Question 1

A 2.0 kg-book is on the floor of an elevator. If the elevator is accelerating downward at a rate of , the normal force acting on the book by the floor is
A) 0 N.
B) 2.0 N.
C) 10 N.
D) 20 N.
E) 30 N.

Accepted Answer

The normal force is the force exerted by a surface to support the weight of an object resting on it. When the elevator accelerates downward, the normal force is reduced by the force due to the acceleration. The weight of the book is 20 N (2.0 kg * 10 m/s²). If the elevator is accelerating downward at 10 m/s², the normal force becomes zero because the downward acceleration matches gravitational acceleration.

Question 2

A 2.0 kg-book is on the floor of an elevator. If the elevator is accelerating upward at a rate of , the normal force acting on the book by the floor is
A) 2.0 N.
B) 10 N.
C) 20 N.
D) 30 N.
E) 40 N.

Accepted Answer

The normal force is the sum of the gravitational force and the force due to the elevator's acceleration. The gravitational force is 2.0 kg * 9.8 m/s² = 19.6 N. If the elevator accelerates upward at 5 m/s², the additional force is 2.0 kg * 5 m/s² = 10 N. Thus, the normal force is 19.6 N + 10 N = 29.6 N, which rounds to 30 N.

Question 3

A 2.0-kg book is at rest on the horizontal desk. The normal force exerted on the book by the desk is
A) 2.0 kg.
B) 2.0 N.
C) 20 N.
D) 200 N.
E) 20 kg.

Accepted Answer

The normal force exerted by the desk is equal in magnitude and opposite in direction to the weight of the book. Using the equation F=ma, where a=0 because the book is at rest, the normal force is equal to the weight of the book, which is (2.0 kg)(9.8 m/s^2) = 19.6 N. Therefore, the closest answer choice is C, 20 N.

Question 4

A 2.0 kg-book is at rest on the horizontal floor. You are applying a 10 N downward force, on the book, at an angle of 60^o from the horizontal. The normal force on the book exerted by the floor is A) 5.0 N. B) 10 N. C) 25 N. D) 28 N. E) 30 N.

Accepted Answer

The answer of A 2.0 kg-book is at rest on...

Question 5

A 2.0 kg-book is at rest on a 30^o inclined plane. The normal force acting on the book by the inclined plane is A) 2.0 N. B) 5.0 N. C) 10 N. D) 17 N. E) 20 N.

Accepted Answer

The normal force is the component of the gravitational force perpendicular to the inclined plane. It is calculated as $ N = mg \cos(\theta) $. For a 2.0 kg book, $ N = 2.0 \times 9.8 \times \cos(30^\circ) \approx 17 $ N.

Question 6

A 2.0-kg book is on the floor of an elevator. If the elevator is moving upward at a constant speed of 5.0 m/s, the normal force acting on the book is
A) 0 N.
B) 2.0 N.
C) 5.0 N.
D) 10 N.
E) 20 N.

Accepted Answer

The normal force equals the gravitational force when moving at constant speed. The gravitational force is mass (2.0 kg) times gravity (9.8 m/s²), which is approximately 20 N.

Question 7

The net force acting on an object is zero. You can therefore definitely conclude that the object is
A) at rest.
B) moving in a straight line at constant speed.
C) moving in a circle at constant speed.
D) undergoing acceleration.
E) either at rest or moving in a straight line at constant speed.

Accepted Answer

If the net force on an object is zero, it means that there is no acceleration acting on the object. This can only happen if the object is either at rest or moving in a straight line at a constant speed. So, the correct answer is either at rest or moving in a straight line at constant speed, which is option E. Option A describes one of the possibilities, but option E is more inclusive as it also includes the possibility of the object moving in a straight line at constant speed.

Question 8

A horizontal force acts on a mass m that lies on a horizontal surface. The acceleration of m is . The coefficient of kinetic friction µ_k between mass m and the surface can be calculated from A) µ_k = a/g. B) µ_k = F/mg) - a/g). C) µ_k = F/mg) + a/g). D) µ_k = 0. E) None of these is correct.

Accepted Answer

We know that the force of friction (Ff) is given by: Ff = µ_k * N where N is the normal force exerted by the surface on the object. In this case, since the object is on a horizontal surface, the normal force is equal to the weight of the object: N = mg The horizontal force applied on the object causes an acceleration, given by: a = F / m where F is the applied force. Using Newton's second law, we can write: F - Ff = ma Substituting the expressions for Ff and N, we get: F - µ_k * mg = ma Solving for µ_k, we get: µ_k = (F/mg) - (a/g) Therefore, the correct choice is B.

Question 9

A block of mass m_b rests on a horizontal surface and is accelerated by means of a horizontal cord that passes over a frictionless peg to a hanging weight of mass m_w. The coefficient of kinetic friction between the block and the horizontal surface is µ and the tension in the cord is T. The acceleration of the block is given by A) T - m_wg)/m_w + m_b). B) T/µm_bg + m_b). C) T - µm_b)/m_b_. D) m_wg - T)/m_w. E) T/m_b - µm_bg.

