Exam 2

Question 1

Newtons First Law

Answer 1

An object at rest remains at rest as long as no net force acts on it

Question 2

Newtons Second Law

Answer 2

The summation of force equals mass times acceleration (either in x or y components)
The acceleration of an object is proportional to the force acting on it, and inversely proportional to its mass

Question 3

Newtons Third Law

Answer 3

For every force that acts on an object, there is a reaction force acting on a different object that is equal in magnitude and opposite in direction.
Always on two DIFFERENT objects

Question 4

According to Newtons first law,
If the net force on an object is zero, its velocity is _________

Answer 4

If the net force on an object is zero, its velocity is constant

Question 5

Imagine a ball being held up between two strings. If there is slow force applied to lower string, which string will break?

Answer 5

The top string. The force traveled up the strings and the top string was holding the weight of the ball, which now has force.

Question 6

Imagine a ball being held up between two strings. If there is sudden force applied to the lower string, which string will break?

Answer 6

The lower string. The force did not have time to travel up to the top string. Because of the inertia of the ball, the top string was unaffected

Question 7

A net force is required to make an object ___________

Answer 7

A net force is required to make an object accelerate. No net force = no acceleration and vice versa because F=ma.

Question 8

An object that is moving will continue to move with________ velocity unless a net force acts on it.

Answer 8

An object that is moving will continue to move with the same velocity unless a net force acts on it. This constant velocity can be 0

Question 9

Acceleration is proportional to _______ and inversely proportional to ________

Answer 9

Acceleration is proportional to force and inversely proportional to mass.

ex: as force increases, so does acceleration. Double the force you will double the acceleration.
As weight increases, acceleration slows down. Double the weight you will cut the acceleration in half. (1/2)a

Question 10

What about Newtons Second Law equation must we remember?

Answer 10

It is the SUM of the Forces that equal Mass times Acceleration. Not just force, but the SUM of the forces.
Net force = Sum of forces
Usually need to break force into x and y components

Question 11

If an object’s acceleration is zero, its velocity must be ___________

Answer 11

If an object’s acceleration is zero, its velocity must be constant. If the net force on an object is zero, the object moves with constant velocity. This is Newton’s first law.

Question 12

Forces are measured in what? What are the components of this unit?

Answer 12

Newton. (appx 1/4 lb)
N= Kg . m/s^2

One newton is defined as the force required to give one kilogram of mass an acceleration of 1 m/s^2.
Dividing a N by its Mass equals its acceleration (kg will cancel leaving m/s^2)

Question 13

The net force acting on a Cessna 172 airplane has a magnitude of 1900 N and points in the positive x direction. If the plane has a mass of 860 kg, what is its acceleration?

Answer 13

1900 N / 860 kg = 2.2 m/s^2

Question 14

What is a free body diagram?

Answer 14

A sketch of the object that include every external force acting ON the object.

Question 15

What is the Normal Force?

Answer 15

acts perpendicular to the surface.
“support force” .

Question 16

What are the steps to building a free body diagram?

Answer 16

Sketch the forces, Isolate the object of interest, choose a convenient coordinate system, Resolve forces into components, Apply newtons second law to each coordinate (F=ma)

Question 17

All forces come in _____ but _____ _____

Answer 17

All forces come in equal but opposites pairs

Question 18

Newtons third law regarding Forces:
the forces in a pair always act on _______ objects, are ________ in magnitude, and point in _________ directions

Answer 18

The forces in a pair always act on different objects, are equal in magnitude, and point in opposite directions

Question 19

Newtons third law - force must be on two different objects. How is this drawn in a free body diagram?

Answer 19

Only draw the force that works on the object in question. Do not draw its pair force because it is on a different object

Question 20

Weight vs Mass

Answer 20

W=ma, weight includes gravitational force. Weight is measured in Newtons. Weight is considered a force, therefore a vector

Mass- the measure of the inertia of an object, does not change due to gravity. Measured in kg

Question 21

Equation for weight: Fg=

Answer 21

Fg=m * g
use logic to determine sign for g

Question 22

Applied force

Answer 22

any force action being applied on an object.

Question 23

What are the 3 common forces on an object?

