physics exam 2

mass is defined as

the amount of matter in an object and is measured in kilograms

force is defined as

• push or pull exerted on an object and is measured in newtons

• 1 N=1kg x m/s²

forces are considered

vectors! added by vector addition

what explains the behavior of all forces

newtons 3 laws

newtons first law

• if an object has constant velocity or is at rest the total force acting on the object must be zero

if an object is moving at a constant seed in a straight line _____

• the net force is zero

• object will tend to stay at zero

• newtons first law applies

if an object has constant velocity (including v=0) then either:

1. no forces are acting on it

2. the forces acting on it cancel out (the more common occurrence)

mathematics of newtons first law

if v=constant

then ∑F=0

newtons second law described

• if an object is accelerating, there must be a net force acting on it

• an object will accelerate in direct proportion to its mass

mathematics of newtons second law and how the first law applies

∑F= M x A

• when only force acts in an object this equation is true

• newtons first law is when acceleration is zero, therefor the sum of the force is also zero

newtons 3rd law

• for every action force, there is an equal and opposite reaction force

• for the two forces tp be action-reaction pair the must both be the same type of force (gravity, friction etc)

• each of the forces in a pair acts on a diff object else they’d cancel out

the force of gravity (weight)

• gravity acts on all objects with mass attracting them to each other (newtons third law)

how do we define weight

the strength of the force of gravity on an object

how do we find the force of gravity

F_g=mg

• F_g is the force of gravity pulling down - measured in newtons (N)

  • can also be called weight

• m is mass in kg

• g= 9.8 m/s² on earth

“normal force”

• caused by two different surfaces pushing against one another

• consequence of electron-electron repulsions

• changes depending on how much force is applied against it

• always acts perpendicular to the surfaces in contact with

• key word “apparent weight”

tension

• force transmitted along a string, rope, wire cable or chain

• caused ny atomic bonding within a material (atoms pulling on each other to stay together when stressed)

• tension force can grow stronger or weaker

• same tension force always acts in both directions along a string (newtons third law)

if two cubes are the same size, but different mass, and both free falling what is the force exerting on each other? (assuming no air resistance)

net force is ZERO!

• acceleration is gravity acting on both cubes

in the absence of a net force (net force = 0) an object must not be

accelerating!

newtons second law of motion states when a net force is acting upon an object the acceleration is

inversely proportional to the objects mass

(ex. smaller animals have larger acceleration vs bigger animals)

when the sum of net forces is zero what does this imply

• the object is NOT accelerating

• could be at rest or could be at constant velocity

an objects inertia is solely dependent upon

its mass

the more mass an object has…

the more inertia

inertia is described as

the tendency of an object to resist changes in its state of motion

is weight a measure of inertia?

NO, weight is a the force of gravity and mass. gravity has no effect on an objects inertia

the inertia of an object is ____ of its location

independent!!! can be anywhere, only depends on MASS

is inertia considered a property of force?

no. strictly a property of mass

• only describes how much an object is resisting its change in motion

which of the following are NOT examples of force?

• gravity on a stone

• friction between ice and hockey puck

• inertia of a car

• pushing a door open with arm

• attraction between two magnets

• heat from stove

• energy used to turn the tv on

• tension of a stretched string

• light from a lamp

• light from lamp

• energy used for tv

• heat from stove

• inertia of car

difference between mass and weight

MASS: units of kg, not dependent on gravity, inertia of an object

WEIGHT: dependent fully on gravity, measured in newtons

Lucy boards a crowded bus as it sits in the station, there are no available seats so Lucy stands in the center aisle next to a support pole. as the bus starts moving forward and picks up speed, Lucy holds the pole tightly to avoid falling backwards.

1. When the bus is speeding up, what is the direction of the total force on Lucy?

2. As the bus drives down a straight road at a constant speed, what is the direction of the total force on Lucy?

3. When the bus comes to a sudden stop at a red light and Lucy grips the pole tightly to avoid falling over what is the direction of the total force on Lucy?

1. Forward (the bus is moving forward, Lucy is on the bus and the pole is acting upon her)

2. zero, constant velocity

3. backwards, deceleration

the force of earths gravity pulls down on a stone after someone tosses it off a cliff what is the equal and opposite force during this reaction and which law is it

• newtons third law

• the force of the stones gravity pulling “up” on the earth

force is defined as

a push or pull on object

• defined in newtons

• newtons are quite small

when you a driving your car in constant velocity…. what is happening with the forces

they are canceling each other out

free body diagram of airplane

• Fg = force of gravity (x axis)

• Flift = against force of gravity (y axis)

• f engine = x axis forward

• f drag = x axis backwards, same force opposite direction - fluid drag

N in free body diagram

represents normal force (against gravity)

when drawing free body diagram IF the object has constant velocity the forces

MUST cancel out

if only one force is acting on an object we can write the equation as

force = MxA

newtons third law pair and the force of gravity

• any and all objects with mass are attracted to one another

• the strength of the force of gravity is defined as weight

Fg=mg means

weight!

• fg is the force of gravity pulling on an object in newtons

• m= mass

• g= 9.8 m/s2 on earth

normal force

• caused by two different surfaces pushing against one another

• consequence of electron electron repulsion

• normal force always acts perpendicular to surfaces in contact with

if a problem asks us to find the “apparent weight” of an object

it is asking for the normal force

friction overall definition

• force opposing motion between surfaces in contact

• most common when pushing or pulling

• felt before object begins moving

friction is caused by

electron sharing between neighboring surfaces

kinetic friction acts on objects which are

sliding and opposes their motion

static friction

acts on objects which are stationary and prevents them from moving

kinetic friction

acts on objects which are sliding and opposes their motion

static friction

acts on objects which are stationary and prevents them from moving

kinetic friction math

f_k=U_KN

• also long as you know U_k you can always find fk with this

• you can also find fk from sumF=0 or sumF=ma if provided with enough info

Static friction math

• f_s < U_sN

• can only be used to find F_s if you know that the maximum value has been reached

• otherwise you have to find F_s from sum of F=0