Physics 161, section 02
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
Chapters 3 and 5, and in another chapter I posted from Birkett and Elby, entirely on
Law. (I recommend that one! In fact, I like it so much I’m going to use two problems from it.)
Newton’s Laws are all about a refined definition of
, including the idea that a force is always applied
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.
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
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.
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
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.