Download Physics Newton`s 3 Laws of Motions

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Transcript
Physics
Newton’s 3 Laws of Motions
Newton’s 1st Law
An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same velocity, unless the
object is acted upon by an unbalanced force.
-OR“An object will remain at rest or continue to travel at a constant velocity in the same direction unless acted upon by a
net or unbalanced force.”
•
If NO Forces act on an object- its at rest.
•
If BALANCED Forces act on an object- remains at rest, or continues to travel at a constant velocity.
•
If UNBALANCED Forces act on an object- accelerates or decelerates.
Inertia
This is the resistance an object has to change in its state of motion.
The inertia of an object is a quantity which is solely dependent upon its mass. The greater the mass of an object the
greater its inertia.
Newton’s 2nd Law
The second law states that the acceleration of an object is dependent upon two variables - the net force acting upon the
object and the mass of the object. The acceleration of an object depends directly upon the net force acting upon the
object, and inversely upon the mass of the object.
The net force is equated to the product of the mass times the acceleration.
 F=mxa
Momentum
Momentum is the physical quantity which takes into account both the mass of an object and its velocity.
Linear Momentum (p) of an object is the product of the mass (m) of the object and its velocity (v)
p=mv
“Mass in motion”
Impulse
In a collision, an object experiences a force for a specific amount of time that results in a change in momentum. The
result of the force acting for the given amount of time is that the object's mass either speeds up or slows down (or
changes direction). The impulse experienced by the object equals the change in momentum of the object. In equation
form, F • t = m • Δ v.
Impact Forces
Forces which result from changes in momentum are known as impact forces. Impact forces occur when one object
strikes another, for example when a footballer kicks a ball.
Consider an object of mass, m and initial velocity, v1. A retarding force act on the object such that after t seconds, its
velocity reduces to v2.
Therefore, it’s initial momentum = m x v1
and final momentum after t seconds = m x v2.
The change in momentum (∆p) = mv2 – mv1
= m (v2 – v1)
The rate of change of momentum = change in momentum
time
= m (v2 – v1)
time
= mass x rate of change of velocity
= mass x acceleration = Force
Force is equal to the rate of change of momentum
Conservation of Momentum
Providing that the vector sum of the external forces acting on a system is zero (Ʃ F = 0). This is when the total linear
momentum of that system remains constant during collisions. This is a statement of the Principle of Conservation of
Linear Momentum.
Consider the head on collision between two objects m1 and m2.
Momentum before collision: Ʃ pb= m1v1 – m2v2
Momentum after collision: Ʃ pa= m1v’1 – m2v’2
**Since momentum is conserved, m1v1 – m2v2 = m1v’1 – m2v’2
Momentum and Collisions
Collisions play a central role in many areas of physics. Collisions may be elastic (bounce off) or inelastic (stick together).
Elastic: Momentum conserved, kinetic energy conserved and total energy conserved.
Inelastic: Momentum conserved, kinetic energy not conserved, total energy conserved.
Newton’s 3rd Law
For every action there is an equal and opposite reaction.
It is important to note, forces exist in pairs- ‘equal and opposite action-reaction pairs’