Physics Chapter 4: Force and Motion
a ball is attached to a string at the top and the bottom -how would you pull to get the upper string to break -how would you pull to get the lower string to break
- to break the upper sting...you keeping pulling down straight with a large force so there is more tension in the upper string to break it -to break the lower string, pull it with a sudden jerk. by 3rd law, the ball will tend to remain at rest so the tension in the upper string will not increase as much as the tension in the lower string and the lower string will break
What are the 4 fundamental forces? Which fundamental force do we focus on in 211
. gravitational force (the weakest by far) • electromagnetic force (we study this next semester) • weak force (used to explain decay of atoms • strong force (interaction between particles in the nucleus) We are focusing on gravity
Mew (u) is always less than or close to BUT NEVER OVER
1
3 things to note of acceleration in relation to mass and force
1. Acceleration is proportional to force 2. Acceleration is pointing in the same direction as force 3.acceleration is inversely proportional to lass
What Newton's laws are qualitative
1st and 3
What Newton's laws are quantitative
2
If mass is doubled what happens to acceleration?
Acceleration is halved since acceleration is inversely proportionate to mass
Equilibrium in physics means
All forces acting on object add to zero ( balanced forces)
Suppose the masses of two objects are the same and that the objects are moving toward each other at the same speed: Suppose the masses of two objects are the same and that object 1 is moving toward object 2, but object 2 is at rest Suppose the mass of object 1 is much less than that of object 2 and that it is pushing object 2 which has a dead motor so that both objects move in the same direction at speed v. Predict the relative magnitudes of the forces between object 1 and object 2.
All forces of object 1 are equal and opposite to object 2
A ball sitting still on a table is a balanced or unbalanced force?
Balanced force since the ball exerting force is equal to the table exerting force right na m
Doll being pulled at an angle on either side Equilibrium or not Equilibrium
Both Equilibrium or not Equilibrium Equilibrium in the x direction most likely but NOT in the y
When throwing a mass, what do you notice with center of mass? Why do I care to know this, in relation to something I do!
Center of mass, literally it finest really change as its being thrown When we draw our free body diagram, we are shrinking the object down so that all its mass is located at the center of mass.
Book at rest on an elevator Equilibrium or not Equilibrium
Equilibrium
Book on an elevator at a constant velocity Equilibrium or not Equilibrium
Equilibrium
If a doll is being pulled F on the left and F on the right Equilibrium or not Equilibrium?
Equilibrium
Schaffer's Newton's First Law
Every object continues in its state of rest or of uniform *velocity* (motion at uniform speed in a straight line) unless acted upon by an unbalanced force (a net force). Conversely, if an object is at rest or in motion with uniform velocity, all forces that may be acting on it must cancel so that the net force is zero.
Is the force that makes you move need to be internal or external?
External force!!!
T or f: constant motion requires a constant cause Give an example to explain your reasoning
False What is keeping a ball moving through the air? Nothing!!! The pitcher let's go, the ball cant move itself, and air resistance only slows it down
If a question asked you to find the weight or Find "Fg".... what do you have to do to find Fg?
Fg= mass times force of gravity
once the object is sliding,,....the force of friction changes to _________________friction,=... this has a magnitude of
Fk=umkN
Newton's Second Law
Fnet=ma Force and acceleration or vectors and mass is a scalar
How do you find Fnet? ( hint: you do it the same way if you would want to find a resultant vector)
Fnetx=m x ax Fnety= m x ay Then find the magnitude from Fnetx and Fnety
Newton's Third Law
For every action(force) there is an equal and opposite reaction (force)
What does "dyna" in dynamics stand for?
Forces
constant forces
Forces that are a result of directly touching (physical contact) ex: open a door or kick a ball
Microscopic slides are difficult to separate bc a lot of......due to
Friction due to bonds that are forming btw them both
Svetlana's summarized 1st law of motion as an equation
If Fnet=0....v is constant and acceleration is zero
In answer to the question "What keeps the earth moving around the Sun?", your friend asserts that inertia keeps it moving. Correct your friends erroneous assertion?
Inertia isn't a force! It is a characteristic of an objects resistance to change If there is no force on earth, the earth will go in a straight line but the earth has a changing velocity
How can you break a meter stick by whipping it?
It you whip it and stop it quickly, the acceleration may be to great and break the stick
Newton's standard unit
Kg x m per second 2
Kinetic friction is always..._________ no matter what the objects velocity is of weight
Kinetic friction is always a constant value, unlike static which is always changing
How do I know if m2 to m1 is exerting a force?
