Physics 121
Wednesday, September 23, 2009
Last class:
Momentum is p = mv. Total momentum, summed over all objects in a system, is conserved.
Today:
More momentum! And vectors, too!
There’s a handout on vectors, §3.2 of Physics for Scientists and Engineers by Tipler &
Mosca. It is completely optional reading, possibly useful if you haven’t worked with vectors
before.
Some problems we might work through in class.
1. Someone standing on a cart throws balls off of it. Which way does the cart move?
2. Now there is a wall on the cart. The balls bounce off the wall before flying away. Now
which way does the cart more?
3. A 1000 kg car is moving along at 10 km
. A 2000 kg truck crashes into it, from behind,
hr
km
at a speed of 30 hr . If the truck and car stick together after the collision, what is their
final speed?
4. I bounce a 50 g superball on a table. Its velocity just before hitting the floor is −5 ms ,
and its velocity just after bouncing is +5 ms collision).
(a) By how much did its momentum change?
(b) What about conservation of momentum?
5. A superball, flying through space in the +x-direction, hits another ball which is initially
at rest. The superball flies off in the direction shown. Qualitatively, what can we say
about the motion (if any) of the second ball?
?
6. A 150 g hockey puck is sliding due north at 10 ms on the surface of a frozen lake. Another
150 g hockey puck is sliding due east at 10 ms . The sides of the pucks are covered in
Velcro. The pucks collide and stick together. What direction are they going now? How
fast?
7. The velocity of a truck is 50 ms in a direction shown below. What are the components
of its velocity, vx and vy ?
y
v
o
75
x
8. A ball has velocity components vx = 15 ms and vy = −20 ms . What is its speed, v? In
what direction is it moving?
9. A 1 kg bomb is sitting at rest. Suddenly it explodes, breaking into three pieces. One
piece, with mass 200 g, flies off due east at 300 ms . Another piece, with mass 500 g, flies
off 40◦ south of west at 200 ms . What can we say about the third piece?