assignment4solutions - Physics 161, section 02 Hammer...

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Physics 161, section 02 Hammer Assignment 4 Due Oct 9, start of class Reference reading: For the next couple of weeks we’ll be talking about Newton’s Laws and a Newtonian definition of force. There’s relevant stuff in Knight Chapters 4 and 5 (and 6, but 6 is mostly kinematics), in Understanding Physics Chapters 3 and 5, and in another chapter I posted from Birkett and Elby, entirely on Newton’s 3 rd Law. (I recommend that one! In fact, I like it so much I’m going to use two problems from it.) 1) Newton’s Laws are all about a refined definition of force , including the idea that a force is always applied by one object on another object. In many, many situations that makes perfect sense, and you can identify the object responsible for the force. But in some situations you have to do a little work to reconcile your intuition and Newton’s Laws. Here are a couple of examples, to get you started. a) If you’re standing on a train and the train starts moving, you feel “thrown backward.” How can you account for that experience using Newton’s Laws? It can’t be that there’s a force on you backward, because there’s no other object that could be exerting that force — that’s the Newtonian definition of force. What are the other objects? The bus, the air in the bus, and the earth (gravitational attraction). That’s it. The earth, of course – you are pulled downward. The air… there’s not much of a net force from the air on the bus, or you’d feel a wind. And the bus is touching you, but it’s pushing you forward , to get you to speed up in that direction. So the floor pushes forward on your feet, and everything around you starts moving forward, and you’re taking a bit to get started moving. So – relative to the bus, you move backward a little. Relative to the ground outside, though, you’re starting to move forward. b) We think of the controls inside a car as causing its motion — the gas pedal makes it speed up (or move), the brakes slow it down, the steering wheel makes it change directions. But…by Newton’s Second Law, if there’s a change in the car’s velocity it’s because there’s a net (total) force on the car by some other object. If Newton’s Laws are right, there must be another object outside the car pushing on it. Is there? What other objects are touching the car? The air, the ground… the air sure isn’t pushing it forward. It must be the ground! The tires push backward on the ground; the ground pushes forward on the tires. It’s the same kind of thing when you walk — or to exaggerate that think of jumping forward.
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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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assignment4solutions - Physics 161, section 02 Hammer...

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