one medicine ball (3 kgkg) falls off the wagon. What happens to the speed of the wagon?

The speed of the wagon does not change, because no external force is exerted on the wagon.

The gravitational force that Earth exerts on an object causes an impulse of +10N⋅s+10N⋅s in one experiment and +1N⋅s+1N⋅s on the same object in another experiment. In both experiments, the body retains its shape. How can this be?

The time intervals during which the force was exerted are different.

If the sum of the components of the forces is not zero, the object will have ---- acceleration.

Translational

You are going to balance a sharp wooden pencil on your fingertip, point down. How will you use pocketknife do this?

You stick the blade of the pocketknife into the pencil close to the tip where the pivot is to lower the center of mass of the system.

The center of mass of a thin rectangular shaped metal block of uniform density is located

at the intersection of the two diagonals.

A person is standing in one end of a paddle board in still water. He then walks forward towards the dock and keeps standing. The paddle board is now...

behind its original position and not moving

Two battery-powered fan carts are resting on a frictionless horizontal track. The mass of cart AA is twice the mass of cart BB. The carts are released so they start moving to the right.Which answer and explanation correctly compare the linear momentum of the carts after traveling the same distance? Assume that the force exerted by the air on the carts is the same for both carts and remains constant while the carts are moving.

pA>pBpA>pB because the air exerts a force on cart AA for a longer time interval than on cart BB.

If the sum of the torques about an arbitrary axis is not zero, the object will have --- acceleration.

rotational

What are the conditions of equilibrium?

∑FonOx=0, ∑FonOy=0, ∑τ=0

A wooden box is pulled horizontally on a rough carpet, and as a result it is moving with constant velocity through a distance d. Considering the box as the system, which of the following expressions shows the work done by external forces on the box?

( fpull - fk carpet on box)x d

Select the examples of situations in which an object is starting to rotate even though the sum of the forces exerted on the object is zero.

-A CD that has just been inserted into a CD player. -Two forces being exerted on the opposite ends of a meter stick; the forces have the same magnitude but are in opposite direction, the lines of action of the forces do not coincide. -A boy turning the wheel of a parked bicycle.

Why does an inflated balloon shoot across a room when air is released from it?

-Because the air inside the balloon pushes on the balloon, exerting the same force that the balloon exerts on the air. -Because the momentum of the balloon-air system is constant.

How would you convince somebody that the momentum of an isolated system is constant?

-Derive it from Newton's second and third laws. -Use it to make predictions about a experiment (for example, collision of two bodies), and then compare the outcome to the prediction.

According to a report on traumatic brain injury, woodpeckers smack their heads against trees at a force equivalent to 1200 gg's without suffering brain damage. Identify the mistakes in this statement.

-The quantity "1200 gg" is a measure of acceleration, not force. -Woodpeckers smack their beaks (not head) against the trees.

Which of the following statements are true? Select all correct responses.

-The vector sum of forces exerted on a particle equals the rate of change of momentum of the particle with respect to time. -The total momentum of any number of particles is equal to the vector sum of the momenta of the individual particles. -The total momentum of an isolated system is constant.

When a flea (mm = 450 μgμg) is jumping up, it extends its legs 0.5 mmmm and reaches a speed of 1 m/sm/s in that time. How high can this flea jump? Ignore air drag and use gg = 10 m/s2m/s2.

0.050 m

A bullet fired at a door makes a hole in the door but does not open it. Your finger does not make a hole in the door but does open it. Why?

A finger exerts a smaller force but the time interval is much longer.

Which answer best represents the system's change in energy for the following process? The system includes Earth, two carts, and a compressed spring between the carts. The spring is released, and in the final state, one cart is moving up a frictionless ramp until it stops. The other cart is moving in the opposite direction on a horizontal frictionless track.

Elastic potential energy to kinetic energy and gravitational potential energy

Person B does twice the work of person A, and in one-half of the time . How does the power output of person B compare to person A?

Person B has four times the power output of person A.

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 has the greater momentum change?

The ball has the greater momentum change.

Two clay balls are moving toward each other. The balls have equal kinetic energies (K1=K2K1=K2), but ball 1, which is coming from the left, is moving faster than ball 2 (v1>v2v1>v2).

The balls will move to the left because p2>p1p2>p1 before the collision.

A person exerts a horizontal 50 N force on a crate, causing it to move horizontally at a constant speed through a distance of 10 m. What is the work done by the person on the crate?

The work done on the crate is 500 J.

A falling leaf usually flutters while falling. However, we have learned that the force that Earth exerts on an object is exerted at its center of mass and thus should not cause rotational motion.

There are other forces exerted on the leaf as it falls besides the force exerted by Earth.

When can you apply the idea that momentum is constant to solve a problem?

