Chapter 5

Question 1

Determine the accelerations that result when a 12-N net force is applied to a 3-kg object and then to a 6-kg object.

Answer 1

A 3-kg object experiences an acceleration of 4 m/s/s. A 6-kg object experiences an acceleration of 2 m/s/s.

Question 2

A net force of 15 N is exerted on an encyclopedia to cause it to accelerate at a rate of 5 m/s2. Determine the mass of the encyclopedia.

Answer 2

Use Fnet= m * a with Fnet = 15 N and a = 5 m/s/s.

So (15 N) = (m)*(5 m/s/s)

And m = 3.0 kg

Question 3

Suppose that a sled is accelerating at a rate of 2 m/s2. If the net force is tripled and the mass is doubled, then what is the new acceleration of the sled?

Answer 3

Answer: 3 m/s/s

The original value of 2 m/s/s must be multiplied by 3 (since a and F are directly proportional) and divided by 2 (since a and m are inversely proportional)

Question 4

Suppose that a sled is accelerating at a rate of 2 m/s2. If the net force is tripled and the mass is halved, then what is the new acceleration of the sled?

Answer 4

Answer: 12 m/s/s

The original value of 2 m/s/s must be multiplied by 3 (since a and F are directly proportional) and divided by 1/2 (since a and m are inversely proportional)

Question 5

Newton second law , in terms of the acceleration changing due to m and fnet.
How does it change ?

Answer 5

The analysis of the table data illustrates that an equation such as Fnet = m*a can be a guide to thinking about how a variation in one quantity might affect another quantity. Whatever alteration is made of the net force, the same change will occur with the acceleration. Double, triple or quadruple the net force, and the acceleration will do the same. On the other hand, whatever alteration is made of the mass, the opposite or inverse change will occur with the acceleration. Double, triple or quadruple the mass, and the acceleration will be one-half, one-third or one-fourth its original value.

Question 6

Newtons first law

Answer 6

Newton’s first law of motion predicts the behavior of objects for which all existing forces are balanced. The first law - sometimes referred to as the law of inertia - states that if the forces acting upon an object are balanced, then the acceleration of that object will be 0 m/s/s. Objects at equilibrium (the condition in which all forces balance) will not accelerate. According to Newton, an object will only accelerate if there is a net or unbalanced force acting upon it. The presence of an unbalanced force will accelerate an object - changing its speed, its direction, or both its speed and direction.

An object that is at rest will remain at rest, or an object that is moving will continue to move in a straight line with constant velocity, if and only if the net force acting on the object is zero.
from ppw

Question 7

Newtons second law

Answer 7

Newton’s second law of motion pertains to the behavior of objects for which all existing forces are not balanced. The second law states that the acceleration of an object is dependent upon two variables - the net force acting upon the object and the mass of the object. The acceleration of an object depends directly upon the net force acting upon the object, and inversely upon the mass of the object. As the force acting upon an object is increased, the acceleration of the object is increased. As the mass of an object is increased, the acceleration of the object is decreased.

Question 8

newtons first law and keeping an object in motion

Answer 8

Newton’s first law of motion declares that a force is not needed to keep an object in motion. Slide a book across a table and watch it slide to a rest position. The book in motion on the table top does not come to a rest position because of the absence of a force; rather it is the presence of a force - that force being the force of friction - that brings the book to a rest position. In the absence of a force of friction, the book would continue in motion with the same speed and direction - forever (or at least to the end of the table top)! A force is not required to keep a moving book in motion; and a force is not required to keep a moving sled in motion; and a force is not required to keep any object horizontally moving object in motion.

Question 9

newton 1st law is also known as …

Answer 9

the law of inertia

remains unchanged .

Question 10

do we need a cause for the object to move ?

and why ?

Answer 10

No cause is needed for an object to
move!
why?

Question 11

A non-zero net force causes …..

Answer 11

A non-zero net force causes the change
of the object’s velocity (the speed
and/or the direction)
hence causes acceleration

Question 12

what is a inertial reference frame ?

