Q: A 2 kg and a 5 kg mass are hanging with a string in equilibrium. What is the tension in the string holding the 5 kg mass?

$$T = 5\ \mathrm{kg} \times 9.8\ \mathrm{m/s}^2$$

Q: A force of 8.0 N acts at an angle of 45° to the x-axis. What is the x-component of this force?

$$F_x = 8.0\ \mathrm{N} \times \cos 45^\circ$$

Question 1

A plane of mass 51,000 kg accelerates from rest to a speed of 70 m/s over a distance of 940 m. What is the magnitude of the net force acting on the plane during its acceleration?

Accepted Answer

$$F = m a_x = (51,000\ \mathrm{kg})(2.61\ \mathrm{m/s}^2) = 133,000\ \mathrm{N}$$

Question 2

Which kinematic equation is used to determine the acceleration of the plane as it travels down the runway?

Accepted Answer

$$v_{x1}^2 = v_{x0}^2 + 2 a_x \Delta x$$

Question 3

A 5 kg object is lowered at constant speed by a rope. What is the tension in the rope?

Accepted Answer

$$T = m g$$

Question 4

A skier is being towed up a snow-covered hill by a rope. Which of the following forces does NOT act on the skier?

Accepted Answer

Spring force from the rope

Question 5

Which statement is true about the net force on an object moving at constant velocity?

Accepted Answer

$$\vec{F}_{\mathrm{net}} = 0$$

Question 6

When drawing a free-body diagram, which of the following is NOT a correct step?

Accepted Answer

Include forces that do not actually act on the object

Question 7

A book is pushed across a table by a hand. Which forces act on the book?

Accepted Answer

Hand force, normal force, friction, and weight

Question 8

A ball is tossed straight up and is just after leaving the hand (ignore air resistance). What is the only force acting on the ball?

Accepted Answer

Gravity

Question 9

Which equation correctly expresses Newton's second law for motion in two dimensions?

Accepted Answer

$$\vec{F}_{\mathrm{net}} = m \vec{a}$$

Question 10

A 2 kg and a 5 kg mass are hanging with a string in equilibrium. What is the tension in the string holding the 5 kg mass?

Accepted Answer

$$T = 5\ \mathrm{kg} 	imes 9.8\ \mathrm{m/s}^2$$

Question 11

A force of 8.0 N acts at an angle of 45° to the x-axis. What is the x-component of this force?

Accepted Answer

$$F_x = 8.0\ \mathrm{N} 	imes \cos 45^\circ$$

Question 12

If a skier is being towed up a hill at constant speed, what can be said about the net force acting on the skier?

Accepted Answer

The net force is zero

Question 13

Which of the following is a long-range force acting on a skier being towed up a hill?

Accepted Answer

Weight (gravity)

Question 14

A plane accelerates down a runway with constant acceleration. Which equation would you use to find the acceleration if you know the initial and final velocities and the distance traveled?

Accepted Answer

$$a_x = \frac{v_{x1}^2 - v_{x0}^2}{2 \Delta x}$$

Question 15

Which of the following is NOT a contact force?

Accepted Answer

Weight

Question 16

A block is pulled across a rough surface at constant velocity. Which forces must be equal in magnitude?

Accepted Answer

Applied force and friction force

Question 17

Which of the following best describes the direction of the friction force acting on a skier being towed up a hill?

Accepted Answer

Down the hill, opposite to the direction of motion

Question 18

A free-body diagram shows three forces acting on an object: $$F_1 = 10$$\ \mathrm{N}$$ at 30$$^\circ$$ above the x-axis, F_2$ = 8\ \mathrm{N} at$$ $$45^\circ$$ below the x-axis, and $$F_3 = 5$$\ \mathrm{N}$$ along the x-axis. What is the magnitude of the net force in the x-direction?

Accepted Answer

$$F_{\mathrm{net},x} = 10\ \mathrm{N} \cos 30^\circ - 8\ \mathrm{N} \cos 45^\circ + 5\ \mathrm{N}$$

Question 19

Which of the following is the correct vector equation for the sum of forces acting on an object in equilibrium?

Accepted Answer

$$\sum \vec{F} = 0$$

Lecture 9: Newton's Laws, Free-Body Diagrams, and Force Analysis

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