Answer 23

Normal force (N), applied force (F), and gravitational force (W for weight or FG)

Question 24

Dynamic vs static equilibrium

Answer 24

Dynamic is moving constantly. Velocity is constant but not zero.

static is stationary- velocity is constantly zero

Question 25

What is the sum of the forces when in equilibrium?

Answer 25

always zero. Because acceleration is zero in equilibrium (a= change in v/t and change in v is zero), so sum of forces=m*a, a=0 so sum of F=0

Question 26

True or False: Every force has one and only one 3rd law pair force

Answer 26

True Every force has one and only one 3rd law pair force They must act on different objects and in different directions

Question 27

True or False: The two forces in each pair may have different physical origins (for instance, one of the forces could be due to gravity, and its pair force could be a normal contact force).

Answer 27

False Each type of physical interaction (e.g., gravity, friction, or electrostatic) always produces a pair of forces. If it did not, the center of mass of the universe would accelerate as a result of the unmatched forces

Question 28

What does it mean if the Normal force is negative?

Answer 28

the tension force is exceeding the Weight force (Fg). There is no normal force, the object would have been raised off the ground by the tension force.

Question 29

What is the equation for FN when on an incline?

Answer 29

m g cosθ Know how to derive this

Question 30

How is mass related to the force? How is acceleration related to force? If you double the mass how does this affect the force? If you double the acceleration and the mass, how does it affect the force?

Answer 30

Doubling the mass will double the force. F=ma F=2*1=2 Doubling the mass and acceleration will increase the F by a factor of 4. F=2*2=4

Question 31

What is inertia?

Answer 31

a tendency of an object to resist or maintain a state of rest. a larger object (more mass) will maintain its inertia better than a light object (less mass)

Question 32

Two forces have magnitudes F1 and F2. If these forces are added together, which of the following statements about the net force is correct?

Answer 32

The net force can have a magnitude less than F1+F2

Question 33

What do I need to remember about work?

Answer 33

Scalar! Not directional. Negative means negative

Question 34

If F and d are in opposite directions, what does that mean about your work?

Answer 34

It should be negative. W= F cos(180) * d Cos(180)= -1

Question 35

True or False: A net force is required to make an object accelerate

Answer 35

True

Question 36

True or False: An object that is moving will continue to move with the same velocity unless a net force acts on it

Answer 36

True

Question 37

What does the phrase "no net force" mean

Answer 37

No forces act on the object or Forces act on the object but they sum to zero

Question 38

Two boxes—one large and heavy, the other small and light—rest on a smooth, level floor. You push with a force F. Is the contact force between the two boxes (a) the same in either case, (b) larger when you push on the large box, or (c) larger when you push on the small box?

Answer 38

The contact force is larger when you push on the small box. Because F=ma. A is the same for both but the mass differs and m is inversely proportional to a. To put it another way, the contact force is larger when it must push the larger box

Question 39

An object is acted on by a single force that is at an angle of 35° above the positive x axis. The object’s x component of acceleration is 1 m/s2 . Is the y component of the object’s acceleration greater than, less than, or equal to 1 m/s2. Explain.

Answer 39

Less than 1 m/s2 because the y component of a vector is less than the x component when the vector is only 35° above the x axis

Question 40

Two identical springs are attached to two different masses, MA and MB, where MA is greater than MB. The masses lie on a frictionless surface. Both springs are compressed the same distance, d, as shown in the figure. Which of the following statements describes the energy required to compress spring A and spring B?

Answer 40

Spring A requires the same amount of energy as spring B. Mass is not a factor in springs energy Espring= 1/2 k d^2

Question 41

When is the FN of an object equal to the W force?

Answer 41

When the object is not moving FN-W=ma FN-mg=0 FN=mg

Question 42

Describe the components of normal force when on an incline. Where are your FN, F sinθ, F cosθ, W, and θ?

Answer 42

Whenever a surface is tilted by an angle θ the weight W makes the same angle θ with respect to the negative y axis W always to the center of the universe as that is the pull of gravity To find its coordinates (because your plane has changed) Fcosθ is your Wy (which is your FN) and Fsinθ is your Wx FN is perpendicular to your surface

Question 43

Driving down the road, you hit the brakes suddenly. As a result, your body moves toward the front of the car. Explain, using Newton’s laws.

