Date post: | 01-Jan-2016 |
Category: |
Documents |
Upload: | kirtana-devaj |
View: | 22 times |
Download: | 0 times |
Sir Isaac NewtonPhilosophiae Naturalis Principia Mathematica (1687)
Opticks (1704)"Nature and nature's laws lay hid in night;God said 'Let Newton be' and all was light."
Newton’s Three LawsInertia:
“Every body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by a force impressed on it.”
Force, Mass, Acceleration (F=ma):“The change in motion is proportional to the motive force
impressed; and is made in the direction of the right line in which that force is impressed.”
“Action = Reaction”:“To every action there is always opposed an equal reaction;
or, the mutual actions of two bodies are always equal, and directed to contrary parts.”
Newton’s Law Summary1. Velocity is zero or constant when net force is zero.
2. F=ma
3. Action = Reaction (in opposite direction)
Net ForceNet force (or total force) is the sum of all the forces applied to an object.
For example, if there are three people, A, B and C pushing the crazy kid. The net force on him is:
First Law and Net ForceThe First Law deals with cases when there is no net force.
“Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by a force impressed on it.”
Puck on iceIce has very little friction (so no net force), so the inertia keeps the puck moving once it is set in motion.
Mass or Inertia Inertia is the tendency of an object to
remain at rest or in motion with constant speed along a straight line.
Mass (m) is the quantitative measure of inertia. Mass is the property of an object that measures how hard it is to change its motion.
Units: kg
Mass vs. Weight
Mass is an intrinsic property of an object. A rock has same mass whether it is on the moon
or on Earth. Mass does not change
Weight is the force exerted on an object by gravity: W=mg This is different depending upon the strength of
the gravitational force. You weigh less on the Moon than on Earth.
ExampleWhat is the weight of a man of mass 70kg on Earth?
Weight is measured in N.(Pound: 1lb = 4.448N)
Newton’s 1st LawThere are many forces act on the plane, including weight (gravity), drag (air resistance), the thrust of the engine, and the lift of the wings. At some point the velocity of the plane is constant. At this time, the total (or net) force on the plane:
1. is pointing upward2. is pointing downward 3. is pointing forward 4. is pointing backward5. is zero
lift
weight
drag thrust
correct
Newton’s 1st LawNewton's first law states that if no net force acts on an object, then the velocity of the object remains constant. Since the velocity is constant, the total force on the plane must be zero, according to Newton's first law. lift
weight
drag thrust
Newton’s Second LawF = ma
Deals with the effect of a non-zero net force.Non-zero net force causes acceleration.
Unit: N (Newton)
F means NET FORCE!!!
The F in F = ma is the net force on the object. If you are careful, you may write instead:Fnet = ma
Always remember to find the net force first!
When Fnet is non-zero
When Fnet ≠ 0, since F=ma, we have a≠0.
No net force, no acceleration.Net force leads to acceleration.
If an object is accelerating, there must be a non-zero net force.
T-mg or mg-T ?In non-vector notation, we usually assume the variable a represents the magnitude of the acceleration, and is therefore positive whether it is up or down. In other words, unless stated otherwise, we will not use the up/positive, down/negative convention.
With this new convention, whenever you have opposing forces, the forces pointing in the same direction as a comes first, minus the forces in the opposite direction as a.
T-mg or mg-T ?
Another way to remember, if a > 0, you want:(big number) - (small number) = maor(same direction) - (opposite direction) = ma
Newton’s Third Law For every force, there is an equal and
opposite force every “action” has a “back-reaction” these are precisely equal and precisely
opposite
Newton’s Third Law
You cannot push without being pushed back just as hard
In tug-of-war, each side experiences the same force (opposite direction)
When you push on a brick wall, it pushes back on you!
Force Pairs Illustrated
Force on person by box
Force on floor by box Force on box by floor
Force on box by person
Force on personby floor
Force on floorby person
Not shown are the forces of gravity and the associated floor forces
Don’t all forces then cancel?
Force on boxby floor
Force on box by person
Net Forceon box
How does anything ever move (accelerate) if every force has an opposing pair?
Action and reaction force act on different objects.
Exercise: Action/Reaction Suppose a tennis ball (m= 0.1 kg) moving at a velocity v = 40
m/sec collides head-on with a truck (M = 500 kg) which is moving with velocity V = 10 m/sec. During the collision, the tennis ball exerts a force on the
truck which is smaller than the force which the truck exerts on the tennis ball. TRUE or FALSE ?
The tennis ball will suffer a larger acceleration during the collision than will the truck. TRUE or FALSE ?
Suppose the tennis ball bounces away from the truck after the collision. How fast is the truck moving after the collision?
< 10 m/sec = 10 m/sec > 10 m/sec ?
Exercise: Action/Reaction solution During the collision, the tennis ball exerts a force on the
truck which is smaller than the force which the truck exerts on the tennis ball. TRUE or FALSE ?
Equal and opposite forces!
The tennis ball will suffer a larger acceleration during the collision than will the truck. TRUE or FALSE ?
Acceleration = Force / mass
Suppose the tennis ball bounces away from the truck after the collision. How fast is the truck moving after the collision?
< 10 m/sec = 10 m/sec > 10 m/sec ?
Force from the ball causes deceleration.
Normal ForceForce from a solid surface (e.g. wall, ground) providing support for an object.“Normal” means “perpendicular”, Normal Force is always perpendicular to the solid surface.
Notations:FN, Fn, N or n