Force and Motion

Question 1

Describe Newton’s First Law in Motion.

Answer 1

If no net force acts on a body (F_net = 0), the body’s velocity cannot change; that is, the body cannot accelerate.

Question 2

Describe the principle of superposition for forces.

Answer 2

A single force that has the same magnitude and direction as the calculated net force would then have the same effect as all the individual forces. This is the reason why we can break down forces into components then find the net force in that direction.

Question 3

This reference frame is one in which Newton’s laws hold.

Answer 3

inertial reference frame.

Question 4

A frame in which see a non existent force is what type of reference frame?

Answer 4

Noninertial reference frame.

Question 5

How is acceleration related to mass?

Answer 5

inversely related.

a = F / m

so as m increases a decreases.

Question 6

What is Newton’s Second Law state?

Answer 6

The net force on a body is equal to the product of the boy’s mass and its acceleration.

F_Net = ma

or

F = dp/dt.

Question 7

True/False: The acceleration component along a given axis is caused only by the sum of the force components along that same axis, and not by force components along any other axis.

Answer 7

True.

Question 8

A system consists of one or more bodies, and any force on the bodies inside the system from bodies outside the system is called what?

Answer 8

external force.

Question 9

Define the gravitational foce.

Answer 9

F_g = mg

A certain type of pull on a body that is directed toward a second body.

An attraction of masses.

Question 10

Define weight.

Answer 10

The magnitude of the net froce required to prevent the body from falling freely, as measured by someone on the ground.

F_W = mg.

Question 11

Define the normal force, F_N.

Answer 11

When a body presses against a surface, the surface (even a seemingly rigid one) deforms and pushes on the body wiht a normal force F_N that is perpendicular to the surface.

Question 12

Define frictional force.

Answer 12

A resistant force caused by a boding of a body in motion across a surface.

Question 13

When a cord (or a rope, cabe , or other such object) is attached to a body and pulled taut, the cord pulls on the body with a force T directed away/towards the body and along the cord.

Answer 13

away.

Question 14

Define Newton’s Third Law.

Answer 14

When two bodies interact, the forces on the bodies from each other are always equal in magnitude and opposite in direction.

Question 15

When any two bodies interact in any situation, what type of force is present?

Answer 15

Third-law force pair.

Question 16

Suppose that a cantaloupe sitting on a table are both in an elevator cab that begins to accelerate upward. (a) Do the magnitudes of F_TC and F_CT increase, decrease, or stay the same? (b) Are those two forces still equal in magnitude and opposite in direction? (c) Do the magnitudes of F_CE and F_EC increase, decrease, or stay the same? (d) Are those two forces still equal in magnitude and opposite in direction?

Answer 16

increase; yes. Contact forces increase due to acceleration.

stay the same; yes. Gravitional force = mg (Earth and object interation)

Question 17

When a body presses against a surface, the surface (even a seemingly rigid one) deforms and pushes on the body with a normal force F_N, that is parallel/perpendicular to the surface?

Answer 17

perpendicular

Question 18

In Fig. 5-7, is the magnitude of the normal force F_N greater than, less than, or equal to mg if the block and table are in an elevator moving upward (a) at constant speed and (b) at increasing speed?

Answer 18

F_N - mg = ma_y

Question 19

Answer 19

(a) FcosØ / m = a

Note: F_N + FsinØ - mg = 0 in y-dir. If we plug in F in this equation and find F_N is negative ⇒ it is no longer touching the ground. (Not the case)

(b) F_N + FsinØ - mg = 0, when the block is leaving the ground ⇒ F_N → 0.

F = mg / sinØ

(c) Plugging in our answer of (b) into equation (a) we will get a new acceleration.

Question 20

Answer 20

Due to Newton’s 2nd & 3rd Law:

(a) T₃ = m_Ta, where m_T = m₁ + m₂ + m₃
(b) T₁ = m₁a
(c) T₂ = (m₁ + m₂)a

where m_T and each m_i share the same acceleration.

Question 21

Answer 21

Logic + Ratio:

Let’s look at the ratio of the forces on the outside block of each figure.

F_BA / F_AB = m_B / m_A , this is true since F_applied is the same on each figure.

⇒ m_B = 2m_A and from what given both masses add to 12kg.

With some guessing we get m_B = 8kg and m_A = 4kg, this is the only possible solution to match what is given….

(a) F_BA = m₁a for the acceleration.
(b) F_applied = m_Ta, where m_T = m_A+ m_B.

Question 22

Answer 22

-T₃ + mg = m₃a → 2 unknowns…..

Consider only the F_ext on the entire system.

F_Net = (m_c - m_a)g = m_Ta, where mT = m_A+ m_B +m_C. We now have acceleration.

Now if we plug in the acceleration into our equation of mass 3 we now have the tension on mass 3.