Q: A ball of mass $$m is $$whirled in a horizontal circle of radius $$R at $$constant speed $$v. $$What is the magnitude of the net force acting on the ball?

$$F_{\text{net}} = \frac{m v^2$}{R}$$

Question 1

A car is parked on a flat road. Which of the following statements best describes the forces acting on the car according to Newton’s first law?

Accepted Answer

The sum of all forces acting on the car is zero, so $$\sum \vec{F} = 0.$$

Question 2

A 10 kg box is pulled horizontally with a force of 40 N on a surface with a coefficient of kinetic friction $$\mu_k = 0.3. $$What is the acceleration of the box? (Take $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$a = 0.8\ \mathrm{m/s^2$}$$

Question 3

Which of the following is the correct expression for the maximum static friction force?

Accepted Answer

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

Question 4

A block is at rest on a horizontal surface. A horizontal force is gradually increased until the block just begins to move. Which force is responsible for keeping the block at rest before it moves?

Accepted Answer

Static friction

Question 5

A skydiver jumps from a plane and eventually reaches terminal velocity. Which of the following is true at terminal velocity?

Accepted Answer

The drag force equals the weight: $$D = mg.$$

Question 6

A ball of mass $$m is $$whirled in a horizontal circle of radius $$R at $$constant speed $$v. $$What is the magnitude of the net force acting on the ball?

Accepted Answer

$$F_{	ext{net}} = \frac{m v^2$}{R}$$

Question 7

Which of the following should NOT be included in a free-body diagram?

Accepted Answer

The vector $$m \vec{a}$$

Question 8

A 5 kg block is at rest on a horizontal surface. The coefficient of static friction is $$\mu_s = 0.4. $$What is the minimum horizontal force required to start moving the block? (Take $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$19.6\ \mathrm{N}$$

Question 9

A car rounds a flat curve of radius $$50\ \mathrm{m} at a $$speed of $$20\ \mathrm{m/s}. $$What is the minimum coefficient of static friction required to prevent the car from skidding?

Accepted Answer

$$\mu_s = 0.40$$

Question 10

Which of the following statements about friction is correct?

Accepted Answer

Static friction can vary up to a maximum value, but kinetic friction is constant for given surfaces.

Question 11

A block slides down a rough inclined plane at constant speed. Which of the following is true?

Accepted Answer

The friction force equals the component of gravity parallel to the incline.

Question 12

Which of the following is NOT a fundamental force of nature?

Accepted Answer

Frictional force

Question 13

A 2 kg object is acted on by a net force of $$10\ \mathrm{N} to $$the right. What is its acceleration?

Accepted Answer

$$a = 5\ \mathrm{m/s^2$}$$

Question 14

A block is pulled at constant velocity across a horizontal surface. Which of the following is true about the forces acting on the block?

Accepted Answer

The pulling force equals the kinetic friction force.

Question 15

A ball is attached to a string and whirled in a horizontal circle. If the string suddenly breaks, what path will the ball follow?

Accepted Answer

It will move in a straight line tangent to the circle at the point of release.

Question 16

Which of the following best describes the direction of acceleration and net force for a particle in uniform circular motion?

Accepted Answer

Both are directed toward the center of the circle.

Applying Newton’s Laws: Equilibrium, Dynamics, Friction, and Circular Motion

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