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Friction and Inertia, Explained

Here's how friction and inertia work together to explain nearly every kind of motion around you.

What is friction?

Friction is a force that acts between the surfaces of two objects in contact, opposing their relative motion. It's split into static friction, which acts before an object starts moving, and kinetic friction, which acts on an object that's already moving β€” and the maximum static friction is generally larger than kinetic friction.

What would go wrong without any friction

If there were no friction at all, there'd be no grip between your shoes and the floor, making it impossible to even walk, and a car's tires wouldn't be able to push against the road, just spinning in place. Nails and screws wouldn't be able to hold objects together, either β€” friction is inconvenient at times, but it's also absolutely essential to everyday life.

What is inertia?

Inertia is an object's tendency to keep its current state of motion β€” whether at rest or moving in a straight line at constant speed β€” unless a force acts on it. The greater an object's mass, the harder it is to change its state of motion, so mass is essentially a measure of the amount of inertia an object has.

Why your body lurches when a vehicle stops suddenly

When a bus or car stops suddenly, your body keeps moving forward due to inertia, causing you to lurch forward; conversely, sudden acceleration makes you feel pushed backward. This is precisely the physical reason seatbelts are essential in vehicles.

How to reduce or increase friction

Applying lubricant to a machine's rotating parts, or using bearings, can significantly reduce friction, cutting energy loss and wear β€” while, conversely, adding tread grooves to car tires or patterns to shoe soles increases friction to prevent slipping. Engineering deliberate control of friction, in either direction, is important across many fields.

Why understand friction and inertia together

Friction and inertia are both fundamental concepts involved in nearly every motion we make β€” walking, driving, moving objects. They can seem like opposites, since friction resists motion while inertia is the tendency to keep moving, but in practice the two work together to produce the motion we see all around us.

Conservation of momentum, too

Inertia β€” an object's tendency to keep its state of motion β€” is closely connected to the concept of momentum as well, since momentum is essentially inertia in motion.

Frequently Asked Questions

What does it mean that static friction is greater than kinetic friction?

Think of pushing a heavy piece of furniture: it takes the most force right at the start to get it moving, and once it's moving, less force is needed to keep pushing it. That's because the maximum static friction resisting an object at rest is generally larger than the kinetic friction resisting an object already in motion.

What exactly does it mean that more mass means more inertia?

Greater inertia means it's harder to change an object's state of motion. Just as it's much harder to push or stop a fully loaded shopping cart than an empty one, an object with more mass needs more time or more force to change its speed by the same amount, for the same applied force.