Answer 12

is a coordinate system in which Newton’s laws are valid. Accelerating reference frames are not inertial reference frames. A coordinate system moving with constant velocity relative to an inertial reference frame is also an inertial reference frame.

the object cant accelerate in order to use the inertial reference frame.
remember inertial means no remains the change.

Question 13

what is a mechanical equilibrium ?

Answer 13

An object on which the net force is zero is said to be in mechanical equilibrium.
There are two types of mechanical equilibrium

Question 14

name the two type of mechanical equilibrium

Answer 14

When the object is at rest → static equilibrium (statics) When the object is moving in a straight line with a constant velocity → dynamic equilibrium

Question 15

A constant force applied to object A cause A to accelerate to 5m/s ^2 . The same force applied to object B causes a acceleration of 3m/s^2 . Applied to object C, it causes a acceleration of 8 m/s^2

a) which object has the largest mass?
b) which object has the smallest mass?
c) what is the ration of mass A to mass B ?

Answer 15

a) object B
b) object C
c) 3/5

Question 16

A constant force applied to an object causes the object to accelerate at 10 m/s^2 . What will the acceleration of this object be if

a. the force is doubled
b. the mass is doubled
c. the force is doubled and the mass is doubled
d. the force is doubled and the mass is halved

Answer 16

a. 20 m/s^2
b. 5 m/s^2
c. 10 m/s^2
d. 40 m/s^2

Question 17

A constant force is applied to an object causes the object to accelerate 8m/s^2 . What will the acceleration of this object if

a. the force is halved
b. the mass is halved
c. the force is halved and the mass is halved
d. the force is halved and the mass is doubled

Answer 17

a. 4m/s^2
b. 16 m/s^2
c. 8 m/s^2
d. 2 m/s^2

Question 18

If an object is at rest, can you conclude that there are no forces acting on it . Explain ?
according to newtons first law

Answer 18

No, that is not a reasonable conclusion. Consider a book at rest on a table. The book experiences a force
due to gravity, as well as the normal force from the table. The book remains at rest because the net force is zero,
not because there are no external forces.

Question 19

Is it possible for an object to be in motion if no net force is acting on it? Explain.

Answer 19

Yes, it is possible for an object to be in motion, even if it experiences no net external force. For example,
consider a car moving down the interstate at constant velocity. There is friction between the tires and the road, air
resistance, and wind acting on the car. However, these all can cancel out, causing the net force to be zero. This
implies that the car is accelerating, but it continues to move at constant velocity.

Question 20

An object thrown into the air stops at the highest point in its path. Is it in equilibrium at this point? Explain.

Answer 20

While the velocity is zero at the highest point, it is not at equilibrium. At the highest point of the throw,
gravity is still acting on the object, pulling it downwards. This is the only force, and thus the net force is downward.
If an object experiences a non-zero net force, it is not at equilibrium.

Question 21

are the following inertial references frames ?

a car driving at steady speed on a straight and level road

Answer 21

yes , since its going at a steady speed

therefore, no acceleration = f net 0

Question 22

are the following inertial references frames ?

a car driving at steady speed up a 10 degree incline

Answer 22

yes
steady speed
therefore no acceleration = fnet 0

Question 23

are the following inertial references frames ?

A car speeding up after leaving a stop sign

Answer 23

No. since the car is accelerating

change of velocity over time

Question 24

are the following inertial references frames ?

A car driving at steady speed around the curve

Answer 24

No
Even though the car speed is constant. it is going around a curve. And we learned that if a car goes around a curve, there is acceleration due to change in direction.
so, you have to take into account velocity which is a vector.
so, acc is not 0 and fnet is not equal to 0

Question 25

are the following inertial references frames ?

A hot ballon rising straight up at steady speed

Answer 25

Yes
going up with steady speed.
therefore, no acceleration and no fnet

Question 26

are the following inertial references frames ?

A sky diver just after leaping from a airplane

Answer 26

No.
he is accelerating due to gravity.
which is the only force acting on him during free fall
free fall = no air resistance

Question 27

are the following inertial references frames ?

A space station orbiting the earth.

