# What is the action-reaction pair of static friction?

First of all sorry for this basic question.

Newton's third law states that for every force there is a force, which is equal in magnitude and opposite in direction to those of given force.So by that logic friction should also have a reaction force.

Now my questions are, what is the reaction force for static friction ?

For instance in the picture, what is the direction of reaction force of static friction and on which object will that force act ? ## 3 Answers

There is no reaction force on this free body diagram. This is namely the diagram for the object.

Draw a free body diagram of the Earth, and there you have your reaction force. The static friction pulls in the opposite way in the Earth.

You can think of it like this: the static friction is trying to prevent sliding to happen. So it pulls left in the box, to hold back in it, and it pulls right on the Earth to try to make the Earth follow along with the box, so that there is no sliding.

• Of course i know that there is no reaction force on that diagram because i did not draw those. I just showed a situation for somebody to show me the direction of reaction force of friction. Thanks for the answer. I will edit the question to make it a little clear. $\ddot \smile$ – A---B Aug 30 '16 at 19:23

If the object stays stationary and no force acting on it, there is no frictional force on it (static or kinetic). However when the force is applied from zero, until the object just starts moving, the static frictional force is acting on the object and increasing in magnitude. The direction is equal and opposite to the applied force until critical point is reached (where it starts moving) hence that obeys newton's third law.

• On which body it acts ? is what i asked ? :) – A---B Aug 30 '16 at 19:19
• This answer is an example of Newton's second law, not the third law. – garyp Aug 30 '16 at 19:24
• static frictional force is "equal and opposite"; reflects third law – Kosala Aug 30 '16 at 19:35
• "equal and opposite" also applies to the second law when the object is at rest. The forces that you (correctly) say are equal and opposite are two different forces that happen to be equal and opposite. They are not the two forces associated with a single interaction. They are not an action/reaction pair. – garyp Aug 30 '16 at 19:38

The red arrow is presumably the force on the block due to the table. It is a friction force.

By Newton's third law, there is a force on the table due to the block. It points to the right. It is a friction force.

You drew your diagram with a bit of an ambiguity. You drew the red arrow right on the interface between the block and the table, so it is not clear which object that force is acting on. To be clearer, you should draw the red arrow unambiguously connected to the block. The reaction force would be drawn as an arrow pointing to the right unambiguously attached to the table.