M1 wants to move right and m2 is in the way of M1 and is opposing its motion
What do we assume now with "physics strings" (2 characteristics) why?
Massless and strechless Because if we didn't... string is normally never horizontal bc if you pull a string on either side in the air...only the force of gravity will be acting on it because nothing balances it That means tensions on the string will be at an angle bc the string sags down And angle tension gets messy
Fsmax= (words first then actual equation)
Max force one can apply b4 it starts moving =usN Or Fsmax< or equal to uFN
What are the main 2 forces we will be discussing?
Mechanical and gravitational
What is the "sexy" symbol
N for Newton's
Applied force at an upward angle N compared to mg
N<mg
Applied force at downward angle... How does N compare to gravity
N>mg
If a glove pushes M1 and m1 pushes in m2.... and each are a reaction pair... is the glove and m2 a reaction pair?
No!!! The glove can't be a reaction pair with m2 because they aren't physically touching. A reaction pair is only a thing if the 2 are touching
Types of forces (8)
Normal Force, Contact force, Tension, Weight, Spring, Centripetal, Friction, Drag.
Friction DOES depend on___________contact
Normal force contact
Normal Force (Fn) What does normal mean?
Normal means perpendicular The perpendicular force exerted by a surface on an object in contact with it
If a doll is being pulled 2F to the right and F to the left Equilibrium or not Equilibrium
Not equilibrium
Doll being pulled F on the hand to the right and F on the hand and F on the leg to the right Equilibrium or not Equilibrium
Not equilibrium!!!
Asteroids have been moving through space for billions of years. What keeps them moving?
Nothing keeps asteroids moving. They are "happily" moving through space in a state of dynamic equilibrium. Objects like the Sun and Earth can change their motion but after this brief interaction they again move off at some constant speed
Translation Equilibrium
Occurs when all the forces on a body are balanced Remains at rest or moves at a constant velocity
•Two main kinds of friction
Sliding friction tries to prevent one object from sliding over another... •Air friction tries to prevent objects from moving through the air
What are the only 2 things that can exert a force on an object?
Something touching it or gravity
Friction DOESNT depend on _______ contact
Surface area
Where does tension force point?
Tension force points *inwards* on both sides
What falls under mechanical forces?
Tension, friction, compression
When a bug crashes into a bug and splats, what is the reason why this happens (explain in terms of acceleration)
The bugs acceleration is larger if it is exerting the same as the bus...so when they crash together the bug dies bc the acceleration is too great and the mosquitos structure can not withstand it
net force
The combination, vector sum, of all forces acting on an object
What causes the force of gravity?
The earth
Svetlana of Fa,b in words
The force of A on B
F1,2 represents in words
The force on object one due to object 2
What force makes you move while you walk
The force you your feet on the ground is opposite to the force of the ground on your feet
Force of Friction (Ff)
The friction force is the force exerted by a surface as an object, solid, liquid or gas, moves across it. (whenever 2 medias are in contact with each other)
A farmer urges his horse to pull a wagon. The horse refuses, saying to try would be futile for it would flout Newton's Third Law. the horse concludes that she can't exert a greater force on the wagon than the wagon exerts on her, and therefore won't be able to accelerate the wagon. What is your explanation to convince the horse to pull?
The horse just has to produce a greater force on the ground compared to the cart on the horse....and the horse will move in the net direction forward
The closer 2 surfaces are in contact to each other... the more...
The more friction
Block and pulley system The more pulley string you add,...what happens to the force of needing to pull? What happens to the height lifted with the more pulley strings?
The more pulley strings you add, the easier it is to pull so the less force you need The height lifted with the more pulley strings actually decreases bc with decrease in force needed it will not go as high
Intertia
The natural tendency of an object to maintain a state of rest or remain in uniform motion in a straight line (constant velocity)
Suppose a mosquito is flying through the air and is suddenly hit by the windshield of a school bus. Predict the relative magnitude of forces of them
The objects exert the same force on each other
The action reaction forces in Newton's 3rd law do not act on....
The same object!! The second law is concerned with forces acting on a particular object
What causes the normal force?
The surface in the object
What is the reason for friction???thrings aren't
Things aren't smooth
This chapter ( ch4 forces and friction) focuses on : vectors or scalars
Vectors!!! F=ma (a is a vector)
Newton's 3rd law describes by Schaffer What is this law also known as
Whenever one object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first. Action and Reaction Law
Why do you lurch forward in a bus that suddenly slows?
