Transcript NEWTON`S LAWS OF MOTION Second LAW

PHYSICS 103: Lecture 13
Agenda for Today:
• Review of HW
• Momentum
 Impulse
 Conservation of Momentum
 Collisions
Momentum
• Momentum is the product of mass times velocity of an
object P = m v
• Momentum is a vector quantity (same direction as v)
Units = kg m/s
NEWTON’S LAWS OF MOTION
Second LAW:
The force on an object is equal to the product of
that object’s mass times its acceleration. The
acceleration is in the same direction as the force.
F=m.a
a = Dv/Dt
F = m . Dv/Dt
F . Dt = m . Dv
Impulse =F . Dt
Dp = m . Dv
Change in “momentum”
NEWTON’S LAWS OF MOTION
Third LAW:
For every force that one object exerts on a
second object, there is an equal but oppositely
directed force that the second object exerts on
the first object.
(For every action there is an equal but opposite reaction)
F . Dt = m . Dv
If the external force acting on a system of objects is zero, the
total momentum is conserved.
CONSERVATION OF MOMENTUM
During a collision, there are no external forces, so
momentum is conserved.
This means: total momentum before collision = total
momentum after collision
Types of Collisions
• Elastic
- no kinetic energy is lost during collisions
(things bounce off each other)
• Partially
Inelastic
- some kinetic energy is lost during
collisions
• Perfectly
Inelastic
- objects stick together
Example Collision:
Total momentum before collison = Total momentum after collision
v = 10 m/s
v = 0 m/s
Before:
V’ = 5 m/s
After:
(m  10 m/s)before = (2m  V’)after
This is an example of an inelastic collision
Test your understanding:
A 1-kg cart and a 2-kg cart roll toward the center of a straight
track from opposite ends of the track, each with a speed of 1 m/s.
They collide and stick. The combined mass moves at a speed of
(A) 0 m/s.
(B) 1/2 m/s.
(C) 1/3 m/s.
(D) 1/6 m/s.
(E) 1.5 m/s.
1 m/s
1 m/s
Before:
1 m/s
1 m/s
V=?
After:
(2 kg  1 m/s - 1 kg  1 m/s)before = (3 kg  V)after
V = 1/3 m/s
Main Points from Today’s Lecture
• Momentum
You should understand impulse, momentum,
conservation of momentum in different kinds of
collisions. You should review the example problems in
this lecture.