Frictional Forces

Friction Two or more objects in contact moving past each other will slow each other down. The force which causes them to slow down is friction. Friction: the force that acts to resist the relative motion of objects in contact. Friction is a force between surfaces that depends on the materials in contact with each other and how much force is pushing the objects together Unit: Newtons (N) Ex: Tires on the road, rubbing your hands together, air resistance

Check Your Understanding Why do sports shoes have more tread on them compared to dress shoes? Because more tread leads to more bumps, which leads to more friction. Athletic shoes need more friction to allow for greater acceleration (rapid sprints, sudden stops, and quick turns), whereas dress shoes do not require the same sudden change in motion.

2 Types of Friction There are 2 types of friction, and both ALWAYS oppose motion Static Friction Kinetic Friction

Static Friction: the force exerted between two surfaces when there is no motion between the two surfaces Always with nonmoving objects; always larger than kinetic friction Takes more force to overcome More difficult to overcome at rest inertia Unit: Newtons (N) Ex: trying to make a parked car move, trying to move furniture

Kinetic Friction: the force exerted between two surfaces when they rub against one another because one or both surfaces are moving. Always with at least one object in motion Always smaller than static friction Does note take as much force to overcome Unit: Newtons (N) Ex: a car rolling down the highway, walking on concrete

Check Your Understanding A car is broken down in the parking lot and the driver tries to push the car off to the side but can’t move it. What type of friction is this? Static friction. It won’t be considered kinetic until the driver gets the car moving.

Check Your Understanding If the friction forces between the road and a car on a dry day are 4500 N and 5200 N, which is the frictional force when the car is moving? The 4500 N. Kinetic friction is always smaller than the static friction because it does not have to overcome the car’s initial inertia.

Free Fall and Air Resistance Air resistance is a type of friction because it opposes movement. Air resistance diminishes the net forces acting on an object in free fall. This is why an elephant and a feather will not fall at the same rate on Earth; the air resistance slows down the feather. The air resistance builds up quicker for the small feather, thereby slowing it down quicker

When the air resistance on an object in free fall equals the object’s weight, the object has reached its maximum speed, its terminal velocity, and will have zero acceleration. This is b/c the object is in equilibrium The net force is 0N, so the acceleration is 0 m/s2 But it can still be moving at a constant speed