Q: A 70 kg skydiver reaches terminal velocity. If $$g = 9.8\ \mathrm{m/s^2$}$$, what is the magnitude of the air resistance acting on her?

$$R = m \times g = 70 \times 9.8 = 686\ \mathrm{N}$$

Q: A car of mass 1000 kg accelerates at $$2\ \mathrm{m/s^2$}. $$What is the net force acting on the car?

$$F = 1000 \times 2 = 2000\ \mathrm{N}$$

Question 1

Which of the following statements best describes Newton's First Law of Motion?

Accepted Answer

An object continues in a state of rest or uniform motion in a straight line unless acted upon by a nonzero force.

Question 2

A 5 kg box is pushed with a net force of 20 N. What is its acceleration?

Accepted Answer

$$a = \frac{F}{m} = \frac{20}{5} = 4\ \mathrm{m/s^2$}$$

Question 3

If the net force acting on an object is zero, which of the following must be true?

Accepted Answer

The object moves with constant velocity or remains at rest.

Question 4

A skydiver jumps from a plane. As she falls, air resistance increases. What happens to her acceleration as her velocity increases?

Accepted Answer

Her acceleration decreases.

Question 5

Which equation correctly expresses Newton's Second Law of Motion?

Accepted Answer

$$F = m 	imes a$$

Question 6

A 70 kg skydiver reaches terminal velocity. If $$g = 9.8\ \mathrm{m/s^2$}$$, what is the magnitude of the air resistance acting on her?

Accepted Answer

$$R = m 	imes g = 70 	imes 9.8 = 686\ \mathrm{N}$$

Question 7

Two people, one heavy and one light, jump from the same altitude with identical parachutes. Who reaches the ground first, assuming air resistance is present?

Accepted Answer

The heavy person

Question 8

Which of the following best describes the force of friction?

Accepted Answer

It results from mutual contact of irregularities on surfaces of sliding objects.

Question 9

A feather and a stone are dropped in a vacuum chamber. What happens?

Accepted Answer

Both fall at the same rate.

Question 10

Which formula represents the Universal Law of Gravitation?

Accepted Answer

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

Question 11

When a cannon is fired, why do the cannon and cannonball have different accelerations?

Accepted Answer

The masses are different.

Question 12

Which of the following is true about action-reaction pairs according to Newton's Third Law?

Accepted Answer

Action and reaction forces are always equal in magnitude and opposite in direction.

Question 13

A car of mass 1000 kg accelerates at $$2\ \mathrm{m/s^2$}. $$What is the net force acting on the car?

Accepted Answer

$$F = 1000 	imes 2 = 2000\ \mathrm{N}$$

Question 14

If an object is moving at constant velocity, which of the following must be true?

Accepted Answer

The net force on the object is zero.

Question 15

A skydiver falls faster and faster through the air. What happens to the magnitude of the net force acting on her?

Accepted Answer

It decreases.

Question 16

Which of the following is NOT a consequence of Newton's Second Law?

Accepted Answer

Acceleration is always in the opposite direction of net force.

Question 17

A block slides across a table and eventually comes to rest. Which force is primarily responsible for stopping the block?

Accepted Answer

Friction

Question 18

If the air resistance $$R$$ acting on a falling object is given by $$R = bv$$, where $$b is a $$constant and $$v is $$velocity, what happens to $$R as $$the object speeds up?

Accepted Answer

It increases.

Question 19

Which of the following is a correct statement about the forces involved when you push against a wall?

Accepted Answer

You exert a force on the wall, and the wall exerts an equal and opposite force on you.

Question 20

A 60 kg person stands on Earth. What is their weight?

Accepted Answer

$$W = m 	imes g = 60 	imes 9.8 = 588\ \mathrm{N}$$

module 3 lecture 6: Newton's Laws of Motion, Forces, and Gravity

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