Accepted Answer

The block and the hanging weight experience opposite forces due to the tension in the cord. The net force on the block due to the tension is T - frictional force, where frictional force = µ*N = µ*m_b*g. The normal force N is equal and opposite to the weight of the block, which is m_b*g. Therefore, the net force on the block is T - µ*m_b*g. By Newton's second law, this force is equal to the mass of the block times its acceleration, so: T - µ*m_b*g = m_b*a Rearranging gives: a = (T - µ*m_b*g)/m_b However, we know that the hanging weight is pulling the block with a force of m_w*g, so: m_b*a = T - µ*m_b*g - m_w*g Solving for a gives: a = (m_w*g - T)/(m_w + m_b)

Question 10

An atomic force microscope consists of a cantilever with a very fine tip on one end. The lever is driven by a piezoelectric motor allowing the tip to scan a surface on an atomic scale. As it scans the surface, what is the main frictional force between the tip and the surface?
A) static
B) kinetic
C) rolling
D) all three
E) There is no frictional force.

Accepted Answer

The main frictional force between the tip and the surface will be kinetic friction because the tip is in motion as it scans the surface. Static friction would only come into play if the tip were initially at rest and needed to overcome a static friction force to start moving, which is not the case here. Rolling friction would not be a factor as there is no rolling motion involved in this setup.

Question 11

A block of wood is pulled by a horizontal string across a rough surface at a constant velocity with a force of 20 N. If the coefficient of kinetic friction between the surfaces is 0.2, what is the weight of the wooden block?
A) 20 N
B) 40 N
C) 75 N
D) 100 N
E) 10 N

Accepted Answer

The force of friction is equal to the coefficient of kinetic friction multiplied by the normal force (which is the weight of the block in this case). Since the block moves at constant velocity, the force of friction equals the pulling force of 20 N. Therefore, 0.2 * weight = 20 N, solving gives weight = 100 N.

Question 12

A heavy truck and a light car are traveling at the same speed on the same roadway. If the coefficients of static friction between their tires and the road are the same, which vehicle will be able to stop in the shortest distance? Assume both have the same braking force.
A) the car
B) the truck
C) Both will be able to stop in the same distance.
D) One cannot tell without knowing their coefficients of kinetic friction.
E) One cannot tell without knowing their masses.

Accepted Answer

Since both vehicles have the same coefficient of static friction and the same braking force, they will be able to stop in the same distance because the stopping distance depends only on the coefficient of static friction and the initial speed, not on the mass of the vehicle.

Question 13

The SI units for the coefficient of friction are
A) newtons per meter.
B) meters.
C) newtons.
D) newtons times meters.
E) None of these is correct; the coefficient of friction has no units.

Accepted Answer

The coefficient of friction is a dimensionless quantity, meaning it has no units. It is a ratio of the force of friction between two bodies and the force pressing them together.

Question 14

A 2.0 kg-book is at rest on the horizontal floor. You are applying a 10 N downward force, on the book, at an angle of 30^o from the vertical. The normal force on the book exerted by the floor is A) 5.0 N. B) 10 N. C) 25 N. D) 28 N. E) 30 N.

Accepted Answer

The answer of A 2.0 kg-book is at rest on...

Question 15

A block of mass m is at rest on an inclined plane that makes an angle of 30^owith the horizontal. Which of the following statements about the force of static friction is true? A) f_s > mg B) f_s > mg cos 30^o C) fs = mg cos 30^o D) f_s = mg sin 30^o E) None of these statements is true.

Accepted Answer

The force of static friction (fs) is equal to the component of gravitational force parallel to the inclined plane, which is mg sin 30°.

Question 16

Two objects are sliding at the same speed across a wooden surface. The coefficient of kinetic friction between the first object and the surface is twice that between the second object and the surface. The distance traveled by the first object before it stops is S. The distance traveled by the second object is
A) impossible to determine without knowing the masses involved.
B) 2S
C) S/2
D) S
E) 4S

Accepted Answer

The distance traveled before stopping due to friction is inversely proportional to the coefficient of kinetic friction. If the coefficient for the first object is twice that of the second, the second object will travel twice the distance before stopping, making the correct answer 2S.

Question 17

Four identical blocks are moving on a surface for which the coefficient of kinetic friction between each block and the surface is µ_k. The velocity of each block is indicated by the vector on the block. For which block is the force of friction between the surface and the block greatest? A) 1 B) 2 C) 3 D) 4 E) The force of friction is the same for all blocks.

Accepted Answer

The force of friction on each block will be equal and opposite to the force applied on it to cause its motion (Newton's Third Law). Since all the blocks have the same mass and coefficient of friction, and are moving with the same speed, the force of friction will be the same for all four blocks. Therefore, the answer is E.

Question 18

A block of mass m_b rests on a horizontal surface and is accelerated by means of a horizontal cord that passes over a frictionless peg to a hanging weight of mass m_w. The coefficient of kinetic friction between the block and the horizontal surface is µ_k and the tension in the cord is T. The acceleration of the block is given by A) g. B) . C) . D) . E) .

Accepted Answer

The answer of A block of mass m_b rests on...