You and the bus are moving at the same velocity....so when the bus slows down...you are still moving with the same velocity forward and that's why a seatbelt is needed to keep you in
If you're on an elevator, what will the scale read?
Your normal force
free body diagram
a physical model that represents the isolated forces acting on a system
an unbalanced net force always produces an...
acceleration
forces can cause
acceleration
Condition for translational equilibrium
acceleration=0 therefore the sum of all forces =0
the larger the object and the faster it moves...the more contact it will make with (what)
air molecules
Newton's First Law
an object at rest will stay at rest, an object that is moving will stay moving with a constant velocity ...unless disturbed by an un balenced force.(non-zero)
2 divisions of forces
contact forces and action-at a distance forces
what does force focus on: kinematics or dynamics
dynamics, what causes things to move
net effect is zero when..
forces are cancelled, this may not produce a change in motion
Action-at-a-distance forces
forces that can have an effect on an object without touching it ex: gravitational forces, electrical forces, magnetic forces
rolling friction
friction that occurs when an object rolls over a surface, but does not slip at the point of area or contact
static friction Is it active or passive
frictional force that prevents two surfaces from sliding past each other (a relative motion between 2 objects) It is a passive force ex:the force of static friction between the desk's legs and the floor opposes and equals the force you are applying....no motion....static condition
if there is no applied force...fs=
fs=0
maximum value of static friction
fsmax=usN
because dropping this off the tower of pizza were difficult to measure force for Galileo, to slow things down to observe motion...galileo did what?
galileo used rolling balls inclined planes
Galileo dropped objects to observe motion under the influence of
gravity
if a force acts ALONE the object on which it acts will
have a change in velocity or an acceleration
a massive object has more________, or more resistance to change in motion than does a less massive object
inertia
massis a quantitative quantity of....(galileos redefined term)
inertia
no matter where you go, your inertia......
inertia= mass inertia stays the same
when the magnitude of F exceeded fs, what happens to the object?
it begins moving
how do coefficient of static and kinetic friction compare
kinetic<static max
Consider a ball at rest in the middle of a toy wagon. When the wagon is pulled forward, the ball rolls against the back of the wagon. Interpret this observation in terms of Newton's first law.
law of inertia (1st law) causes the ball to move backward first and then forward because an object at rest will stay at rest unless acted upon by another force. The ball doesn't have any force acting on it, but the wagon does !
name a situation in an auto body shop were friction would need to be reduced
lubrication or moving machine parts..allowing parts to move freely and lessening wear and reducing expenditure of energy
mk is ______________to ms
mk is usually smaller than ms, this is because it takes a lot of force for something to get moving than it is for something to keep moving
if F = Fk...they object will
move at a constant velocity
when several forces act on an object, the interest is their combined effect or...
net force
kinetic friction Is it active of passive
occurs when there is relative motion (when there's sliding) at the interfaces of the motions in contact It is an active force ex: when pushing on a desk, you eventually get it sliding but there is still a great deal of resistance
Schaefer's definition of normal force,
perpendicular to surface, contact forces are always exerted normal from the surface
Inertia is:
resistance to change in motion
asperities
rough places on surfaces where they "catch" each other
an unbalanced force may change an objects
size and shape
for a ball on a frictionless surface, galileo reasoned that a ball would..
slide on the surface indefinitely
*force*
something that is *capable* of changing an objects state of motion, changing its velocity or producing an acceleration capable is significant here bc its ability to produce a change in motion maybe balanced, or canceled, by one or more forces
maximum constant velocity is when....... it is sometimes called
the air resistance equals the objects weight force terminal velocity
force of friction depends on (2)
the kinds of material in contact and how much the surfaces are pressed together (normal force)
if an object is moving at a constant velocity...there is no
there is no net force
a non zero force is considered a
unbalanced force
force is what type of quantity: vector or scalar
vector
a force can produce a change in... (direction and speed). meaning is a ...
velocity meaning face is an acceleration
mass is related to 3 things
weight, inertia, density
the purpose of a car's antilock brakes is to prevent the wheels from locking up so token the car rolling rather than sliding. Why would rolling decrease stopping distance as compared to sliding?
while rolling....static friction is trying to stop you while sliding....the kinetic friction force is trying to stop you and since the static force is grater than the kinetic force, the car will stop in a shorter distance is it is rolling
when speed doubles, the air resistances increases by...?
x4
the force required to move a rocketship at constant velocity in space is..
zero.... due to newtons 1st law
