Download Two Ball Race

Two Ball Race
When coasting downhill on skateboards, who will speed up faster – an adult or a child? Does the weight of a ball
affect how fast it rolls?
Materials
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Pasco cart
Rubber bands
2 track sections mounted side by side
2 identical metal balls (same size)
Some modeling clay
Discussion:
> The speed limit in Centerburg is 35 mph (15 m/sec); at the city limit there is a sign
saying "Resume safe speed." Maria starts accelerating at the sign but it is 5 seconds
before she reaches 70 mph (30 m/sec). What is her acceleration?
> Where should the policeman park, to catch Maria just as she gets up to speed?
Andrea says, "75 meters from the city limits." Betsy says, "115 meters from the city
limits." Carla says, "150 meters from the city limits." Who's right, and why?
>On a graph of position versus time, how can you tell whether the object is moving fast
or slow? How can you tell whether it is accelerating, decelerating, or moving at constant
speed?
> How long does it take for a car to get to 22 mph (10 m/sec) starting from rest (assuming
normal driving, like when you leave a stop light)? What is the corresponding
acceleration?
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>How long does it take to stop a car, when it is going 22 mph (again assuming normal
driving)? What is the corresponding acceleration?
1.
Attach a chain of three rubber bands to a Pasco dynamics cart. Use this to get the
cart moving, and to stop it again. What do you have to do to make the cart accelerate?
2. Put a weight on the cart, or even two
acceleration?
How does mass affect your ability to cause
3. Practice releasing two identical balls at the same time on the parallel track assembly.
Make several trials and develop a method for release so that they arrive
at the end of the ramps at nearly the same time (you will not quite
succeed; please note how accurately you can do this).
a. Are the motions of the two balls the same or very different?
b. Are the final speeds the same or very different?
c. Are the accelerations the same or very different?
4. What will happen in the “ball race” when you use two different weights of balls? Will
one get there much faster than the other? Or will they both get there at about the same
time?
Write your prediction.
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5. After writing your prediction, get another metal ball that is a different size from the
first balls you used.
Test your prediction by releasing two different balls at the same
time. Compare their motions.
a. Are the motions the same or very different?
b. Are the final speeds the same or very different?
c. Are the accelerations the same or very different?
6.
Repeat step 3 above for different track steepness. Does your conclusion still hold?
Check: Discuss the roles that mass and weight play in the acceleration of a falling
ball (or a ball rolling down a ramp). Explain the results of these activities.
7. At some airports there is a subway
that will take you from one terminal to
the next. Here is a graph that shows the
position of the subway car as it goes
from Terminal A to Terminal B, a
distance of 200 meters.
(a) What is the average speed of this
subway car?
(b) What is the maximum speed of this
subway car?
(c) Estimate the acceleration of the
subway car as it starts to move.
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Egg Drop
Why do things break when they fall to the floor?
Make several balls of modeling clay, about 2 cm (1 inch) in diameter. They don't need to
be exactly spherical, but they should be smooth and round.
1. Gently place one ball on the table top. Then drop another ball from a height of 50 cm
(18 inches) onto the table. Compare the flat spots on the bottoms of the two clay balls.
What does the difference imply?
2. The clay ball weighs about 0.1 N. Gently lay the 2 newton weight on the ball that was
not dropped (don't drop the weight on the ball!). What size flat mark does this make on
the ball?
What does this tell you about what happens when the clay ball is dropped on
the table?
3. Similarly drop a ball onto the floor from as high as you can reach. Or throw the ball at
the floor! Again, estimate the force that is needed to squash the ball a similar amount.
4. Describe the forces acting on the ball and the acceleration of the ball at various times:
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before you dropped it;
while it was falling;
just as it hit the floor;
1 second after hitting the floor.
Check: Why does a glass break when you drop it on a concrete floor, but might not break
if you drop it on the rug?
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Summary:
We have met three laws that relate force and motion. These are called "Newton's Laws
of Motion" and are usually referred to by number ("The Second Law").
The First Law -- The Law of Inertia: In the absence of external forces, an object moves
with constant velocity
The Second Law -- The Law of Force and Acceleration: An unbalanced force will cause
an object to accelerate, so that
Net Force = mass x acceleration
The Third Law -- The Law of Interaction: If object A exerts a force on object B, object B
exerts an equal and opposite force on object A.
5. Alice threw an orange straight up. After a while, it came down again. Her group is
trying to decide when the acceleration was the largest.
Barb said, “The acceleration was largest just after it left her hand.”
Clara said, “The acceleration was the largest when the orange was at the highest point,”
Danielle said, “The acceleration was the largest just before the orange touched the floor.”
Eva said, “The orange wasn’t accelerating.”
Fern said, “The orange had constant acceleration.”
Who is right?
orange?
What does the Law of Force and Acceleration have to say about the
As part of your discussion,
>sketch how the position of the orange depends on time
>sketch how the velocity of the orange depends on time
>sketch how the acceleration of the orange depends on time
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6. You can kick a soccer ball, but kicking a bowling ball will just hurt your foot. Explain
why, with reference to today’s activities.
7. We did several different things today: pulling on the Pasco cart; racing balls down
parallel tracks; and dropping clay balls.. What role did the Law of Force and
Acceleration play in each of these?
Reminder of a homework assignment: Please make a moon observation, including the
angle between the sun and the moon, before October 4, 2010. Turn in a copy.
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Each group should hand in one copy of this page at the end of class
Group:
Names of group members present:
8. Suppose a barge carrying 100,000 Kg of coal (a bit more that 100 tons) is moving
down the Ohio river at 10 m/sec when it is notice that there is a fishing boat in the
channel, 100 meters away. The boat guiding the barge goes into reverse and stops the
barge in 20 seconds. What force does the boat have to exert? (This question helps
explain why the "tug" boats push from behind instead of pulling from the front).
9. A 0.20 Kg tennis ball traveling at 15 meters/second encounters a tennis racquet that
causes the velocity to change to 10 meters/second in the opposite direction, in a time
interval of 0.01 seconds. What force did the racquet exert on the ball?
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