Physics 172 – Recitation 02
Spring 2012
You will find more information about the topics covered in today’s problems in
chapter 1 and in sections 2.1 through 2.3 of chapter 2 of the text.
Problem 1.
A child is playing with a lowfriction toy car on the balcony of her mother’s
apartment.
The figure below shows the car’s position at a sequence of equally spaced
times separated by 0.226 seconds.
It shows the car initially running in the
x
direction,
under the guardrail and off the edge of the balcony at time
3
t
.
The
x
axis runs along the
level ground below the balcony.
The
y
axis runs vertically upward and the
z
axis points
toward you.
The gridlines shown are 1 meter apart.
1
3
5
6
2
4
7
y
x
a)
What are the components of the vectors
∆
r
21
and
∆
r
32
representing the displacement
of the toy car as it rolls across the balcony?
Don’t forget to show the units of the
components of these and the other vectors you work with today.
b)
What are the components of the toy car’s average velocity vector between times
1
t
and
2
t
?
Between
2
t
and
3
t
?
Explain how your answers are consistent with Newton’s first
law and what you know about the interactions between the car and its surroundings as it
rolls across the balcony.
c)
What is the toy car’s speed as it rolls across the balcony?
What are the components of
the unit vector ˆ
v
representing the direction of the toy car’s velocity as it rolls across the
balcony?
What are the components of the toy car’s instantaneous velocity at
3
t
just as it
leaves the edge of the balcony?
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 Spring '08
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 Acceleration, Friction, Velocity, hockey, toy car, average velocity vector

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