When the system is isolated.

An extended object is in static equilibrium if __________.

both the net force acting on the object is zero and the net torque acting on the object is zero

What happens to the momentum of the truck at the instant the bar lands on it in that case?

The bar lands on the truck. The horizontal momentum of the whole system does not change, but the system consists of the truck and the bar now, so the momentum of the truck decreases.

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 sitting on will tilt downward.

Jim says that momentum is not a conserved quantity because objects can gain and lose momentum. Explain why you don't agree with him.

-While he is right that objects can gain or lose momentum, with appropriate choice of what the system comprises of, one can show that the total momentum is conserved in a(n) CLOSED system. -For example, consider the collision between two cars. The increase in momentum of one car is equal to the DECREASE of momentum of the other. -If our system consists of both cars, then the TOTAL MOMENTUM OF THIS CLOSED SYSTEM remains unchanged before or after the collision. -On the other hand, if the system consists of only one car, then the other car would be the EXTERNAL agent exerting a force on it.

A 1400-kgkg car is traveling on a level road at a constant speed of 72 mi/hmi/h. The drag force exerted by the air on the car and the rolling friction force exerted by the road on the car tires add to give a net force of 920 NN pointing opposite to the direction of the car's motion.

29 kW

A Hooke's law bowstring is stretched x meters until a force of f newtons is applied, and then held. By what factor will the elastic potential energy of the bowstring be affected if the final force at which the bowstring is held is tripled?

9 times

Choose which statement describes a process in which an external force does negative work on the system. The person is not part of the system.

A person slowly lowers a box from a tabletop to the floor.

Three processes are described below. Choose one process in which there is work done on the system. The spring, Earth, and the cart are part of the system.

A relaxed spring rests upright on a tabletop. You slowly compress the spring. You then release the spring and it flies up several meters to its highest point.

What could it mean if object 1 does +10 JJ of work on object 2?

All of the above

Object A of unknown mass and speed is moving east and has a collision with object B of different unknown mass and speed moving south. If they were to stick together after the collision, in which direction will they be moving?

Between east and south

You push a small cart by exerting a constant force FF along a table. After you push the cart a distance ss, the cart falls off the table and lands on the floor a distance D1D1 away from the table edge. You repeat the same experiment (same FF, same ss) using a heavier cart and obtain a new distance D2D2.Choose the answer that correctly compares the distances D1D1 and D2D2 and gives the best explanation of the outcome.

D1>D2 because the carts had the same kinetic energy before they flew off the table, and consequently the speed of the lighter cart had been greater.

What will happen to Earth if our Sun becomes a black hole (with the same mass)?

Earth will be freezing in the absence of a heat source.

What could it mean if the torque of a 10-N force is zero?

Either of the two choices above.

A meterstick of uniform density is hanging from a string at 0.40 m. An object that experiences a gravitational force 0.20 N is hung at 0.20 m. Which one of the equations below demonstrates that the meterstick is in static equilibrium?

F0.20 (0.2m) = mmeterstick x g x 0.1m

A heavy bar falls straight down onto the bed of a horizontally rolling truck. What happens to the momentum of the truck at the instant the bar lands on it?

How many correct answers do you think are possible? Make sure you think of what "falls straight down" means.When the bar falls straight down, it has zero velocity in the horizontal direction either with respect to the truck or with respect to Earth. Thus, the problem can have two answers.

Energy is a conserved quantity, meaning that...

If no work is done on the system, the energy of the system will not change.

A hammock is tied with ropes between two trees. A person lies in it. Under what circumstances are its ropes more likely to break?

If stretched tightly between the trees.

Where is the center of mass of a donut?

In the center of the hole.

A carpenter's trick to keep nails from bending when they are pounded into a hard material is to grip the center of the nail with pliers. Does this help?

It helps, the middle of the nail is where its center of mass is. Gripping it there with pliers improves its stability.

In which situation does the momentum of a tennis ball change more?

It hits the racket and flies off in the opposite direction.

A toy car with very low friction wheels and axles rests on a level track. In which situation will its speed increase more?

It is hit by a rubber ball with the same mass and velocity of the clay that rebounds in the opposite direction after hitting the car

Sit on a chair with your feet straight down at the front of the chair. Keeping your back perpendicular to the floor, try to stand up without leaning forward. Is it possible to do? Why?

It is impossible. Your center of mass is behind the contact point between your feet and the floor.

Is it more difficult to do a sit-up with your hands stretched in front of you or with them behind your head?

It is more difficult to do a sit-up with your hands behind your head than with your arms stretched out in front of you.

Why do you believe that momentum is a conserved quantity?

Momentum constancy in an isolated system is a natural consequence of Newton's laws. It has been demonstrated in everyday phenomena such as collisions, explosions, and rocket propulsions.

