Homework Help: Forces in space

1. Nov 24, 2012

sgstudent

1. The problem statement, all variables and given/known data
When I drag my feet on the ground (on a frictionless surface), there will be an equal but opposite force acting on another body. What is that body exactly?

2. Relevant equations

none

3. The attempt at a solution

Will it be transferred to some other parts to my body? So in space if i apply i push force the force will be transferred to some other part causing it to rotate? So it means that i might have translational equlibrium but i have a net moment so i can still rotate?

thanks!

Last edited: Nov 24, 2012
2. Nov 24, 2012

Staff: Mentor

It's not clear what force you are referring to. If the ground is frictionless, then you do not exert a friction force on it.

But you do exert other forces.
Not sure what scenario you have in mind. Whenever any two objects interact, they exert equal and opposite force on each other. Whether you begin to rotate or not depends on where that force is exerted on you.

3. Nov 24, 2012

sgstudent

for example i'm in space and i kick forward. Where would my opposite reaction force be? I'm guessing another part of my body, so I don't have a net force on my body?

4. Nov 24, 2012

Staff: Mentor

To kick your leg forward means that one part of your body exerts a force on the other. (Equal and opposite forces, per Newton's 3rd law.) No net force on you, since those forces are internal to you and add to zero.

5. Nov 28, 2012

sgstudent

But in space the leg would still be able to move so why is the net force 0N?

6. Nov 28, 2012

Staff: Mentor

The net force on your body as whole is zero. So your center of mass will not accelerate. When you kick your leg out the net force on your leg is non-zero, so it accelerates. But not for long! (It's attached to you.)

7. Dec 1, 2012

sgstudent

So do you mean the reaction force on the body is not directly behind the leg? Because if it is so then the leg shouldn't be move at all but it also doesn't make sense for the reaction force to be not directly behind it as well.

8. Dec 1, 2012

Staff: Mentor

I don't quite understand what you mean by the reaction force being 'directly behind' the leg or why that implies that the leg shouldn't move at all.

Whenever two things interact, they exert equal and opposite forces on each other. To kick your leg, lets say the lower leg, the rest of your body must exert various forces on your lower leg. Each one of those forces is part of a Newton's third law 'action/reaction' pair.

For your leg to start moving, there must be a net force on it. (There will be an equal and opposite net force on the rest of your body.)

Let's take a simpler example, also out in space. Say you are tied to a bowling ball by a rope. You push the ball to the east, so it pushes you back to the west. Those forces are equal and opposite. The ball moves east and you move west. When the rope becomes taut it exerts a tension force on both of you, pulling you back. So you end up going nowhere.

9. Dec 2, 2012

sgstudent

Oh, but if if the muscles behind my foot exert the force onto my leg won't the reaction force be on those muscles. So since they are so close together that tensional force would be instantaneous? Thanks for the help!

10. Dec 2, 2012

Staff: Mentor

Of course.
Not sure what you mean. Action/reaction forces are simultaneous.

The term 'action/reaction' is a bit misleading, as it seems to imply that one causes the other. Better to think of them as two aspects of a single interaction.

11. Dec 3, 2012

sgstudent

Oh because if the muscles behind the foot exert a force onto the foot itself, then the simultaneous reaction force will be exerted upon those muscles. So how would the foot be able to move forward? Since both bodies are connected to each other unlike the example you gave whereby the rope was intially slack.

Thanks for the help!