Assignment #3
Introduction to Physics (PHSX 1010)
Adam Johnston
DUE: Monday, September 29^th   

Instructions: Completely answer the questions for TWO of the following situations.

Going in circles

You are riding your bicycle in a parking lot, constantly turning around in circles.  You have a constant speed of 20 m/s (almost 100 mph!), but not a constant velocity, since you are always changing your direction around the circle.  Assume, for the purposes of this problem, that you have perfect balance (or training wheels) while riding your bicycle.

• Describe the acceleration of your bicycle.
• Describe the net force on your bicycle.  What causes the force on your bicycle?
• Describe the motion of the bicycle if the force mentioned above were to disappear.  What would cause this force to disappear, anyway?

A weighty topic

Let=s imagine that the contents of the entire universe go away, leaving only you and your dog.  The two of you are floating out in space.  You are three times as massive as your dog.  Each of you feels a gravitational force towards one another, just as Newton=s law of gravitation describes.

• Compare the force that you feel towards your dog with the force your dog feels towards you.  Describe which direction the forces are, and how big they are compared to one another.  Justify your answer.
• If you were being pulled towards your dog with 8 Newtons of force in the previous scenario, what force would you feel if your dog suddenly doubled his mass?  Explain.
• Again, compared to the 8 Newtons of force in the first scenario, how would the force change if your dog (at his original mass) doubled his distance away from you?  Explain.

Elevator physics

You are standing on a scale inside an elevator.  At home, the scale would read 100 lbs.  Currently, the scale reads 120 lbs.  There could be two different reasons for this.  Explain exactly what 2 motions the elevator could be experiencing.  Be sure to describe the velocity and the acceleration of the elevator, and why these make you feel heavier than you really are.

Flying dinosaur

You are standing in a giant, glass school bus that is moving down a perfectly level and smooth road at a constant velocity of 20 m/s.  You have a stuffed dinosaur with you (for comfort and companionship).  If you toss your stuffed purple dinosaur straight into the air, describe his motion from:

• Your perspective
• The perspective of an observer standing on the ground, watching the bus go by.
• You can use a sketch to describe the motion, but also explain (in words) what is taking place.
• Also, while your dinosaur is in midair, what are all the forces on him?  How does he feel?  (If he were standing on a scale while flying through the air, what would the scale read?)

How on Earth?

It is described how the Earth and other planets go about the Sun in elliptical paths; and it is also described how the cause of this can be gravity.  This is all fine.  But, why don't we fall off the Earth if it is moving and spinning at such an outrageous pace?  (Hint: The answer is not gravity.)

Going bananas

It was once argued that rockets would never work because of Newton=s 3rd law: If there is nothing to push against in space, then there will be no way for a rocket to make itself go.  (Certainly, your car won=t work in space, since there are no roads for the tires to push against.)  So how does a rocket work?  Answer the following questions:

Imagine that you are sitting on a raft in the middle of the ocean.  All you have (besides yourself and some old Rick Springfield c.d.=s) is a ton of bananas.  If you are floating on calm waters and not moving, what is the total momentum of you, the raft and the bananas (all included as one system)?  How do you know?

• If there is no current, wind, sharks, or other external forces, what is the total momentum going to be in 10 minutes?  How do you know?
• Those were the easy questions.  Now the hard questions.)  If you throw a banana to the west with a velocity of 10 m/s (or almost 50 mph), exactly what will happen to your raft (assuming a frictionless ocean)?  Assume that one banana is one one-thousandth of the mass of the raft and all its cargo.
• If you want to be moving at 1 m/s to the east, approximately how many bananas should you throw?

(Note: Rockets use the exhaust of fast moving gases instead of bananas.  These gases do not have very much mass, but they have very high velocity.  (Even higher than our bananas!)  The rocket and its exhaust have a total momentum of zero, just like the raft and its bananas.  Something to think about (but not necessary for the homework): Does this violate Newton's 3rd law, as was once claimed?)

The great rivalry

Two rivals (a chemist and a physicist) stand on a frictionless sheet of ice out in the middle of the Salt Lake.  They are stranded for days.  Finally, the chemist gets tired of the uncomfortable situation and gives the physicist a push so that the he will have the ice sheet to himself.  As the physicist slides off the ice sheet, she laughs at the chemist's foolishness.  Explain why.