Q: You pull a 5 kg box across a rough floor with a horizontal force of 30 N. The coefficient of kinetic friction is 0.2. What is the magnitude of the frictional force?

$$F_f = \mu_k F_N = 0.2 \times (5 \times 9.8) = 9.8\ \mathrm{N}$$

Question 1

Which of the following best describes the direction of the normal force acting on an object resting on a horizontal surface?

Accepted Answer

It acts perpendicular to the surface and away from the surface.

Question 2

A 10 kg box is at rest on a rough horizontal floor. Which forces act on the box?

Accepted Answer

Weight, normal force, and friction

Question 3

You pull a 5 kg box across a rough floor with a horizontal force of 30 N. The coefficient of kinetic friction is 0.2. What is the magnitude of the frictional force?

Accepted Answer

$$F_f = \mu_k F_N = 0.2 	imes (5 	imes 9.8) = 9.8\ \mathrm{N}$$

Question 4

A person pushes a couch across a carpeted floor. Which of the following forces does NOT act on the couch?

Accepted Answer

Tension

Question 5

In a free-body diagram, which of the following is the correct way to represent forces?

Accepted Answer

Draw arrows from the object's center, each labeled with the type of force.

Question 6

A 2 kg physics textbook is pushed to the right with a force of 20 N across a flat table. The kinetic friction is 8 N. What is the net force on the book?

Accepted Answer

$$F_{net} = 20\ \mathrm{N} - 8\ \mathrm{N} = 12\ \mathrm{N}$$

Question 7

Which force is always directed toward the center of the Earth?

Accepted Answer

Weight

Question 8

A block is pushed against a rough vertical wall with a force $$F at a$$ $$45^\circ$$ angle upward. Which forces act on the block?

Accepted Answer

Weight, normal force, friction, and applied force

Question 9

If a box is being pulled upward by a rope at constant speed, which statement is true about the forces acting on the box?

Accepted Answer

The tension in the rope is equal to the weight of the box.

Question 10

Which of the following best describes friction?

Accepted Answer

A force that acts parallel to the surface and usually opposes motion.

Question 11

A child swings from a tree branch by a rope. Which forces act on the child at the lowest point of the swing?

Accepted Answer

Weight and tension

Question 12

A box is sliding to the right across a rough table. Which direction does the friction force act?

Accepted Answer

To the left, opposite the direction of motion

Question 13

Which of the following is NOT a common force included in a free-body diagram for an object on a surface?

Accepted Answer

Buoyant force

Question 14

A 3 kg box is pulled across a rough floor with a force of 15 N. The box moves at constant velocity. What is the frictional force?

Accepted Answer

$$F_{friction} = 15\ \mathrm{N}$$

Question 15

Which of the following statements about free-body diagrams is correct?

Accepted Answer

They show only the forces acting on a single object.

Question 16

A 4 kg box is at rest on a table. What is the magnitude of the normal force acting on the box?

Accepted Answer

$$F_N = 4 	imes 9.8 = 39.2\ \mathrm{N}$$

Question 17

A person pushes a box up a rough ramp at a constant speed. Which forces act on the box?

Accepted Answer

Weight, normal force, friction, and applied force

Question 18

If the frictional force is greater than the applied force on an object, what will happen to the object's motion?

Accepted Answer

The object will decelerate and eventually stop.

Question 19

Which of the following best describes the applied force?

Accepted Answer

A force that is exerted by a person or another object directly pushing or pulling.

Question 20

A 6 kg box is pulled horizontally with a force of 40 N. The coefficient of kinetic friction is 0.25. What is the acceleration of the box?

Accepted Answer

$$a = \frac{F_{applied} - F_{friction}}{m} = \frac{40 - (0.25 	imes 6 	imes 9.8)}{6} = \frac{40 - 14.7}{6} = 4.23\ \mathrm{m/s^2$}$$

Fundamentals of Forces and Free-Body Diagrams in Physics

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