Question 19

A block of wood is pulled by a horizontal string across a rough surface at a constant velocity with a force of 20 N. The coefficient of kinetic friction between the surfaces is 0.3. The force of friction is
A) impossible to determine without knowing the mass of the block.
B) impossible to determine without knowing the speed of the block.
C) 0.3 N.
D) 6 N.
E) 20 N.

Accepted Answer

Since the block moves at a constant velocity, the force of friction must equal the pulling force, which is 20 N, to satisfy Newton's first law of motion.

Question 20

A 2.0 kg-book is at rest on a frictionless horizontal floor. You are applying a 10 N downward force, on the book, at an angle of 30^o from the vertical. The acceleration of the book is A) 5.0 m/s². B) 2.5 m/s². C) 1.0 m/s². D) 9.8 m/s². E) 0 m/s².

Accepted Answer

The force can be resolved into its components parallel and perpendicular to the surface of the floor. The perpendicular component does not contribute to the acceleration, as it is balanced by the normal force of the floor on the book. The parallel component, which is $10\sin(30^\circ) = 5\,\mathrm{N}$, contributes to the acceleration. Using $F=ma$ with $F=5\,\mathrm{N}$ and $m=2.0\,\mathrm{kg}$, we get $a = 5/2.0 = 2.5\,\mathrm{m/s^2}$ in the direction of the force.

Quiz 5: Forces and Motion II: Applications

A 2.0 kg-book is on the floor of an elevator. If the elevator is accelerating downward at a rate of , the normal force acting on the book by the floor is

A 2.0 kg-book is on the floor of an elevator. If the elevator is accelerating upward at a rate of , the normal force acting on the book by the floor is

A 2.0-kg book is at rest on the horizontal desk. The normal force exerted on the book by the desk is

A 2.0 kg-book is at rest on the horizontal floor. You are applying a 10 N downward force, on the book, at an angle of 60 o from the horizontal. The normal force on the book exerted by the floor is

A 2.0 kg-book is at rest on a 30 o inclined plane. The normal force acting on the book by the inclined plane is

A 2.0-kg book is on the floor of an elevator. If the elevator is moving upward at a constant speed of 5.0 m/s, the normal force acting on the book is

The net force acting on an object is zero. You can therefore definitely conclude that the object is

A horizontal force acts on a mass m that lies on a horizontal surface. The acceleration of m is . The coefficient of kinetic friction µk between mass m and the surface can be calculated from

An atomic force microscope consists of a cantilever with a very fine tip on one end. The lever is driven by a piezoelectric motor allowing the tip to scan a surface on an atomic scale. As it scans the surface, what is the main frictional force between the tip and the surface?

A block of wood is pulled by a horizontal string across a rough surface at a constant velocity with a force of 20 N. If the coefficient of kinetic friction between the surfaces is 0.2, what is the weight of the wooden block?

A heavy truck and a light car are traveling at the same speed on the same roadway. If the coefficients of static friction between their tires and the road are the same, which vehicle will be able to stop in the shortest distance? Assume both have the same braking force.

The SI units for the coefficient of friction are

A 2.0 kg-book is at rest on the horizontal floor. You are applying a 10 N downward force, on the book, at an angle of 30 o from the vertical. The normal force on the book exerted by the floor is

A block of mass m is at rest on an inclined plane that makes an angle of 30 o with the horizontal. Which of the following statements about the force of static friction is true?

Four identical blocks are moving on a surface for which the coefficient of kinetic friction between each block and the surface is µk. The velocity of each block is indicated by the vector on the block. For which block is the force of friction between the surface and the block greatest?

A block of wood is pulled by a horizontal string across a rough surface at a constant velocity with a force of 20 N. The coefficient of kinetic friction between the surfaces is 0.3. The force of friction is

A 2.0 kg-book is at rest on a frictionless horizontal floor. You are applying a 10 N downward force, on the book, at an angle of 30 o from the vertical. The acceleration of the book is

A 2.0 kg-book is at rest on the horizontal floor. You are applying a 10 N downward force, on the book, at an angle of 60^o from the horizontal. The normal force on the book exerted by the floor is

A 2.0 kg-book is at rest on a 30^o inclined plane. The normal force acting on the book by the inclined plane is

A horizontal force acts on a mass m that lies on a horizontal surface. The acceleration of m is . The coefficient of kinetic friction µ_k between mass m and the surface can be calculated from

A 2.0 kg-book is at rest on the horizontal floor. You are applying a 10 N downward force, on the book, at an angle of 30^o from the vertical. The normal force on the book exerted by the floor is

A block of mass m is at rest on an inclined plane that makes an angle of 30^owith the horizontal. Which of the following statements about the force of static friction is true?

Four identical blocks are moving on a surface for which the coefficient of kinetic friction between each block and the surface is µ_k. The velocity of each block is indicated by the vector on the block. For which block is the force of friction between the surface and the block greatest?

A 2.0 kg-book is at rest on a frictionless horizontal floor. You are applying a 10 N downward force, on the book, at an angle of 30^o from the vertical. The acceleration of the book is