Mechanics 1 Flashcards
Dynamics
the explanation of motion in terms of forces that act on an object
Force
push or pull exterted by one object on another
Summary of Forces
If you pull on a rope attached to a crate, you create a tension in the rope that pulls the crate. When a sky driver is falling through the air, the earth exerts a downward foice called the gravitational force and the air exerts an upward force called air resistance. When you stand on the floor, the floor provides an upward, supporting force called the normal force. If u slide na book across the table, the table exerts a frictional force against the book so the book slows down and eventually stops. Static cling provides an example of the elctrostatic force. (All the forces mentioned above with the exception of gravity are ultamitely due to electromagnetic forces).
Newton’s First Law
An objects state of motion - its velocity - will not change unless a net force acts on the object.That is if no net force acts on an object, thenif the object is at rest, it will remain at restand if the object is moving, then it will continue to move with constant velocity (constant speed in a straight line)no net force = no acceleration
Inertia
property of objects that describes their natural resistance to change in their state of motion. The first law is ofeten referred to as the law of inertia
The first law applies that there is no NET force on an object so that means….
could mean that there are no forces at all but more commonly it means that the forces on an object balance out (total of all the forces in each dimension is zero)
The mass of an object is the quantitive measure of its ________
inertia (mass measures how much matter is contained in an object) Mass is measured in kilograms
Newtons second law
If Fnet is the net or total foece acting on an object of mass m then the resulting acceleration of the object, a satisfies this simple equationFnet = ma
Notice that the first law is really just a special case of
the second law, the case in which Fnet = 0
Forces are represented by ________ because a force has a __________ and a ___________
forces are represented by vectors because force has a magnitude and direction
Net force is simply
the sum all individual forces
Fnet is the sum of all the forces that act on the object namely the object whose mass, m, is on the other side of the equation. Any force exerted by the object is not included in Fnet
(How well do u understand this?)
Because mass is a positive number, the direction of acceleration is always the same as the direction of Fnet ……
Therefore an object will accelerate in the direction of the net force it feel, this does not mean that an object will always move in the direction of Fnet .
What if Fnet = 0? Then a = 0. What does a = 0 mean?
It means that the objects velocity does not change, which is also what Newtons first law says.
Does Fnet = 0 mean that v = 0?
NO!!!!! , Fnet = 0 means that an object will not accelerate not that it wont move (VERY IMPORTANT)
Fnet = ma is a vector equations so it automatically means that the components of both sides must be the same. In other words, Fnet could be written as the sum of a force in the horizontal direction plus vertical direction
(Do u understand this)
What is the unit of force?
Its the unit of mass * the unit of accelerationForce = [m] [a] = kg * m/s2A force of kg * m/s2 is called 1 newton (N).1 N = 1/4 of a pound.
Newtons 3rd Law
If object 1 exerts a force, F 1-on 2, on object 2, object 2 will also ecert a force F 2-on-1 . These 2 forces will have the same magnitude in opposite directions (they act on different objects). The 2 forces are said to form an action reaction pair.Note: does not say that the effects of these forced will be the same (different for diff masses)
Newtons 1st law vs Newtons 3rd Law
1st law: all forces must be acting on a single object, net force = sum of vectors3rd law: each force must be acting on a different object in action reaction pair
Newtons 3rd law confusing concepts
1) just because 2 forces are = and oppisire doe snot mean they form an action reaction pair, forces also have to be from 2 objects acting on each other2) 3rd law will apply when objects experience acceleration, 1 object accelerates other one will push back
What to do when calculating forces exerted on 2 diff objects
remember that its opposite, lets say M = 100 and m = 50 and a = 2, the force exerted by the larger crate on the small one is 50 * 2 = 100 N, smaller crate on larger one is 100 * 2 = 200 N
Mass
intrinsic property of an object and does not change with location
Weight
The weight of an object is the gravatational force exerted on it by the earthweight is a force so we can use f=ma to compute, setting a = gwe get w = mg
Newton’s law of Gravitiation
Every object in the universe exerts a gravitational pull on every other object. The magnitude of this gravitational force is proportional to the objects masses and inversely proportional to the sqaure of the disrance between them
Inverse Square Law (Newtons law of gravitation)
The magnitude of the gravitational force is inversely proportional to the square of the distance between the centers of the objects
we can set the eqautions w=mg and Fgrav = GMm/r2 to get
mg = GmM/r2 then we divide by mass to get
g = GM/r2
What do we use this formula for: g = GM/r2
It tells us how to find the value of the gravitational acceleration. On earth we know that g = 10 m/s2.Ex. moon has a smaller mass making gravity much smaller (about 1.6) So remember the value of little g depends on where you are.
