# What are the equal-and-opposite forces of friction forces?

• thecommexokid
In summary, there are several forces involved in the pictured situation of a block resting on a table and a second block hanging over the side connected by a rope. These forces include the gravitational force of the Earth on each object, normal forces, tension in the rope, and forces of friction. According to Newton's II and III laws, all objects involved are in equilibrium and the friction forces act in opposite pairs to oppose the relative motion between the two surfaces in contact.
thecommexokid

## Homework Statement

A block rests on a table, and a second block, connected to the first with a rope, hangs over the side of the table. See the illustration. (Assume the pulley is massless and frictionless.) All objects pictured are stationary.

Identify every force involved in the pictured situation.

## Homework Equations

Newton II and III.

## The Attempt at a Solution

None of block 1, block 2, the table, or the Earth are accelerating, so Newton II tells us the net force on each of the 4 objects must be 0. I'll identify forces in opposite pairs a la Newton III:
• Gravitational force of Earth exerted on block 1
• Gravitational force of block 1 exerted on earth
• Gravitational force of Earth exerted on block 2
• Gravitational force of block 2 exerted on earth
• Gravitational force of Earth exerted on table
• Gravitational force of table exerted on earth
• Normal force of table exerted on block 1
• Normal force of block 1 exerted on table
• Normal force of Earth exerted on table
• Normal force of table exerted on earth
• Tension of rope exerted on block 1 due to block 2
• Tension of rope exerted on block 2 due to block 1
• Force of friction exerted on block 1 due to table
• ? Force of friction exerted on table due to block 1 ?
• Force of friction exerted on table due to earth
• ? Force of friction exerted on Earth due to table ?
As you might have guessed from all the question marks, I'm confused about the opposite pairs to the friction forces. Obviously I need to include them for the table and the Earth to be in equilibrium, but I don't really know what they mean.

When I say, "The table exerts a force of friction on block 1," what I mean is that the block would be moving laterally across the table, except that the friction force of the table's surface opposes that motion.

But if I try to form the analogous statement, it does not seem to me to be the case that the Earth would be moving laterally beneath the table, except that the friction force of the table's feet opposes that motion. What gives?

Have you drawn your force diagrams for each object?

thecommexokid said:
As you might have guessed from all the question marks, I'm confused about the opposite pairs to the friction forces. Obviously I need to include them for the table and the Earth to be in equilibrium, but I don't really know what they mean.

When I say, "The table exerts a force of friction on block 1," what I mean is that the block would be moving laterally across the table, except that the friction force of the table's surface opposes that motion.

But if I try to form the analogous statement, it does not seem to me to be the case that the Earth would be moving laterally beneath the table, except that the friction force of the table's feet opposes that motion. What gives?
Friction is a force that opposes the relative motion between two surfaces that are in contact. It does not matter which direction the surfaces are moving or would be moving relative to some particular rest frame. What matters is which direction the surfaces are moving or would be moving relative to each other.

thecommexokid
jbriggs444 said:
It does not matter which direction the surfaces are moving or would be moving relative to some particular rest frame. What matters is which direction the surfaces are moving or would be moving relative to each other.

Cool. I think that clears up my confusion.

## 1. What is the equal-and-opposite force of friction?

The equal-and-opposite force of friction is the opposing force that arises when two surfaces rub against each other. It acts in the opposite direction of the applied force, making it difficult for objects to slide or move against each other.

## 2. How is the equal-and-opposite force of friction calculated?

The equal-and-opposite force of friction is calculated using the coefficient of friction and the normal force. The coefficient of friction is a constant value that depends on the materials and surfaces in contact, while the normal force is the force exerted by one surface on another at a right angle.

## 3. What factors affect the equal-and-opposite force of friction?

The equal-and-opposite force of friction is affected by several factors, including the types of surfaces in contact, the roughness of the surfaces, the weight of the objects, and the amount of force applied. The force of friction increases with rougher surfaces and heavier objects, while it decreases with smoother surfaces and lighter objects.

## 4. How does the equal-and-opposite force of friction affect motion?

The equal-and-opposite force of friction acts in the opposite direction of the applied force, making it difficult for objects to move or slide against each other. It can also cause objects in motion to slow down and eventually come to a stop due to the continuous opposing force.

## 5. Can the equal-and-opposite force of friction be eliminated?

The equal-and-opposite force of friction cannot be completely eliminated, but it can be reduced by using lubricants, smoother surfaces, or by decreasing the applied force. However, it is an essential force that helps us walk, drive, and perform many daily tasks without slipping or sliding.

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