1
Physics 2010
Laboratory 3: Momentum and
Energy
NAME ___________________________
Section Day (circle): M
Tu
W
Th
F
Section Time:
8a
10a
12p
2p
4p
TA Name:
________________________
In this experiment you will use air tracks, gliders, and photogate timers to study the concepts of
momentum and kinetic energy and how they behave when objects interact (that is, when they
"collide"). These concepts are critical for many realworld issues, so are valuable even outside the
sciences.
Background Information
Here we discuss the
momentum
and
kinetic energy
of an object.
Imagine you are in a car stopped at
a light. Prof. Dubson comes up behind you, his mind on some obscure physics problem, and rams
into your rear bumper. Call his mass and his car’s mass,
m
1
,
the mass of you and your car
m
2
, and
the velocity with which he hit you
v
1
i
(i for "initial"). You started at rest, so
v
2
i
= 0. Immediately
after the cars collide, both you and he will be moving at new velocities: he moving forward with
velocity
v
1
f
and you moving with velocity
v
2
f
(f for "final").
During the very short time (call it
Δ
t
) that the cars were in contact, we know by Newton's third law
that the force on Dubson’s car (car 1) by your car (car 2) is equal in magnitude and opposite in
direction to the force on your car by Dubson’s car:
F
21
= –
F
12.
(1)
We also know, by Newton's second law, that
F
net
=
m
a
for
each
car, so that during the contact
between the cars:
m
1
a
1
= –
m
2
a
2.
(2)
For constant acceleration
a
=
Δ
v/
Δ
t = (
v
f
–
v
i
)/
Δ
t, so we can rewrite Eq. (2) as:
m
1
(
v
1
f

v
1
i
)/
Δ
t = –
m
2
(
v
2
f

v
2
i
)/
Δ
t
(3)
Multiplying both sides by
Δ
t and moving things around (being careful with negative signs), we get
m
1
v
1
i
+
m
2
v
2
i
=
m
1
v
1
f
+
m
2
v
2
f
(4)
Calling an object's
momentum
p
=
m
v
(note that it's a vector), we can see that the
total momentum
of the system before the collision, that is, the sum of the momenta of both cars, equals the total
momentum of the system afterwards: hence
momentum is conserved
. (Note that the momentum of
each car individually changes! Only the
total
momentum stays the same.)
Another useful quantity is the kinetic energy. This is defined as
KE
=
½
mv
2
, so for example Prof.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
This is the end of the preview.
Sign up
to
access the rest of the document.
 Spring '06
 DUBSON
 Physics, Energy, Kinetic Energy, Momentum, ΔT, Yugo, Prof. Dubson

Click to edit the document details