Answer 43

The force exerted on the car by the brakes causes it to slow down, but your body continues to move forward with the same velocity (due to inertia) until the seat belt exerts a force on it to decrease its speed. First law states a moving object will continue to move at the same velocity until a force makes it slow down

Question 44

You’ve probably seen pictures of someone pulling a tablecloth out from under glasses, plates, and silverware set out for a formal dinner. Perhaps you’ve even tried it yourself. Using Newton’s laws of motion, explain how this stunt works.

Answer 44

Inertia, Newtons first law. There is not enough force from the tablecloth to make the dishware move. "An object at rest will stay at rest until an outside force acts on it"

Question 45

As you read this, you are most likely sitting quietly in a chair. Can you conclude, therefore, that you are at rest? Explain.

Answer 45

No. You are at rest relative to your immediate surroundings, but you are in motion relative to other objects in the universe

Question 46

A young girl slides down a rope. As she slides faster and faster she tightens her grip, increasing the force exerted on her by the rope. What happens when this force is equal in magnitude to her weight? Explain.

Answer 46

When the magnitude of the force exerted on the girl by the rope equals the magnitude of her weight, the net force acting on her is zero. As a result, she moves with constant velocity- constant velocity could be 0

Question 47

Since all objects are “weightless” in orbit, how is it possible for an orbiting astronaut to tell if one object has more mass than another object? Explain.

Answer 47

An astronaut can tell which of two objects is more massive by pushing on both with the same force. Since acceleration is inversely proportional to mass, the more massive object can be recognized by its smaller acceleration.

Question 47

A whole brick has more mass than half a brick, thus the whole brick is harder to accelerate. Why doesn't a whole brick fall more slowly than half a brick?

Answer 47

The whole brick experiences twice the force of gravity, as well as more mass. Because the whole brick has both higher force and higher mass, the ratio is the same causing the acceleration to be the same.

Question 47

The force exerted by gravity on a whole brick is greater than the force exerted by gravity on half a brick. Why, then, doesn’t a whole brick fall faster than half a brick? Explain.

Answer 47

The whole brick has twice the force acting on it, but it also has twice the mass. Since the acceleration of an object is proportional to the force exerted on it and inversely proportional to its mass, the whole brick has the same acceleration as the half brick.

Question 48

You drop two balls of equal diameter from the same height at the same time. Ball 1 is made of metal and has a greater mass than ball 2, which is made of wood. The upward force due to air resistance is the same for both balls. Is the drop time of ball 1 greater than, less than, or equal to the drop time of ball 2?

Answer 48

even though air resistance acts equally on both balls, the metal ball, with its greater mass, is less affected by this resistance and falls faster

Question 49

Is there such thing as a perfectly smooth surface?

Answer 49

No, on a microscopic level there are always "grooves"

Question 50

The force of kinetic friction depends on......

Answer 50

the normal force

Question 51

What to remember about the coefficient of friction: Units and typical values and direction The greater the coefficient the _______ the friction

Answer 51

It is unitless, always positive, usually between 0 and 1, tends to oppose motion The greater the coefficient the greater the friction

Question 52

Kinetic friction things to know: __________to the magnitude of the normal force, N, between the surfaces __________of the relative speed of the surfaces. __________of the area of contact between the surfaces

Answer 52

Question 53

If a system is inclined at an angle θ relative to the horizontal, the normal force responsible for the kinetic friction is _____ than the weight of the object.

Answer 53

If a system is inclined at an angle θ relative to the horizontal, the normal force responsible for the kinetic friction is LESS than the weight of the object.

Question 54

If two materials are similar, does this increase or decrease the amount of friction?

Answer 54

similar materials increase friction, dissimilar will decrease friction

Question 55

What factor will not impact friction?

Answer 55

speed

Question 56

Can static and kinetic friction happen together?

Answer 56

No

Question 57

Static friction vs Kinetic friction

Answer 57

static is stationary. It will stay still until a max friction is reached then it will move and become kinetic friction. Because of this static friction we can only find the max static friction (the point at which it will begin to move.)

Question 58

Which will be stronger (larger in value), an objects static friction or an objects kinetic friction?

Answer 58

Kinetic friction will always be LESS than FsMax. Once an object is moving it then needs less force to continue to move (it has momentum to help unless static) "Static friction is typically stronger than kinetic friction because when surfaces are in static contact, their microscopic hills and valleys nestle down deeply into one another, forming a strong connection between the surfaces"

Question 59

What is the critical angle for friction?

