Physics Help Forum Need help understanding speed, velocity,accleration, force and newton laws.

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Sep 19th 2008, 08:35 PM #1 Junior Member   Join Date: Sep 2008 Posts: 2 Need help understanding speed, velocity,accleration, force and newton laws. 1) A meteor experiences increased speed as it gets nearer to the earth- is it acceleration? 2) a ball hurts much more when it hits close up than further away- is it force? 3) tom's maintains the same speed after turning the corner on his roller skates- is it velocity? 4) jan's toy doll moves about 2m every 30 seconds-is it speed? 5)Using one of newton's laws, explain the cause of the following. When 2 cars crash into the other, the smaller one skids backwards more than the larger one. - is it newton second law? Grandmother is very tired after rolling the bread dough on her kitchen table.newton's third law? After a while, airport runaways show evidence of dark lines as tires are worn down from constant arrivals.- newton's first law?
Sep 20th 2008, 03:27 PM   #2

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 Originally Posted by lkerry 1) A meteor experiences increased speed as it gets nearer to the earth- is it acceleration? 2) a ball hurts much more when it hits close up than further away- is it force? 3) tom's maintains the same speed after turning the corner on his roller skates- is it velocity? 4) jan's toy doll moves about 2m every 30 seconds-is it speed?
1), 3), and 4): An object is accelerating if and only if it's velocity is changing. Note that this can be a change in speed, a change in direction, or both. So for 1) the speed is increasing, etc. For 4) note that the question says the doll moves "about" 2 m every 30 s. This implies that the speed is changing, so it must be accelerating. I'll let you do 3).

2): I should call this an "air resistance" problem, though I'm not certain that the question is addressing this. Anyway, if it hurts more, then we may say that the speed of the ball is higher for a close-in impact than it is farther away, so the change in momentum is larger, hence the impluse the ball transmits is larger, and thus the average force it hits with is greater. (We can say all this using the impulse momentum theorem.)

-Dan
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Sep 20th 2008, 03:37 PM   #3

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 Originally Posted by lkerry 5)Using one of newton's laws, explain the cause of the following. When 2 cars crash into the other, the smaller one skids backwards more than the larger one. - is it newton second law?
It is a combination of Newton's 3rd and Newton's 2nd Laws. Each car has the same magnitude of force acting on it, due to Newton's 3rd. Then the 2nd Law takes over to say that the acceleration of the smaller car must be greater.

 Originally Posted by lkerry Grandmother is very tired after rolling the bread dough on her kitchen table.newton's third law?
Yes. The force she exerts to the dough is equal and opposite to the force the dough exerts on her.

 Originally Posted by lkerry After a while, airport runaways show evidence of dark lines as tires are worn down from constant arrivals.- newton's first law?
I"m guessing that you are correct here. The wheels are trying to slow the plane down, thus there is a force acting on the plane. But I'd say that these skid marks are there because of the heat generated by friction. So I can't really advise you on this one.

-Dan
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 Tags accleration, force, laws, newton, speed, understanding, velocity

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