Physics Help Forum Earths angular momentum and newtons second law.

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 Jul 23rd 2017, 09:42 AM #1 Senior Member     Join Date: Feb 2017 Posts: 203 Earths angular momentum and newtons second law. Newtons second law states: A force applied to an object changes its state of motion. If the object is at rest, it will start moving. If it is moving, it will speed up, slow down, or change direction. The change in motion depends on the mass of the object. Can we interpret this way? The earth actually wants to move in a straight line but the centripetal force of the sun keeps it in orbit. This force makes Earth change direction. Is this correct?
Jul 23rd 2017, 10:14 AM   #2
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 Originally Posted by avito009 Newtons second law states: A force applied to an object changes its state of motion. If the object is at rest, it will start moving. If it is moving, it will speed up, slow down, or change direction. The change in motion depends on the mass of the object. Can we interpret this way? The earth actually wants to move in a straight line but the centripetal force of the sun keeps it in orbit. This force makes Earth change direction. Is this correct?
Yes.

But its much simpler than that. The gravitational force exerted by the sun on the earth changes its momentum, i.e. its motion. Its merely the nature of the gravitational force that makes it a centripetal one. The magnetic force can't be a centripetal force.

Jul 23rd 2017, 10:49 AM   #3
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 Originally Posted by avito009 Newtons second law states: A force applied to an object changes its state of motion. If the object is at rest, it will start moving. If it is moving, it will speed up, slow down, or change direction. The change in motion depends on the mass of the object. Can we interpret this way? The earth actually wants to move in a straight line but the centripetal force of the sun keeps it in orbit. This force makes Earth change direction. Is this correct?
No

Why do you think Newton's first law is required?

Many have tried to dispense with N1 over the centuries, but it is necessary.

 Jul 23rd 2017, 11:13 AM #4 Senior Member     Join Date: Feb 2017 Posts: 203 Earths and Moons mass Also according to this second law of newton since the Earth has more mass than the moon, so more force is required to make the Earth move than the force required to move the moon by the Earths gravity. Since mass of Earth is greater than the moons mass.
Jul 23rd 2017, 11:33 AM   #5
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 Originally Posted by avito009 Also according to this second law of newton since the Earth has more mass than the moon, so more force is required to make the Earth move than the force required to move the moon by the Earths gravity. Since mass of Earth is greater than the moons mass.
Your first post was wrong and this one is even more wrong.

Jul 23rd 2017, 11:37 AM   #6
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 Originally Posted by studiot Why do you think Newton's first law is required?
He referred to Newton's second law, not the first. Newton's second law is F = dp/dt

Jul 23rd 2017, 11:41 AM   #7
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 Originally Posted by studiot Your first post was wrong and this one is even more wrong. Did you not read my reply?
Both of his responses are correct. If you disagree please state why. Your comments are confusing to me.

Jul 23rd 2017, 12:45 PM   #8
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 Originally Posted by Pmb Both of his responses are correct. If you disagree please state why. Your comments are confusing to me.
It seems to me that the OP was asking for approval of his way of putting what he referred to as Newtons Second law.

You have also stated a version of N2.

However the OP version would go a long way towards Newton's First Law, N1, which is why I asked what I did.

The OP then posted a second rephrasing which refers to gravity.

None of Newton's 3 laws of motion refer to gravity, that is another Law.

So if you really wish to help the OP, explain what N1, N2 and N3 are as well as Newton's Law of Universal Gravitation (to give it its correct title).

Last edited by studiot; Jul 23rd 2017 at 12:48 PM.

 Jul 23rd 2017, 01:44 PM #9 Senior Member   Join Date: Nov 2013 Location: New Zealand Posts: 534 I can't see what was said that violated Newton's first law. From what I can tell N1 is talking about a scenario with no forces or equal and opposite forces giving a net force of zero. The earth-sun system doesn't have a zero net force. https://www.grc.nasa.gov/www/k-12/airplane/newton.html Unless studiot was arguing that zero net forces don't change a system......
Jul 24th 2017, 12:00 AM   #10
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F=ma

 Originally Posted by Pmb He referred to Newton's second law, not the first. Newton's second law is F = dp/dt
p=m*v

F = dp/dt = d(m*v)/dt

m (mass) is a constant here, so it can be taken out (the product rule at work here!)

Therefore, F = m*(dv/dt) = ma (because acceleration a = dv/dt), which is basically Newton's Second Law defining force as the product of mass and acceleration.

 Tags angular, earths, law, momentum, newtons

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