Net Force & Motion: A 9th Grader's Guide

[proyasha]

Homework Statement

will a body necessarily remain at rest if the net force acting on it is zero?give a reason for ur answer...and I've just passed my ninth grade so if u cud please it to me explain at that level...

Homework Equations

The Attempt at a Solution

 

[Google_Spider]

will a body necessarily remain at rest if the net force acting on it is zero?

It will have a constant velocity. If it was initially at rest, it will be at rest. If it was moving with a certain velocity, it will continue moving with the same velocity.

 

[princesspriya]

it would not always mean it will be at rest because it can also be moving at constant velocity since the definition of force is something that would change the velocity of an object therefore making it accelerate but if the net force is zero then it could either be moving at constant velocity or be at rest since there is no acceleration.

 

[rock.freak667]

It will have a constant velocity. If it was initially at rest, it will be at rest. If it was moving with a certain velocity, it will continue moving with the same velocity.

it would not always mean it will be at rest because it can also be moving at constant velocity since the definition of force is something that would change the velocity of an object therefore making it accelerate but if the net force is zero then it could either be moving at constant velocity or be at rest since there is no acceleration.

And Newton's first law says this directly.

 

[physixguru]

Your question is just the question Aristotle asked in a different way.
He said that a force is necessary to keep a body in motion.
As the honorary members above already have told you that Newton's first law says this directly that a force is needed only to accelerate a body.

Of course Aristotle's assumption was wrong.

 

[rock.freak667]

Of course Aristotle's assumption was wrong.

If memory serves me correctly, the end result of Aristotle's assumption was that F=mv while Newton said that F=ma

 

[physixguru]

I am talking about "Aristotle's Fallacy".

 

[proyasha]

can you give an everyday example to support the answer?

 

[physixguru]

can you give an everyday example to support the answer?

Answer me this first.

If Newton was correct then why does the Earth revolve around the sun? The gravitational pull of the sun makes the Earth go about it.But this contradicts Newton's law that a force is not necessary to keep a body in motion? :p

Answer me the logic behind this phenomenon.I am not making a statement.I am asking a question.

 

[proyasha]

well,i think that the sun's gravitational force provides the centripetal force which makes the Earth go about it.had the gravitational pull not been there then the Earth would have traveled in a linear path...is that what u want me to answer?..that why does the Earth REVOLVE around the sun?...and I've got another question: is there any difference between a body which has acceleration and abody which is in motion?

 

[physixguru]

A body in motion may not have acceleration.It may have constant velocity.
A body in acceleration may have positive or negative acceleration.This will affect its speed.
Also a body having acceleration may possesses zero velocity.

 

[proyasha]

okay...what about my answer?is it "logical" enough??

 

[physixguru]

There is a theory that the planets don't actually "revolve" around the sun.

According to Einstein's theory, any mass warps space around it. (Imagine a heavy lead ball placed on a bed. The surface of the bed will bend/sag below the ball and the ball will sink into a depression. The ball represents the mass, and the bed represents space.)

Therefore, the planets, relative to themselves, are actually following a straight line. But since the sun (a mass) warps space around it, the planets appear to follow a circular path around the sun.

But to answer your question at our level, any two masses exert gravitation forces on each other (Newton's Law of Universal Gravitation). Therefore the force of attraction between the sun and the Earth is large enough to make the Earth veer off from the straight line path that it would have otherwise followed by Newton's First Law and to make it follow an ellipsis.

 

[proyasha]

ok... i just read that on some site the other day..(no complains there)...but what abt my example...i'm not crystal clear abt it

 

[sourav86]

Answer me this first.

If Newton was correct then why does the Earth revolve around the sun? The gravitational pull of the sun makes the Earth go about it.But this contradicts Newton's law that a force is not necessary to keep a body in motion? :p

Answer me the logic behind this phenomenon.I am not making a statement.I am asking a question.

I think you forgot to consider circular motion as an accelerating motion. Newton's right.
Aristotle forgot to consider frictional force.
That's where he went wrong.