Newton's 1st law - pulling force

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Hello

If an object of certain mass is being pulled upwards at constant velocity (and with a constant force) then by
Newton's first law of motion, the net force acting on the object is
zero.

So the force pulling the object upwards must be equal to the force pulling/pushing it down.

The force pulling up the object could be tension in a string, for example.

But then what would be the forces pulling it down? I'm thinking there must be a force in addition to gravity pulling it down, since the pulling force is greater than the force of gravity.

TIA
 

Answers and Replies

  • #2
An object cannot have a constant velocity and have a constant force applied to it. Any object which has a force applied to it is accelerating. (By Fnet=ma)
 
  • #3
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That doesn't make sense. A car can move at constant velocity by generating constant forces to counteract the resistive forces it faces.

If I push a book on a table to make it move at constant velocity, the net force will not be zero initially but once the book gets moving my force will equal the foce of friction faced by the book.
 
  • #4
Doc Al
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So the force pulling the object upwards must be equal to the force pulling/pushing it down.

The force pulling up the object could be tension in a string, for example.

But then what would be the forces pulling it down? I'm thinking there must be a force in addition to gravity pulling it down, since the pulling force is greater than the force of gravity.
Why do you think the pulling force is greater than gravity?
 
  • #5
Dale
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An object cannot have a constant velocity and have a constant force applied to it. Any object which has a force applied to it is accelerating. (By Fnet=ma)
This is incorrect as written. Here is what you should have said:

"An object cannot have a constant velocity and have a [strike]constant[/strike] net force applied to it. Any object which has a net force applied to it is accelerating. (By Fnet=ma)"

You certainly can have a constant velocity and a constant force provided that it is balanced by an equal and opposite constant force. In such a situation the net force is 0 and the object does not accelerate. This condition is known as equilibrium.
 
  • #6
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Why do you think the pulling force is greater than gravity?

If it was equal to gravity the object will lie still, right? :confused:
 
  • #7
Doc Al
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If it was equal to gravity the object will lie still, right? :confused:
No. If it's equal to gravity the net force will be zero and the object will continue at the same velocity.
 
  • #8
Even better:
An object cannot have a constant velocity and have a constant net force applied to it. Any object which has a net force applied to it is accelerating. (By Fnet=ma)

Net force is just the sum of the individual forces acting on each axis. This is why having a free body diagram is important; it helps you visualize all the forces acting on an object. When using Fnet=ma, you will want the net force, not the individual forces. It can be confusing if it is worded the way I worded my previous statement. I take for granted that when I think of forces I almost always mean net force.

Net forces only directly effect acceleration, not velocity. The net forces equal zero, which means zero acceleration, not necessarily zero velocity.
 
  • #9
arildno
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It is, however true, that in your book example, you'll need to push with a greater force than the maximum force of friction can exert in order for the book to accelerate up to some non-zero velocity.
If you then adjust the applied force to be equal to the resistive force (net force=0, then!), the book will continue to move at a constant velocity.
 
  • #10
Indeed. There are two kinds of friction, static and kinetic. Static friction is always greater than kinetic friction. To accelerate the book from rest, (to give it a velocity greater than zero) the force you apply to the book at rest must be greater than the opposing force of static friction. Once the book has a velocity greater than zero, the force you apply to the book to keep it moving at a constant velocity only has to equal the opposing force of kinetic friction.

Constant velocity = Sum of forces equal to zero.
 
  • #11
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Thank you all. It's hard to accept that an object moving at constant velocity is essentially moving by itself!
 
  • #12
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Thank you all. It's hard to accept that an object moving at constant velocity is essentially moving by itself!

Do you have trouble picturing a wagon being pushed at a constant speed down a road? No matter how hard you push (within human limits), at some speed the friction in the wheels and the air resistance will balance your force, and the speed will level off to a constant.

Pulling something up against gravity and air resistance is exactly the same thing, just a different direction. As long as gravity plus air resistance is less than the pull upward, the object will accelerate. When the forces balance, it will continue upward at constant speed.
 
  • #13
russ_watters
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Thank you all. It's hard to accept that an object moving at constant velocity is essentially moving by itself!
Ever been ice skating?
 

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