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 or materials in contact.  Friction is a force between surfaces that depends on the materials in contact with each other.  SI unit: Newtons (N)  Ex: Tires on the road, rubbing your hands together, air resistance

Forces are needed to overcome any friction that may be present to get an object moving. When friction is present, an object may move with a constant velocity even if an outside force is applied to it.  If the NET FORCE is 0, there is NO acceleration, but it can still be moving

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 on one surface by a second surface when there is no motion between the two surfaces  Always with nonmoving objects; always larger than kinetic friction  More difficult to overcome at rest inertia  SI unit: Newtons (N)  Ex: trying to make a parked car move, trying to move furniture

Kinetic Friction: the force exerted on one surface by another surface when the two surfaces rub against one another because one or both surfaces are moving.  Always with one object in motion  Always smaller than static friction  SI 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.

Coefficient of Friction The friction between objects depends on the materials the objects are made of  Ex: rubber shoes have more friction on asphalt than ice on asphalt A surface with more microscopic bumps will have more friction Each surface-on-surface contact has its own ratio  The coefficient of friction  Usually a decimal point Between 0 and 1 No units!!!; just a number

Just like with static and kinetic friction, the coefficient of static friction is ALWAYS bigger than the coefficient of kinetic friction.

Check Your Understanding Why is the coefficient of static friction always bigger than the coefficient of kinetic friction, even though it’s the same material? Because static friction has to overcome an object’s initial inertia, thus making it more difficult to move.

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/s 2 But it can still be moving at a constant speed

Terminal velocity: the maximum speed a free falling object can obtain due to air resistance; when the air resistance against an object equals the object’s weight  The acceleration equals zero because it is moving at a constant speed  Ex: a piece of paper slowly falling in a classroom full of air

Check Your Understanding What is the acceleration due to gravity on Earth in a vacuum? About 10 m/s 2. What is the acceleration of a piece of paper floating towards the ground in a classroom filled with air? Zero! It has already reached its terminal velocity by the time it has left your hand. Because of the small amount of weight of the paper, it does not take long for the air resistance to equal the paper’s weight.

Check Your Understanding What is the net force acting on a piece of paper in free fall in a room full of air? Zero! Since its acceleration is zero (terminal velocity), its net force must also be zero (remember Newton’s 2 nd law of motion).