Announcements
•
Exam 1 is being graded.
•
Solutions will be available before end of today.
•
Hope to post grades on e-learning by Friday.
•
You will get back your exam on March 2nd.
•
If you forgot to bring ID during exam, you must
see Prof. Chan with your ID or your exam will not
be graded.
Momentum of an object
Momentum
r
∆p m(v f − v i ) r
=
= Fnet
∆t
∆t
alternative statement of
Newton’s second law
The time rate of change of momentum of an
object is equal to the net force acting on it
p
t
Impulse
„
When a single, constant force F acts on
the objectr for time ∆t, there is an
impulse I delivered to the object:
r r
I = F∆t
Using
F
= area under F-t curve
r
r ∆p
F=
∆t
Impulse
r
r
r
r r
I = ∆p = m(v f − vi ) = F∆t
t
Average Force in Impulse
„
Definition: The average force = the constant
force that would give the same impulse to
the object in the time interval as the actual
time-varying force gives in the interval
r
r
r
I = ∆p = Fav ∆t
20. A pitcher throws a 0.15-kg baseball so that it
crosses home plate horizontally with a speed of 20 m/s.
The ball is hit straight back at the pitcher with a final
speed of 22 m/s. (a) What is the impulse delivered to the
ball? (b) Find the average force exerted by the bat on the
ball if the two are in contact for 2.0 × 10–3 s.
pitcher
20 m/s
positive
batter
22 m/s
Collision of 2 objects
Conservation of Momentum
„
Momentum in an isolated system in
which a collision occurs is conserved
„
„
An “ isolated” system: no external forces
the total momentum before the collision
equals the total momentum after the
collision
Conservation of Momentum
„
„
„
Momentum is conserved for the system
of objects. You must define the isolated
system
Assumes only internal forces are acting
during the collision
Can be generalized to any number of
objects
Types of Collisions
„
„
Momentum is conserved in any collision
Inelastic collisions
„
„
Kinetic energy is not conserved
„ Some of the kinetic energy is converted
into other types of energy such as heat,
sound, work to permanently deform an
object
Perfectly inelastic collisions occur when the
objects stick together
„ Not all of the KE is necessarily lost
Types of Collisions
„
Elastic collision
„
„
both momentum and kinetic energy
are conserved
Actual collisions
„
Most collisions fall between elastic
and perfectly inelastic collisions