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