Exam 2 Flashcards
Newtons Laws and Energy
Newtons First Law
An object at rest remains at rest as long as no net force acts on it
Newtons Second Law
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
Newtons Third Law
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
According to Newtons first law,
If the net force on an object is zero, its velocity is _________
If the net force on an object is zero, its velocity is constant
Imagine a ball being held up between two strings. If there is slow force applied to lower string, which string will break?
The top string. The force traveled up the strings and the top string was holding the weight of the ball, which now has force.
Imagine a ball being held up between two strings. If there is sudden force applied to the lower string, which string will break?
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
A net force is required to make an object ___________
A net force is required to make an object accelerate. No net force = no acceleration and vice versa because F=ma.
An object that is moving will continue to move with________ velocity unless a net force acts on it.
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
Acceleration is proportional to _______ and inversely proportional to ________
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
What about Newtons Second Law equation must we remember?
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
If an object’s acceleration is zero, its velocity must be ___________
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.
Forces are measured in what? What are the components of this unit?
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)
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?
1900 N / 860 kg = 2.2 m/s^2
What is a free body diagram?
A sketch of the object that include every external force acting ON the object.
What is the Normal Force?
acts perpendicular to the surface.
“support force” .
What are the steps to building a free body diagram?
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)
All forces come in _____ but _____ _____
All forces come in equal but opposites pairs
Newtons third law regarding Forces:
the forces in a pair always act on _______ objects, are ________ in magnitude, and point in _________ directions
The forces in a pair always act on different objects, are equal in magnitude, and point in opposite directions
Newtons third law - force must be on two different objects. How is this drawn in a free body diagram?
Only draw the force that works on the object in question. Do not draw its pair force because it is on a different object
Weight vs Mass
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
Equation for weight: Fg=
Fg=m * g
use logic to determine sign for g
Applied force
any force action being applied on an object.
What are the 3 common forces on an object?
Normal force (N), applied force (F), and gravitational force (W for weight or FG)
Dynamic vs static equilibrium
Dynamic is moving constantly. Velocity is constant but not zero.
static is stationary- velocity is constantly zero
What is the sum of the forces when in equilibrium?
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
True or False:
Every force has one and only one 3rd law pair force
True
Every force has one and only one 3rd law pair force
They must act on different objects and in different directions
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).
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
What does it mean if the Normal force is negative?
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.
What is the equation for FN when on an incline?
m g cosθ
Know how to derive this
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?
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
What is inertia?
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)
Two forces have magnitudes F1 and F2. If these forces are added together, which of the following statements about the net force is correct?
The net force can have a magnitude less than F1+F2
What do I need to remember about work?
Scalar!
Not directional. Negative means negative
If F and d are in opposite directions, what does that mean about your work?
It should be negative.
W= F cos(180) * d
Cos(180)= -1
True or False:
A net force is required to make an object accelerate
True
True or False:
An object that is moving will continue to move with the same velocity unless a net force acts on it
True
What does the phrase “no net force” mean
No forces act on the object
or
Forces act on the object but they sum to zero
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?
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
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.
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
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?
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
When is the FN of an object equal to the W force?
When the object is not moving
FN-W=ma
FN-mg=0
FN=mg
Describe the components of normal force when on an incline.
Where are your FN, F sinθ, F cosθ, W, and θ?
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
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.
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
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.
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”
As you read this, you are most likely sitting quietly in a chair. Can you conclude, therefore, that you are at rest? Explain.
No. You are at rest relative to your immediate surroundings, but you are in
motion relative to other objects in the
universe
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.
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
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.
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.
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?
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.
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.
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.
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?
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
Is there such thing as a perfectly smooth surface?
No, on a microscopic level there are always “grooves”
The force of kinetic friction depends on……
the normal force
What to remember about the coefficient of friction:
Units and typical values and direction
The greater the coefficient the _______ the friction
It is unitless,
always positive, usually between 0 and 1, tends to oppose motion
The greater the coefficient the greater the friction
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
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.
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.
If two materials are similar, does this increase or decrease the amount of friction?
similar materials increase friction, dissimilar will decrease friction
What factor will not impact friction?
speed
Can static and kinetic friction happen together?
No
Static friction vs Kinetic friction
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.)
Which will be stronger (larger in value), an objects static friction or an objects kinetic friction?
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”
What is the critical angle for friction?
The angle at which an object just begins to slide
What to remember about free body diagrams with strings?
Each object connected gets its own free body diagram
String:
Different masses will cause different _________ but same ________
Different masses will cause different directions but same acceleration
Assumptions for strings:
3 things to know
String is massless,
Tension is consistent
Force tensions are equal even if masses are unbalanced
Hooke’s Law
Fspring= k * change in position
When talking about springs, what are the units and what is meant by the delta x?
Units are N / meter.
Delta x is how far the spring is from its “happy place” or its equilibrium state.
When solving for mew s (static friction coefficient), what do we need to remember about a?
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)
A car drives with its tires rolling freely. Is the friction between the tires and the road kinetic or static?
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.
Which car would be safer to ride in?
I. A car with its front wheels locked.
II. A car with its rear wheels locked.
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.
If you double the extension exerted on a spring what does that do to the force?
Springs exert a force proportional to the amount of stretch or compression.
The larger the value of k (spring constant), the )________ the spring.
The larger the value of k, the stiffer the spring.
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?
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
The direction of centripetal acceleration is ____________
and the equation for its magnitude is _______
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)
Tangential acceleration vs Centripetal acceleration
Tangential changes the speed
Centripetal changes the direction (because always points to radius)
Define work
Units
how does speed affect work?
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.
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)
Kinetic Energy
formula
units
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)
What is the work-energy Theorem
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
How do you find the total work done on an object?
How do you find the work done by each object?
Work is cumulative. So Wtotal=Change in Energy= Final-Initial
Each object has its own work done to it: W=Fd cosθ
Define power
Units
The _____ work that is done in a _______ time, the greater the power.
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.
Is it possible to do work on an object that remains at rest?
No. Work requires that a force acts
through a distance
Potential Energy
Types and equations
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.