Answer 27

no , its not.
its orbiting.
acceleration due to change of direction

Question 28

whats the notation for gravitational force ?

Answer 28

F_G

Question 29

whats the notation for spring force ?

Answer 29

F_Sp

Question 30

What’s the notation for tension ?

Answer 30

T

Question 31

what’s the notation for normal force ?

Answer 31

n

Question 32

What’s the notation for static friction

Answer 32

f_s

Question 33

what’s the notation for kinetic friction ?

Answer 33

f_k

Question 34

what’s the notation for drag ?

Answer 34

F_drag

Question 35

what’s the notation for thrust ?

Answer 35

F_thrust

Question 36

Spring force ?
what is it ?
how do you should it on a diagram ?

Answer 36

Springs exert one of the most common contact forces. A spring can either push (when compressed) or pull (when stretched). Figure 5.4 shows the spring force, for which we use the symbol F⃗ SP. In both cases, pushing and pulling, the tail of the force vector is placed on the particle in the force diagram.

Question 37

tension force ?

Answer 37

When a string or rope or wire pulls on an object, it exerts a contact force that we call the tension force, represented by a capital T⃗ The direction of the tension force is always along the direction of the string or rope, as you can see in Figure 5.5. The commonplace reference to “the tension” in a string is an informal expression for T, the size or magnitude of the tension force.

Question 38

normal force ?

explain it
and how do we define it

Answer 38

If you sit on a bed, the springs in the mattress compress and, as a consequence of the compression, exert an upward force on you. Stiffer springs would show less compression but still exert an upward force. The compression of extremely stiff springs might be measurable only by sensitive instruments. Nonetheless, the springs would compress ever so slightly and exert an upward spring force on you.

Figure 5.6 shows an object resting on top of a sturdy table. The table may not visibly flex or sag, but—just as you do to the bed—the object compresses the spring-like ­molecular bonds in the table. The size of the compression is very small, but it is not zero. As a consequence, the compressed “molecular springs” push upward on the ­object. We say that “the table” exerts the upward force, but it is important to understand that the pushing is really done by molecular bonds.

we define the normal force as the force exerted perpendicular to a surface (the agent) against an object that is pressing against the surface.

Question 39

friction ?

Answer 39

Friction, like the normal force, is exerted by a surface. But whereas the normal force is perpendicular to the surface, the friction force is always parallel to the surface. It is useful to distinguish between two kinds of friction:

KINETIC FRICTION, denoted f⃗ k, appears as an object slides across a surface. This is a force that “opposes the motion,” meaning that the friction force vector f⃗ k, points in a direction opposite the velocity vector v⃗ (i.e., “the motion”).

STATIC FRICTION, denoted f⃗ s, is the force that keeps an object “stuck” on a surface and prevents its motion. Finding the direction of f⃗ s is a little trickier than finding it for f⃗ k. Static friction points opposite the direction in which the object would move if there were no friction. That is, it points in the direction necessary to prevent motion.

Question 40

drag
what is it ?
when do you use drag in problems ?

Answer 40

Friction at a surface is one example of a resistive force, a force that opposes or resists motion. Resistive forces are also experienced by objects moving through fluids—gases and liquids. The resistive force of a fluid is called drag, with symbol F⃗ drag Drag, like kinetic friction, points opposite the direction of motion.

For objects that are heavy and compact, that move in air, and whose speed is not too great, the drag force of air resistance is fairly small. To keep things as simple as possible, you can neglect air resistance in all problems unless a problem explicitly asks you to include it.

Question 41

thrust ?

Answer 41

A jet airplane obviously has a force that propels it forward during takeoff. Likewise for the rocket being launched in Figure 5.10. This force, called thrust, occurs when a jet or rocket engine expels gas molecules at high speed. Thrust is a contact force, with the exhaust gas being the agent that pushes on the engine. The process by which thrust is generated is rather subtle, and we will postpone a full discussion until we study Newton’s third law in Chapter 7. For now, we will treat thrust as a force opposite the direction in which the exhaust gas is expelled. There’s no special symbol for thrust, so we will call it F⃗ thrust.