Friction

Friction is the force that resists the motion of one surface relative to another with which it is in contact. It is parallel to the contact surfaces and opposite to the direction of motion or impeding motion.

SI unit of friction is newton (N). It is a vector quantity.
Viscous force is the equivalence of friction in fluids that resists the relative motion of a body through the fluid.

Body in Motion

When a body is in motion, friction will tend to slow it down.

Example: Pushing a box along the table. The box will eventually come to rest.
The friction comes from the microscopic surface irregularities of the two surfaces (of the box and the table). The surface irregularities catch onto each other and resist motion.

Body in Motion (With pushing force)

When the magnitude of the pushing force is equal to the magnitude of frictional force on the object, the object will travel at a constant speed.

Why? Since the magnitude of the pushing force is equal to the magnitude of the frictional force on the object, there will be zero resultant force. From Newton’s 2nd law of motion ($F = ma$), there will be zero acceleration (i.e. constant speed).

Body at Rest

When a body is at rest, friction will have to be overcome before the body can start to move.

Why? For a body to embark on a state of motion, there needs to be acceleration. From Newton’s 2nd law of motion ($F = ma$), we will require a non-zero resultant force to be acting on the body. Hence, the pushing force (or driving force) must be greater than the frictional force.
In the picture above, the pushing force of 4N is not enough to overcome the frictional force. Hence the body is at rest.

Advantages and Disadvantages of friction

Advantages of friction	Disadvantages of friction
Able to start of stop motion (Used in braking pads to slow down cars)	Reduces speed of motion (A box sliding along the floor gradually comes to a stop)
Prevents slipping when walking	Causes wear and tear (Gears get worn out and require replacements)
Essential for rolling motion (You cannot roll a ball if there is no friction!)	Energy is wasted as work done to overcome friction

Methods to reduce friction:

Lubricate the surfaces in contact
Smoothen the surfaces in contact (E.g. By polishing)
Place ball bearings, rollers between surfaces
Use an air cushion between surfaces (E.g. Hovercraft)

Example:

A man pushes a trolley forward with a force of 10 N. The floor exerts a frictional force of 7 N on the trolley. Find the acceleration of the trolley, given that the mass is 10 kg. If there is no friction, what would be the new acceleration?

Show/Hide Answer

$$\begin{aligned} \text{Net resultant force: } F_{\text{res}} &= F_{\text{man}} \, – F_{\text{fric}} \\ &= 10-7 \\ &= 3 \text{ N} \end{aligned}$$

Net resultant acceleration is given by Newton’s 2nd law of motion,

$$\begin{aligned} a &= \frac{F_{\text{res}}}{m} \\ &= \frac{3}{10} \\ &= 0.3 \text{ ms}^{-2} \end{aligned} $$

If there is no friction,

Net resultant force $F_{\text{new res}} = F_{\text{man}}$

Net resultant acceleration,

$$\begin{aligned} a &= \frac{F_{\text{new res}}}{m} \\ &= \frac{10}{10} \\ &= 1.0 \text{ ms}^{-2} \end{aligned}$$

Self-Test Question

Can you lean against a wall if frictional forces are absent?

Show/hide Answer

No! You will slide down the wall.

When you are leaning against a wall, frictional forces help to “neutralise” part of the gravitational forces that are acting on your body.

Additional Notes:

The major cause of friction between solids appears to be the forces of attraction, known as adhesion (electromagnetic forces), between the contact regions of the surfaces, which are always microscopically irregular. Friction arises from shearing these “welded” junctions and from the actions of the irregularities of the harder surface plowing across the softer surface.
Sliding friction arises because of relative motion between surfaces. There are two kinds of sliding friction: static and kinetic. Static sliding friction refers to the amount of force that needs to be overcome to start motion. On the other hand, kinetic sliding friction is the force that needs to be overcome in order to maintain relative motion between the surfaces.
The force required to start motion, or to overcome static friction, is always greater than the force required to continue the motion, or to overcome kinetic friction.
From experiments, sliding friction is found to be proportional to the load or weight of the body in contact with a surface and is nearly independent of the area of contact.
Rolling friction occurs when a wheel, ball, or cylinder rolls freely over a surface. (Not possible to roll without friction!)
Sliding friction is generally 100 to 1,000 times greater than rolling friction. Thus, a body on a sledge is harder to pull than the same body on wheels.