Friction
when 2 materials ar ein contact, there is an electrical attraction between the atoms of 1 surface with those of the other , this attraction will make it difficult to slide 1 object relative to the other.
Static Friction
when there is no relative motion between the surfaces that are in contact (when there is no sliding)
Kinetic (sliding friction)
when there is relative motion between the surfaces ( when there is sliding)
Normal Force
perpindicular component of the contact force exerted by a surface on an object
Normal force and gravitational force do not form an action reaction pair when
they act on the same object, remember forces on an action reaction pair always act on DIFFERENT objects
For an object on a horizontal surface that feels no downward force, the normal force will be equal to
the weight of the object
Coefficient of kinetic friction
The constant of proportionality that depends on what the surface is made of and what the object is made of, called coefficient of kinetic friction. This is an experimentally determined position with no units and the greater its value, the greater the force of kinetic friction
maximum coefficient of static friction
2 differences are maximum coeffiecient of static friction is always greater than coeficcient of kinetic friction, kinetic fricttion must be greater than static friction. There is no general formula for force fo static friction (all we have is max). Static friction is capable of supplying any necessary but only up to a certain max.
inclined plane
when object is on a ramp
incline angle
the angle that the plane makes with the horizontal
magnitudes of the components w=mg on a ramp
are mgsinθ (parallel to the ramp) and mgcosθ (perpendicular to the ramp)
Pulley
a device that changes the direction of the tension ( the force exerted by stretched string, cord, or rope)
We use FT or T to denote
tension force
Pulleys are used to
decrease the force neccesary to lift an object
Relating to tension, pulling down on one side will result in a
tension force throughout the entire string. As a result, there will be 2 tension force, each of magnitude FT pulling up to lift the block so we only need to exert half as much force to lift the block
For every single pulley, there will be
2 tension forces on a single pulley, one on each side
If you pull downward on a pulley, tension forces will be pulled
upward
When writing down forces, take the direction of the objects motion to be
POSITIVE so other force is by default negative
Accleration of 1 on 2
force / mass of 2
Acceleration of 2 on 1
force / mass of 1
Direction of friction is opposite to the
direction of motion
Force due to gravity acting parallel to inclined plane
mgsinθ
Force due to gravity acting perpendicular to inclined plane
mgcosθ, where θ is measured between the incline and the horizontal
When does w=mg?
For an object near the surface of the earth
The normal force is always
perfectly perpendicular to contact force
Normal actually means perpenduclar
Elevator
going up an elevator you feel heavier, going down an elevator you feel lighter
feel heavier: add acceleration
feel lighter: subtract acceleration
First of all, eliminate choices “mass 4 kg and weight 40 N” and “mass 9 kg and weight 90 N”. The mass will not change by bringing the object to a new location. Now, if the altitude is 2REarth, then the distance from the center of the Earth is 3REarth, so the gravitational acceleration will decrease by a factor of 32 = 9 (because g =GM/R2). This means the weight will decrease by a factor of 9, which in this case means the new weight is 360/9 = 40 N.
Memorize: if the altitude is 2R earth, then the distance fromthe center of the earth is 3R Earth
Are lbs in weight or pounds
they are in weight so use formula mass = weight / gravity
How do you calculate mass from weight and grams
mass is equal to weight DIVIDED by grams
