Q: A kneecap is held in equilibrium by three forces: two tendons pulling at angles and the femur pushing outward. If both tendons have a tension of 60 N, one at $$10^ ext{o}$$ and the other at $$70^ ext{o}$$, what is the magnitude of the femur's force on the kneecap?

$$F = \frac{(60\,\text{N})\cos 10^ ext{o} + (60\,\text{N})\cos 70^ ext{o}}{\cos 30^ ext{o}} = 92\,\text{N}$$

Q: A car weighing 15,000 N is towed up a frictionless $$20^ ext{o}$$ slope at constant velocity. What is the tension in the tow rope?

$$T = 15,000\,\text{N} \sin 20^ ext{o} = 5,100\,\text{N}$$

Q: Which equation represents Newton’s second law for a particle in equilibrium?

$$\vec{F}_\text{net} = \sum \vec{F}_i = 0$$

Q: A 25 kg wood box is pushed across a wood floor at a steady speed of 2.0 m/s. What is the kinetic friction force acting on the box? (Use $$\mu_k = 0.20$$ and $$g = 9.8\,\text{m/s}^2)$$

$$f_k = \mu_k m g = 0.20 \times 25 \times 9.8 = 49\,\text{N}$$

Q: A steel file cabinet (50 kg) is on the back of a dump truck tilted at $$20^ ext{o}. If$$ $$\mu_s = 0.80$$, what is the maximum static friction force before the cabinet slips?

$$f_{s,\text{max}} = \mu_s n = 0.80 \times 50 \times 9.8 \times \cos 20^ ext{o} = 370\,\text{N}$$

Q: Which equation describes the drag force experienced by an object moving through air?

$$F_{\text{drag}} = \frac{1}{2} C \rho A v^2$

Question 1

Which of the following statements best describes mechanical equilibrium for a particle?

Accepted Answer

The particle is at rest or moves with constant velocity, and the net force is zero.

Question 2

A kneecap is held in equilibrium by three forces: two tendons pulling at angles and the femur pushing outward. If both tendons have a tension of 60 N, one at $$10^	ext{o}$$ and the other at $$70^	ext{o}$$, what is the magnitude of the femur's force on the kneecap?

Accepted Answer

$$F = \frac{(60\,	ext{N})\cos 10^	ext{o} + (60\,	ext{N})\cos 70^	ext{o}}{\cos 30^	ext{o}} = 92\,	ext{N}$$

Question 3

A car weighing 15,000 N is towed up a frictionless $$20^	ext{o}$$ slope at constant velocity. What is the tension in the tow rope?

Accepted Answer

$$T = 15,000\,	ext{N} \sin 20^	ext{o} = 5,100\,	ext{N}$$

Question 4

A 1500 kg car is pulled by a tow truck with a tension of 2500 N, opposed by a 200 N friction force. If the car starts from rest, what is its speed after 5.0 s?

Accepted Answer

$$v = 7.7\,	ext{m/s}$$

Question 5

Which equation represents Newton’s second law for a particle in equilibrium?

Accepted Answer

$$\vec{F}_	ext{net} = \sum \vec{F}_i = 0$$

Question 6

According to Newton’s law of gravity, what is the force between two masses $$m_1$$ and $$m_2$$ separated by distance $$r$$?

Accepted Answer

$$F = G \frac{m_1$ m_2$}{r^2$}$$

Question 7

What is the free-fall acceleration $$g$$ near the surface of a planet of mass $$M$$ and radius $$R$$?

Accepted Answer

$$g = G \frac{M}{R^2$}$$

Question 8

A spring scale measures the weight of an object at rest. Which equation describes the equilibrium condition?

Accepted Answer

$$F_{	ext{sp}} = mg$$

Question 9

A man stands on a spring scale in an elevator accelerating upward. How does his weight compare to $$mg$$?

Accepted Answer

Greater than $$mg$$

Question 10

Which of the following best describes weightlessness experienced by astronauts in orbit?

Accepted Answer

The astronauts are in free fall, so the scale reading is zero.

Question 11

Static friction acts to prevent motion. What is the maximum static friction force for an object with normal force $$n$$ and coefficient $$\mu_s$$?

Accepted Answer

$$f_{s,	ext{max}} = \mu_s n$$

Question 12

A 25 kg wood box is pushed across a wood floor at a steady speed of 2.0 m/s. What is the kinetic friction force acting on the box? (Use $$\mu_k = 0.20$$ and $$g = 9.8\,	ext{m/s}^2)$$

Accepted Answer

$$f_k = \mu_k m g = 0.20 	imes 25 	imes 9.8 = 49\,	ext{N}$$

Question 13

A steel file cabinet (50 kg) is on the back of a dump truck tilted at $$20^	ext{o}. If$$ $$\mu_s = 0.80$$, what is the maximum static friction force before the cabinet slips?

Accepted Answer

$$f_{s,	ext{max}} = \mu_s n = 0.80 	imes 50 	imes 9.8 	imes \cos 20^	ext{o} = 370\,	ext{N}$$

Question 14

What is the direction of the kinetic friction force relative to the motion of an object?

Accepted Answer

Opposite to the direction of motion

Question 15

Which equation describes the drag force experienced by an object moving through air?

Accepted Answer

$$F_{	ext{drag}} = \frac{1}{2} C ho A v^2$

Question 16

A falling object reaches terminal speed when:

Accepted Answer

The upward drag force equals the downward gravitational force.

Question 17

Which of the following is NOT a property of mass?

Accepted Answer

Mass depends on the object's location.

Question 18

If a box is at rest and a pulling force is applied, which force prevents the box from moving until a threshold is reached?

Accepted Answer

Static friction

Question 19

Which coefficient of friction is typically smallest for steel on steel?

Accepted Answer

Rolling friction $$\mu_r$$

Dynamics I: Motion Along a Line – Forces, Equilibrium, Friction, and Drag

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