Topic 4 of 4
Friction and motion
Friction acts between surfaces in contact. It opposes their relative sliding, or their tendency to slide.
Always identify the body whose force you are describing. The direction of friction must follow the particular contact, just like any other force on a free-body diagram.
Friction can slow a sliding object
A box slides to the right across a stationary floor. Its lower surface slides right relative to the floor, so friction from the floor on the box acts left. If this is the only horizontal force, the resultant is leftwards and the box slows.
Friction on a box sliding right
Selected body: box
The box slides right over a stationary horizontal floor.
Friction opposes the relative sliding at this contact. Arrow lengths are schematic.
If you pull the box steadily across the same floor, the pull can balance friction. The box then moves at constant velocity even though both horizontal forces are present.
Friction can prevent sliding
A gentle horizontal push does not always make a box move. Static friction can balance the push while the surfaces remain at rest relative to each other. For example, if a 2 N push is balanced by 2 N of friction, the horizontal resultant is zero.
Static friction adjusts to the situation up to a limit. Increasing the push may increase the balancing friction while the box remains still; it does not mean friction always has one fixed magnitude. If the available friction cannot balance the push, the box starts sliding.
Friction can help you move forwards
As you push off to start walking forwards, your planted foot pushes backwards on the ground. The foot would tend to slip backwards without enough grip. Friction from the ground on the foot acts forwards, helping accelerate you forwards.
Friction helps a person start walking
Selected body: planted foot
When pushing off, the foot tends to slip backwards (left) over the ground.
Ground-on-foot friction acts forwards. Only this contact force is shown.
This is why "friction always opposes the object's motion" is too broad. In the sliding-box case it acts backwards; in this walking case it acts forwards. In both, the direction follows the relative sliding or tendency to slide at the contact.
Grip, braking and resistance
Friction provides grip between a tyre and the road. Friction between brake pads and a rotating wheel or brake disc helps slow the rotation. It can also cause unwanted wear and resistance in moving machine parts. Reducing friction can make some movements easier, while reducing grip where it is needed can make slipping more likely.
Air resistance is a resistive force from the surrounding air. Its direction opposes motion relative to the air. An object falling downwards through still air experiences upward air resistance.
Optional explanation: a box stays still under a 3 N horizontal push
Assume the floor's friction is the only other horizontal force. What is the friction on the box, and why?
The box remains at rest, so horizontal acceleration and resultant force are zero. Friction is therefore 3 N opposite to the push. There is no horizontal motion, but there is still a force of friction.