What does it mean if the torque of a force is positive?

None of these choices is necessarily correct.

How can satellites stay in orbit without any jet propulsion system? Choose the correct explanation.

Since the gravitational force is perpendicular to the direction of velocity at every instant, it does not work on the satellite, thus the kinetic energy of the satellite remains constant, as thus the speed does not change.

When would a ball hitting a wall have a greater change in momentum: when it hits the wall and bounces back at the same speed or when it hits and sticks to the wall?

The ball that rebound has a greater change in momentum.

A small ball of mass mm rolls at a constant speed vv along a horizontal surface, then rolls down an incline and continues rolling along a lower horizontal surface with a constant speed 2v2v (see (Figure 1) ). Analyze the motion of the ball (the system) using a momentum approach. Assume that the ball is in contact with the surface at all times and that it does not slip. Choose the correct description of the segments of the ball's motion where the ball's momentum is changing (either in magnitude or in direction)

The ball's momentum changes direction when the ball enters the incline and leaves the incline. The magnitude of momentum increases as the ball rolls down the incline. The final momentum is twice the initial momentum.

Which one of these objects has the greater magnitude of linear momentum? A baseball of mass 150 grams moving at 40 m/s or a soccer ball of mass 500 grams moving at 12 m/s

The baseball and the soccer ball have the same momentum.

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

The force exerted at the doorknob exerts the greater torque.

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.

In which of the following is positive work done by a person on a suitcase?

The person lifts a heavy suitcase.

Two objects are moving at equal speed along a level, frictionless surface. The second object has twice the mass of the first object. They both slide up the same frictionless incline plane. Which object rises to a greater height? Choose the answer with the best explanation.

The two objects rise to the same height.

In which case does the wall exert a greater force on the ball: when it hits the wall and bounces back at the same speed or when it hits and sticks to the wall? Assume the time intervals over which the force is exerted are the same in both cases.

The wall exert a greater force on the ball that hits the wall and bounces back.

A meterstick of uniform density hanging from a string at 0.7 m, had two masses of 0.50 kg and 1.00 kg hung at 0.1 m and 1.0 m respectively to obtain static equilibrium. A calculation for placement of the masses to obtain static equilibrium was done prior to the setup and it came out to: (0.5 kg x 9.8 N/kg x 0.6 m) - (1.00 kg x 9.8 N/kg x 0.3 m) = τnet = 0 N*m However, the setup failed and static equilibrium was not achieved. What do you think went wrong with the setup and/or equation?

The weight of the meterstick is missing in the equation and it would have provided a CCW torque.

An object (the system) weighing 20 N moves horizontally toward the right a distance of 5.0 m. What is the work done on the object by the gravitational force?

The work done on the object by the gravitational force is zero joules.

Choose the case of an object that is not in equilibrium when the net force exerted on it is zero.

Two forces perpendicular to the rod exerted on its different ends with the same magnitude in opposite directions.

Identify which one of these systems can be considered as constituting an isolated system.

Two pucks colliding on a frictionless ice surface. The two pucks are the system and the ice surface is external to the system.

In the equation Ug=mgyUg=mgy, the gravitational potential energy is directly proportional to the distance of the object from a planet. In the equation Ug=−GmpmrUg=−Gmpmr, it is inversely proportional. How can you reconcile those two equations?

Ug=−Gmpmr=−GmpmRE+h≈−GmpmRE(1−hRE)=Ug0+mghUg=−Gmpmr=−GmpmRE+h≈−GmpmRE(1−hRE)=Ug0+mgh, where Ug0Ug0 is our reference point.

You pull on a spring, which obeys Hooke's law, in two steps. In step 1,you extend it by distance xsxs. In step 2, you further extend it by the same distance xsxs. Choose the answer that correctly compares the elastic potential energy of the spring after the first step (Us1Us1) with that after the second step (Us2Us2). Assume that the spring is initially undeformed.

Us2>2Us1 because the average force exerted on the spring in step 2 is larger than the average force exerted on the spring in step 1

A ball (the system) is thrown straight upward. How does the sign of the work done by the gravitational force while the ball is traveling upward compare with the sign of the work done by the gravitational force while the ball is traveling downward?

Work done by the gravitational force is negative while the ball is traveling upward and positive while the ball is traveling downward.

An object modeled as a rigid body is said to be in static equilibrium if

the sum of the horizontal components as well as the vertical components of the forces acting on the object is zero, and the sum of the clockwise and counterclockwise torques acting about any axis of rotation is also zero.

For the net translational force on an object to be zero,

the sum of the horizontal components of all the forces acting on the object must be zero and the sum of the vertical components of all the forces acting on the object must be zero.