assignment5solutions

assignment5solutions - Physics 161, section 02 Hammer...

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Physics 161, section 02 Hammer Assignment 5 Due Oct 16, start of class Reference reading: Still Newton’s Laws. If you’re using Knight, add Chapter 8 on Newton’s Third Law to your reading; if you’re reading Understanding Physics add Chapter 6. 1) Newton’s Third Law says that if there’s a force by object A on object B, there is at the same instant and equal and opposite force by object B on object A. In other words, forces always come in pairs, as symmetrical interaction between two objects. This, again, is about understanding the Newtonian definition of force. And again, in many situations it will make perfect sense, but in some situations you have to do a little work to reconcile it with your intuition. (That chapter by Birkett and Elby is especially helpful on this point.) So here are a couple of questions along those lines. Explain them so they make sense! To you! a) If the force is always equal and opposite, how come all games of tug-of-war don’t end in a tie? (A game of tug of war is when two or more people stand holding opposite ends of a rope, and they try to pull each other across a line in the middle. The side that gets pulls across the line loses.) Because the two forces that are equal and opposite act on different objects . The force by A on B is equal and opposite to the force by B on A. You don’t win a tug of war if the force you exert on your opponent is larger than the force your opponent exerts on you — those two forces have to be equal. You win if the force you exert on your opponent is larger than the sum of the other forces on your opponent. I stole this picture from Birkett and Elby: Note the forces on Ben are by (1) the rope, (2) the earth (gravity), and (3) the ground – the normal force (the ground pushing back from being compressed) and a force of friction to the left. If the force by the rope to the right is greater than the force by the ground to the left, Ben will accelerate to the right. If the force by the ground to the left is greater than the force by the rope to the right, Ben will accelerate to the left. (I drew the tension in the rope as a little bigger in my diagram here, so that would mean Ben’s accelerating to the right. If from there the tension force by the rope is only equal to the friction to the left, Ben will keep moving, and he’ll lose.) But in all cases, the force by the rope on Ben is equal to the force by Ben on the rope. (Some people ask me where they should draw the vectors — do you put the tail of the vector on the object, or the head of it, and do you put it at the point of contact, etc. The answer is it doesn’t matter: A vector has the same value wherever you draw it! But you want to be able to keep track of which vector is which, so draw them in such a way than you can do that…) b) If a baseball bat hits a baseball, how does the force of the bat on the ball compare to the force of the ball on the bat? How can that be?
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This note was uploaded on 04/07/2008 for the course PHYS 161 taught by Professor Hammer during the Fall '07 term at Maryland.

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assignment5solutions - Physics 161, section 02 Hammer...

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