Physics - Final Exam

Can an object exert a force on itself?

No object can exert a force upon itself. If it could, it would accelerate itself without interacting with its surroundings.

Can a force of static friction do work?

No, If we apply some force on a body, due to the static friction the body does not move. So there is no work to be done. By increasing the applied force, the static friction is converted to kinetic friction.

Does a larger net force exerted on an object always produce a larger change in the momentum of an object compared with a smaller net force?

No, Momentum=Force x Time It depends on how long the force is being acted on the object.

It it possible to add a vector quantity to a scalar quantity?

No, a vector has both a magnitude and a direction while a scalar has no direction.

Can kinematic equations be used in a situation in which the acceleration varies in time? Can they be used when the acceleration is zero?

No, the kinematic equations can only be used when the acceleration is constant. Yes they can be used when the acceleration is zero because zero is a constant.

Can the magnitude of a vector have a negative value?

No, the magnitude of the vector can never have a negative value.

A child is pushing on a sled on a frozen lake. At some point in time a sled's instantaneous velocity is eastward and it is accelerating due north. Assuming that push from the child is only horizontal force acting on the sled, in which direction is the force that the child is exerting on the sled?

North

If you drive your car at the "design speed" around a banked curve, what forces provide the centripetal force on your car?

Only the normal force between your car and the road.

Three masses are attached to each other with ropes. Left to right (50kg) rope a (30kg) rope b (10kg) rope c. Which of the ropes has the least amount of tension if rope c is the front where it's being pulled from?

Rope A

The most famous horse race in the galazy is held at churchill downs in louisville, kentucky. The oval of the track measures 1 mile in circumference. Since the race is 1 1/4 miles long, the horses begin the race 1/4 mile from the finish line, then travel an additional one lap around the track. The race takes 2 minutes. Which of the following statement below is true?

The average speed of the horse is larger than the magnitude of the average velocity.

A rubber ball is dropped on the floor. What force causes the ball to bounce?

The ball is exerting a force on the ground. The ground will then exert an equal yet opposite force on the ball so the ball bounces back. This is Newtons Third Law.

A passenger sitting in the rear of a bus claims that she was injured when the driver slammed on the brakes, causing a suitcase to come flying toward her from the front of the bus. If you were the judge in this case, what disposition would you make?

The bus is moving forward. If the brakes are slammed on, the force acting on the passengers and the objects inside the bus will be in the forward direction. Due to this force, the objects in the bus will move forward, not backward. The passenger's claim is INCORRECT, so the case can be denied.

When the net force on an object is NOT zero, which of the statements below must be true?

The direction of the net force is the same as the objects acceleration.

If the average velocity at some time interval is zero, what can you say about the displacement of the object at that interval?

The displacement would be zero because Avg Velo=Displacement/Time

Mathematically we can tell that an object is slowing down when?

its velocity and acceleration are in opposite directions

Two cars are moving in the same direction in parallel lanes along a highway. At some instant, the velocity of car A exceeds the velocity of car B. Does that mean that the acceleration of car A is greater than that of car B? Explain

If both cars are moving parallel to each other, they they do not neccesarily have the same velocity If car A has a velocity of 50km/h, car B has a velocity of 30km/h, then car A's velocity is higher than car B's. And if car B is accelerating while car A passes, it will still be slower. Under the same paramaters, if car B decelerates, then its acceleration will be higher than car A's (in the opposite direction), and car B will have a slower velocity. Under normal circumstances, if car A has greater positive acceleration, it will pass car B at some point in time.

If a car is traveling eastward, can its acceleration be westward?

If the car is traveling eastward and the acceleration is westward, the car slows down. If the car is traveling westward and the acceleration is westward, the car will speed up.

A truck loaded with sand accelerates along a highway. The driving force on the truck remains constant. What happens to the acceleration of the truck as its trailer leaks sand at a constant rate through a hole in its bottom?

It increases at a steady rate

Can kinetic energy be negative?

Kinetic energy is ALWAYS positive. K=(1/2)mv^2 Therefore, mass cant be negative and the velocity can't be negative. A moving object always has positive work.

The centripetal force acting on an object:

1. Does not change the speed of the object. 2. Always points towards the center of the objects circular motion. 3. Is perpendicular to the instantaneous velocity vector of the object. *All the above statements are true*

Cite two examples in which a force is exerted on an object without doing any work on the object.

1. Force of tension on a ball rotating on the end of a string. The force acts on it directed toward the center of rotation. However, there is no work on the object 2. Normal force and gravitational force on an object at rest or moving across a level floor.

