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phy 201
Your 'question form' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
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If you exert a force of 20 Newtons on a 10-kg object and it accelerates in the direction of your force at 1.5 m/s^2, then how do you know there are other forces acting on the object besides your own?
If a 10 kg object accelerates at 1.5 m/s^2, then the net force on the object is
F_net = m a = 10 kg * 1.5 m/s^2 = 15 kg m/s^2 = 15 N.
If your force was the only one acting on the object, then the net force on it would be 20 N and it would be accelerating at 2 m/s^2, not at 1.5 m/s^2.
What is the total of all those forces and in what direction does this total act?
If you are exerting a force of 20 Newtons on the object and the net force is only 15 Newtons, this means that the other forces acting on the object must total -5 Newtons. That is, there has to be a 5 N force opposing you.
&&&&does the opposite for always have to equal my force? or does that mean Fnet = f + Ffrict, so Fnet= 20 + (-15) = 5N? &&&&&
@& The net force on an object is the sum of all the forces acting on it.
Your force might just be one of a number of forces.
The acceleration of the object is
a = F_net / mass.
If you know the acceleration and the mass, you easily find the net force, as you have done.
If that net force is equal to the force you exert, it means that all the other forces add up to zero. This could either mean that there are no other forces, or that there are other forces but they happen to add up to zero.
In the present example this leads to the conclusion that the other forces add up to -5 Newtons. Since you are exerting a force of 20 Newtons, which is positive, the -5 Newton force opposes you.
Had the net force been, say, 27 newtons, the conclusion would be that there is an additional force of 7 Newtons. That foce is positive, as is your 20 Newton force, so the other force would be in the same direction as yours.
F_net = f + F_frict,
where f is the force you exert and F_frict is a frictional force, is a good way to express the situation when the additional force is the result of friction.
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