Practice problems in motion

Woo Hoo – it’s physics problems and questions! OH YEAH!!
You will likely be able to do many of these problems, but probably not all.  Fret not, physics phriends!  We will cover all the material in class.  Do those you can.  Answers are below


1. Determine the average speed of your own trip to school: in miles per hour. Use GoogleMaps or something similar to get the distance, and try to recall the time from your last trip. Use your trip from home to Towson, or something that makes sense to you. If possible, do it in miles per hour AND m/s.

2. Consider an echo-y canyon. You stand 200-m from the canyon wall. How long does it take the echo of your scream (“Arghhhh! Curse you Physics!!!”) to return to your ears, if the speed of sound is 340 m/s? (Sound travels at a constant speed in a given environment.) Also, keep in mind that the sound has to travel away from AND back to the source.

3. What is the difference between traveling at an average speed of 65 mph for one hour and a constant speed of 65 mph for one hour? Will you go further in either case?

4. What is the meaning of instantaneous velocity? How might we measure it?

5.  How far will a light pulse (say, a cell phone radio wave) travel in 1 second? In one minute? In one year? You don't have to work this out, but you should show HOW it would be calculated. Keep in mind that the light pulse travels AT the speed of light.

6.  What is the acceleration of a toy car, moving from rest to 6 m/s in 4 seconds?

7. What does a negative acceleration indicate?

8. Consider an automobile starting from rest. It attains a speed of 30 m/s in 8 seconds. What is the car’s acceleration during this period?

9. In the above problem, how far has it traveled in the 8 seconds?  This problem may be a little tricky. If you're feeling ambitious, feel free to read ahead in the blog - note that I didn't cover all of the material in tonight's class.

10. Review these ideas. Write down answers, if it would be helpful.
a. standards for the m, kg, and s. Know the original meaning of the standard, and the current standard (approximate meaning - don't worry about the crazy numbers)

 We will cover these next 3 in our next class.  Can't wait!

11.  What exactly is gravitational acceleration and what is the significance of 9.8 m/s/s?

12.  A ball is dropped from rest from a great height. After 3 seconds, how fast is it traveling? How far did it fall in this time?  (You may use an approximate value of 10 m/s/s for g, and assume that there is no air resistance.)

13.  Revisit problem 12.  If this has been done on the Moon at the same height, would it take more, less or the same time to hit the ground?  How about on Jupiter?

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Answers:

1.  Answers will vary for this problem.  I will not ask you to convert to m/s on a test, but the conversion factor is approximately:  1 m/s = 2.25 mile/hour.

2.  v = d/t      Note that the speed of sound is constant, so that you don't need an equation of motion, but rather just v = d/t.

340 = 2(200)/t

t = 1.2 sec

3.  no difference

4.  velocity at a given "instant", though there is no easy way to define an "instant."  Essentially, we mean the velocity at a practical "instant," say, at a particular second.

5.  Since v = d/t, d = v t.

Note that the speed of light is constant, so that you don't need an equation of motion, but rather just
d = v t

So, the distance that light will travel will be equal to the speed of light (3 x 10^8 m/s) multiplied by the appropriate number of seconds.

In 1 second, d = 3 x 10^8        (answer will be in meters)
In 1 minute, d = (3 x 10^8) x 60        (answer will be in meters)
In 1 year, d = (3 x 10^8) x 60 x 60 x 24 x 365.35        (answer will be in meters)

6.  a = (vf - vi)/t

a = (6-0)/4 = 1.5 m/s/s

7.  "slowing down" relative to the direction you think of as positive.  For example, driving forward and hitting the brakes.

8.  a = (30 - 0)/8 = 3.75 m/s/s

9.  d = 0.5(vi + vf) t = 0.5(0 + 30) x 8 = 120 meters

10, 11.  see notes.

12.  Approximating g at 10 m/s/s:  v = 30 m/s.  d = 45 m

13.  :-)

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