Transcript Class 13 - Physics at Oregon State University

Stone A (2 kg) is sliding due east at 6.0 m/s across
frictionless ice when it collides with stone B (3 kg),
which is sliding due north at 3.0 m/s. The stones do not
stick together. Immediately after the collision, stone A
has a speed of zero. The speed of stone B is then:
1.
2.0 m/s.
2.
3.0 m/s.
3.
4.0 m/s.
4.
5.0 m/s.
5.
None of the above.
(Follow-up: In what direction is stone B moving?)
10/27/14
Oregon State University PH 211, Class #13
1
A lump of clay is a projectile at the peak of its
parabolic arc when it suddenly explodes into three
lumps of equal mass. The initial velocities of those
lumps at the moment just after the explosion are:
Lump A: 4.00 m/s traveling due east;
Lump B: 3.00 m/s traveling due west;
Lump C: 2.00 m/s due north.
Find the velocity of the lump (both magnitude and
angle) just before the explosion.
1.
2.
3.
4.
5.
2.24 m/s  63.4°
3.00 m/s  45.0°
5.00 m/s  30.0°
2.24 m/s  26.6°
None of the above.
10/27/14
Oregon State University PH 211, Class #13
2
A 1000 kg boxcar is collecting 10kg of
rain per hour. What will the velocity of
the box car be after 3 hours? (Ignore
friction/drag)
vi = 22 m/s
10/27/14
Oregon State University PH 211, Class #13
3
If +x is to the right, this cart’s
change of momentum is
1. –30 kg m/s.
2. –20 kg m/s.
3. –10 kg m/s.
4. 10 kg m/s.
5. 30 kg m/s.
10/27/14
Oregon State University PH 211, Class #13
4
A person attempts to knock down a large wooden
bowling pin by throwing a ball at it. The person has
two balls of equal size and mass, one made of
rubber, the other of putty. The rubber ball bounces
back, while the ball of putty sticks to the pin.
Which ball is more likely to topple the bowling pin?
1.
2.
3.
4.
10/27/14
the rubber ball
the ball of putty
makes no difference
need more information
Oregon State University PH 211, Class #13
5
Suppose you are on a cart, initially at rest on a
track with very little friction. You throw balls
at a partition that is rigidly mounted on the cart.
If the balls bounce straight back as shown in
the figure, is the cart put in motion?
1. Yes, it moves to the right.
2. Yes, it moves to the left.
3. No, it remains in place.
10/27/14
Oregon State University PH 211, Class #13
6
Momentum, by itself, is not that useful (or
exciting). It gets more interesting when it’s
changing. For example, to hit a baseball a long
way (by changing its momentum in a big way),
you’d do best to increase both the force of the bat
on the ball (how hard you swing), and the time of
contact between the bat and ball (follow-through.)
Indeed, Newton defined impulse as the (average)
force on an object multiplied by the time during
which the force is applied: J = FΔt
10/27/14
Oregon State University PH 211, Class #13
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Impulse-Momentum Theorem
“The impulse applied to an object is the change in
momentum of that object.”
J = FavgΔt = mΔv = Δp
To catch a water balloon, egg, or (barehanded) a
baseball, you need to change its momentum to
zero. This is a fixed value, Δp, for a given mass
and impact. “Cushioning” your catch extends the
interaction time, thus decreasing the force.
10/27/14
Oregon State University PH 211, Class #13
8
Or…
Note that the units for impulse will be N s.
These must be the same as kg·m/s!
The time-function version of the theorem lets us
do problems when the force is not a constant
value—useful if the force can be approximated as
a function of time, perhaps the thrust of a solid
booster rocket engine.
10/27/14
Oregon State University PH 211, Class #13
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To summarize:
Problems dealing with collisions or explosions of
multiple objects (comparing the “before” picture to
the “after” picture) use conservation of momentum.
For problems analyzing the processes during the
collision (especially for a single object at a time),
you’re most likely going to use the ImpulseMomentum relationship:
 Fdt  p
10/27/14
Oregon State University PH 211, Class #13
10
A truck with a total mass of 5000 kg traveling
at 3.0 km/h hits a loading dock and comes to a
stop in 0.6 s. What is the average force exerted
by the dock on the truck?
What is the peak force?
Fdock.truck
Notice the units of the
area under the graph.
Impulse  J
t
10/27/14
Oregon State University PH 211, Class #13
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