Physics 121 Exam with Complete
Solutions (Graded A+)
A car travels forward along a level road at constant velocity through the air with
measurable air resistance. When a tire rolls the force the ground exerts on it is due to
static friction, because the surface of the tire is not moving relative to the ground. Under
these conditions, the force of friction between the road and the tires rolling on the road
is - Answer-less than the normal force multiplied by the coefficient of static friction but
greater than zero.
Since it is traveling at constant velocity, the net force must be zero. There is a resistive
air drag on the car, so the tires must be exerting a compensating forward force, so the
force is not zero. The force must also be less than the maximum static frictional force,
which is the coefficient of static friction times the normal force.
When a horse pulls a wagon, the force that causes the horse to move forward is the
force - Answer-the ground exerts on the horse.
The force the horse exerts on the wagon accelerates the wagon. The force the wagon
exerts on the horse pulls the horse backward. The force gravity exerts on the horse just
pulls it down and doesn't accelerate it forward. The force the horse exerts on the ground
acts on the ground, not on the horse. The force the ground exerts on the horse is the
one that provides a forward acceleration.
If only three forces are acting on an object which is moving in the x direction, but not
accelerating, which of the following is true? - Answer-The resultant of two of these
forces must equal the negative of the third.
If the object is not accelerating, then there is no net force acting on the object. This
means that the sum of the three forces must be zero, which is case (b). The fact that it
is moving in the x-direction does not affect the answer to the question.
A karate expert smashes his fist against a board, breaking it in two. Which of the
following is correct? - Answer-The force of the board on his fist is equal to the force of
his fist on the board.
forces must always be equal and opposite
, static friction tries to - Answer-keep things together
if an object is accelerating - Answer-the forces should not balance out
You are riding is the passenger seat of a car as it goes around a tight turn. You slide
across the seat to the passenger side door. Which statement below properly describes
what happens? - Answer-The car seat is exerting a centripetal force on you, but not
enough to keep you in place
Friction between yourself and the seat is the centripetal force that the car exerts on you.
If the friction is not large enough, you slide outward until you run into the door that then
exerts the rest of the centripetal force necessary to bring you around the corner with the
car.
An object is moving in a circle at a constant speed. From this you can be certain that -
Answer-There is at least one force acting on the object
There must be at least one force on the object acting as a centripetal force, toward the
center of the circle.
Under ordinary road conditions, the shortest distance in which an automobile can be
stopped by the application of a constant braking force is directly proportional to -
Answer-the square of the speed of the automobile
The work done by the braking force is the force times the distance. By the Work-Kinetic
Energy theorem that must equal the initial kinetic energy (because the final KE is zero),
which is proportional to the square of the speed of the automobile.
A farmhand carrying a 5 kg bucket of milk dashes 5 meters across a barnyard at a
constant speed of 4 m/s. How much work was done on the bucket in that process? -
Answer-0
net force is 0 because of the constant speed so W = f*d is 0 since f is 0
An object starting with a certain speed slides with friction on a horizontal surface until it
comes to rest. Doing which of the following would double the distance it travels before
coming to rest? - Answer-none
addition and subtraction sig figs - Answer-least number of decimal places
multiplication and division sig figs - Answer-least # of values
Object A is dropped from a height half a meter above object B. Object B is released
when object A reaches its height. Which hits the ground first - Answer-A because the
acceleration is greater and has a nonzero velocity
Solutions (Graded A+)
A car travels forward along a level road at constant velocity through the air with
measurable air resistance. When a tire rolls the force the ground exerts on it is due to
static friction, because the surface of the tire is not moving relative to the ground. Under
these conditions, the force of friction between the road and the tires rolling on the road
is - Answer-less than the normal force multiplied by the coefficient of static friction but
greater than zero.
Since it is traveling at constant velocity, the net force must be zero. There is a resistive
air drag on the car, so the tires must be exerting a compensating forward force, so the
force is not zero. The force must also be less than the maximum static frictional force,
which is the coefficient of static friction times the normal force.
When a horse pulls a wagon, the force that causes the horse to move forward is the
force - Answer-the ground exerts on the horse.
The force the horse exerts on the wagon accelerates the wagon. The force the wagon
exerts on the horse pulls the horse backward. The force gravity exerts on the horse just
pulls it down and doesn't accelerate it forward. The force the horse exerts on the ground
acts on the ground, not on the horse. The force the ground exerts on the horse is the
one that provides a forward acceleration.
If only three forces are acting on an object which is moving in the x direction, but not
accelerating, which of the following is true? - Answer-The resultant of two of these
forces must equal the negative of the third.
If the object is not accelerating, then there is no net force acting on the object. This
means that the sum of the three forces must be zero, which is case (b). The fact that it
is moving in the x-direction does not affect the answer to the question.
A karate expert smashes his fist against a board, breaking it in two. Which of the
following is correct? - Answer-The force of the board on his fist is equal to the force of
his fist on the board.
forces must always be equal and opposite
, static friction tries to - Answer-keep things together
if an object is accelerating - Answer-the forces should not balance out
You are riding is the passenger seat of a car as it goes around a tight turn. You slide
across the seat to the passenger side door. Which statement below properly describes
what happens? - Answer-The car seat is exerting a centripetal force on you, but not
enough to keep you in place
Friction between yourself and the seat is the centripetal force that the car exerts on you.
If the friction is not large enough, you slide outward until you run into the door that then
exerts the rest of the centripetal force necessary to bring you around the corner with the
car.
An object is moving in a circle at a constant speed. From this you can be certain that -
Answer-There is at least one force acting on the object
There must be at least one force on the object acting as a centripetal force, toward the
center of the circle.
Under ordinary road conditions, the shortest distance in which an automobile can be
stopped by the application of a constant braking force is directly proportional to -
Answer-the square of the speed of the automobile
The work done by the braking force is the force times the distance. By the Work-Kinetic
Energy theorem that must equal the initial kinetic energy (because the final KE is zero),
which is proportional to the square of the speed of the automobile.
A farmhand carrying a 5 kg bucket of milk dashes 5 meters across a barnyard at a
constant speed of 4 m/s. How much work was done on the bucket in that process? -
Answer-0
net force is 0 because of the constant speed so W = f*d is 0 since f is 0
An object starting with a certain speed slides with friction on a horizontal surface until it
comes to rest. Doing which of the following would double the distance it travels before
coming to rest? - Answer-none
addition and subtraction sig figs - Answer-least number of decimal places
multiplication and division sig figs - Answer-least # of values
Object A is dropped from a height half a meter above object B. Object B is released
when object A reaches its height. Which hits the ground first - Answer-A because the
acceleration is greater and has a nonzero velocity