A box is being pushed across a frictionless horizontal surface by two forces. The force, A, is directed 10 degrees south of east. Force, B, is directed 30 degrees west of due north. The acceleration of the box is due north. This means that?

1. The north south components of the two forces are equal in magnitude and in the same direction. 2. The north south components of the two forces are opposite in direction and equal in magnitude. 3. The east west components of the two forces are equal in magnitude and in the same direction. 4. The east west components of the two forces are equal in magnitude and in the opposite directions. *None of the above statements are true*

A 30-N box rests on a horizontal table and is connected by a rope to a 10-N weight hanging over the side of the table as shown. If the box and hanging weight are moving with constant velocity, what is the tension in the rope? Assume that the pulley is frictionless.

10 N

An athlete grips a light rope that passes over a low friction pulley attached to the ceiling of a gym. A sack of sand precisely equal in weight to the athlete is tied to the other end of the rope. Both the sand and the athlete are initially at rest. The athlete climbs the rope, sometimes speeding up and slowing down as he does so. What happens to the sack of sand?

Action and reaction forces do not act on the same body. They act on two different bodies. The net force acting on a body is the vector sum of all individual forces acting on it. So the net vector force on any object does not necessarily add to zero.

Explain why changing the axis of rotation of an object changes its moment of inertia?

Because of the change in its distance from the axis of rotation. The moment of inertia is larger for the large distances, and smaller for the small distances, because the mass of the object remains the same.

If a car is traveling due westward with a constant speed of 20 m/s, what is the resultant force acting on it?

F=Mass x Acceleration Therefore, the acceleration of the car is traveling with a constant speed of 20 m/s, which means the acceleration is ZERO. Since the acceleration is zero, the force is zero.

Can a normal force do work?

The normal force never moves the object, each object experiences a force normal to the plane which it lays. The work done by a normal force is always zero.

Why does a long pole help a tightrope walker stay balanced?

The performer can shift the pole left or right to keep the center of mass. This allows him to move the pole instead of moving his body. The pole sags down at the ends to lower the systems center of gravity.

An open box slides across a frictionless, icy surface of a frozen lake. What happens to the speed of the box as water from a rain shower falls vertically downward into the box?

The velocity of the box will decrease because the water falling into the box will increase the weight of the box which will effect the velocity of the box.

A pail of water can be whirled in a vertical path such that no water is spilled. Why does the water stay in the pail, even when the pail is above your head?

The walls of the cylinder exert an inward force to keep everything inside moving in a circular path.

Suppose you drop a stone from the top of a tall building. Your friend throws a ball downward at the same time. After leaving your and your friends hands and before hitting the ground, which object will have the larger acceleration?

Their accelerations will be the same

Does a larger net force always produce a larger change in kinetic energy than a smaller net force?

Yes the larger net force always produces a larger change in kinetic energy than the smaller force.

You are sitting in a chair with your feet off the floor and a book on your lap. The floor exerts a normal force:

Upward on the chair, you, and the book.

Describe two examples in which the force of friction exerted on an object is in the direction of motion of the object:

Walking is an example of where friction is in the direction of motion. The force into the ground which results in walking, by pushing the ground in the direction. So the friction acts in a forward direction. The same principle holds true for a car's wheels, in which the friction of the ground acts in the direction of motion.

If you know the position vectors of a particle at two points along its path and also know the same time interval during which it moved from one point to the other, can you determine the particle's instantaneous velocity?

We cannot calculate instantaneous velocity but we can calculate it's average velocity.

Discuss the work done by a pitcher throwing a baseball. What is the approximate distance through which the force acts as the ball is thrown?

Work is only done in accelerating the ball from rest. The work done over the effective length of the pitchers' arm; the distance his hand moves through wind up and until release.

If the velocity of a particle is nonzero, can the particle's acceleration be zero?

Yes, if the velocity of the particle is nonzero, then when it is moving with uniform velocity then its acceleration becomes zero.

If the velocity of a particle is zero, can the particles acceleration be zero?

Yes, though the velocity of the particle is zero, its acceleration can be non-zero. Suppose the ball is thrown upward with velocity v. The velocity of the ball will decrease with the increasing weight. At maximum height, the velocity of the ball is zero. However, the constant acceleration (gravity) is acting on the ball, causing it to return to the ground.

Can the instantaneous velocity of an object be greater in magnitude than the average velocity?

Yes.

Your hands are wet, and the restroom towel dispenser is empty. What do you do to get drops of water off your hands? How does the motion of the drops exemplify one of newtons laws? Which one?

You would shake your hands to dry them, this is newtons 1stlaw. This is the law of inertia. The drops are originally at rest and they would stay at rest if you didn't shake your hands.