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# Surface forces

## Reaction force

The reaction force is the force opposing any force exerted on a surface.

When you push with your finger against the wall, the reaction force will push against your finger so that the finger and the wall both remain stationary.

The reaction force is the result of the interaction between the subatomic particles on the surface of the two bodies in contact with each other.

The reaction force of a stationary object resting on a surface is perpendicular to the surface.

When you are standing on the ground the reaction force is perpendicular to the ground.

When you are walking the reaction force has some component that is parallel to the ground.

The component of the reaction force that is perpendicular to the surface is called the normal force.

The reaction force on two different stationary objects

## Friction as a resistance to motion

Friction is the force that resists the motion of a surface when it is in contact with another surface.

If you take your foot off the pedal while driving, the car will begin to slow down. This is due to friction.

Friction is parallel to the area of contact between each surface and opposite to the direction of motion of each surface.

Like all forces, the SI unit of friction is the newton $(\text{N})$.

Friction applies on all surfaces that are in contact. It prevents movement of a stationary object and resists the movement of a moving object.

Pushing a heavy table lightly results in no movement as friction keeps it in place.

A cyclist needs to keep pedalling all the time on flat ground to avoid slowing down due to friction.

Friction is greater between rougher surfaces. Friction can be reduced by coating surfaces in substances such as oil or vaseline.

Friction always acts parallel and along the surface of the objects sliding against each other.

## Different kinds of friction

Friction (or frictional force) is the surface force resisting the movement of two objects in direct contact with each other.

• Dry friction is the friction between two solid bodies.

• Static friction is the friction between two stationary objects.

• Kinetic friction is the friction arising when one object slides over another.

The magnitude of static friction is equal to the magnitude of the force it opposes (up to a certain threshold).

If a force pushing an object exceeds this threshold, the object starts to move.

This threshold is called the limiting friction.

The limiting friction is, in most cases, higher than the kinetic friction. This reflects the everyday experience that it is harder to get something to start moving than to maintain its movement.

The kinetic friction between an object and a solid surface is not affected by speed. This is because the microscopic forces between the two surfaces remain roughly constant regardless of speed.

Friction always acts parallel and along the surface of the objects sliding against each other.

## Fluid friction

Fluid friction (or viscous force) is the friction between at least one fluid (i.e. a liquid or gas) and an object or another fluid.

Viscosity is a measure of the resistance of a fluid to movements within it. A "thick" fluid has high viscosity.

Honey has high viscosity while water has much lower viscosity. The viscosity of air is only a fraction of the viscosity of water.

It is more difficult to stir a cup of honey than a cup of water (as there is greater force resisting the movement of the spoon)

Fluid friction depends on the relative velocity between the fluid and the other objects. When the fluid flows in smooth, parallel layers (i.e. laminar flow), the viscous force is directly proportional to the relative velocity.

Air drag (or simply drag), also called air resistance, is the viscous force of air on moving objects.

A mass falling into a fluid with low viscosity (above) and high viscosity (below). The more viscous fluid is much less affected the by the collision of the mass with fluid.