Answer 59

The angle at which an object just begins to slide

Question 60

What to remember about free body diagrams with strings?

Answer 60

Each object connected gets its own free body diagram

Question 61

String: Different masses will cause different _________ but same ________

Answer 61

Different masses will cause different directions but same acceleration

Question 62

Assumptions for strings: 3 things to know

Answer 62

String is massless, Tension is consistent Force tensions are equal even if masses are unbalanced

Question 63

Hooke's Law

Answer 63

Fspring= k * change in position

Question 64

When talking about springs, what are the units and what is meant by the delta x?

Answer 64

Units are N / meter. Delta x is how far the spring is from its "happy place" or its equilibrium state.

Question 65

When solving for mew s (static friction coefficient), what do we need to remember about a?

Answer 65

Its zero in both the x and the y. It has not started to move yet in the x and does not move in the y. This is important because Mass cancels, Gravity cancels, a=0, so to find the mew its just tan θ (must understand how to get here, show your work)

Question 66

A car drives with its tires rolling freely. Is the friction between the tires and the road kinetic or static?

Answer 66

The friction is static because the bottom of the tire is in static contact with the road. If the car skids, then it is kinetic.

Question 67

Which car would be safer to ride in? I. A car with its front wheels locked. II. A car with its rear wheels locked.

Answer 67

It would be safer in the car with the front wheels locked. The front wheels will have static friction and stay in front of the car since it is the stronger force. If the rear wheels locked, they would have static friction, a stronger force, and the car would spin out.

Question 68

If you double the extension exerted on a spring what does that do to the force?

Answer 68

Springs exert a force proportional to the amount of stretch or compression.

Question 69

The larger the value of k (spring constant), the )________ the spring.

Answer 69

The larger the value of k, the stiffer the spring.

Question 70

When a ball is attached to a string and spun around in a circle at a constant speed. How does it have acceleration if its a constant speed?

Answer 70

Acceleration is the change in direction. As the ball spins around and around, the force is directed to the center of the circle, therefore is force has a change of direction. This is centripetal acceleration

Question 71

The direction of centripetal acceleration is ____________ and the equation for its magnitude is _______

Answer 71

The direction of centripetal acceleration is always toward the center of the circle. The equation for its magnitude is a= v^2 / r so if F cent= ma cent, then Fcent= mass* (v^2/r)

Question 72

Tangential acceleration vs Centripetal acceleration

Answer 72

Tangential changes the speed Centripetal changes the direction (because always points to radius)

Question 73

Define work Units how does speed affect work?

Answer 73

Force times Distance, scalar W=Fd cos θ (cosθ used when force is in a different direction than distant) N*m=Joule Speed does not affect work done.

Question 74

Is work positive, negative, or zero when : The force is in the same general direction as the displacement (-90 < θ< 90) The force is generally opposite to the displacement. (+/- 90) The force is at right angles to the displacement. (90 < θ < 270)

Answer 74

Question 75

Kinetic Energy formula units

Answer 75

Energy of motion K= 1/2 mv^2 Joules- it is scalar Always positive or zero, can never be negative, (m and v are always postive)

Question 76

What is the work-energy Theorem

Answer 76

work–energy theorem says that when a force acts on an object over a distance—doing work on it—the result is a change in the speed of the object, and hence a change in its energy of motion. The total work done on an object is equal to the change in its kinetic energy. Wtotal=Delta Ke

Question 77

How do you find the total work done on an object? How do you find the work done by each object?

Answer 77

Work is cumulative. So Wtotal=Change in Energy= Final-Initial Each object has its own work done to it: W=Fd cosθ

Question 78

Define power Units The _____ work that is done in a _______ time, the greater the power.

Answer 78

How quickly work is done. Watts or J/s P=w/t The more work that is done in a shorter time, the greater the power.

Question 79

Is it possible to do work on an object that remains at rest?

Answer 79

No. Work requires that a force acts through a distance

Question 80

Potential Energy Types and equations

Answer 80

Potential energy due to gravity - mgh Potential energy due to springs - 1/2 k d^2 Units are Joules. d and h can be negative, so potential energy can be neg.