Forces Mass, Weight, and Friction

Weight Weight: force of gravity on an object - on Earth your weight is a direct measure of the planet’s force pulling you toward the center - measured in Newtons (N) Weight can be found by using Newton’s 2nd Law F = ma *REMEMBER - force of gravity on an object is present whether the object is falling, resting, or being lifted

Weight g = 9.8 m/s 2 F = ma W = mg m = kgg = m/s 2 W = Newtons (N) On Earth: 1 kg object = 9.8 N (weight)

Friction Any force that opposes motion is called Friction - brakes on a car, pushing a box along the floor, skydiver in the air Friction: force that opposes the motion between 2 surfaces that are in contact, parallel to surface What is µ (mu)? The level of friction that different materials exhibit is measured by the coefficient of friction, given the symbol µ. The higher µ is, the more force resists motion if two objects are sliding past each other. µ is UNITLESS - it DOES NOT have a unit!

Friction Two Types of Friction: Sliding (Kinetic) Friction: force between 2 surfaces in relative motion - this is LESS then static friction *A car will stop faster if wheels are not skidding symbol: µ k Static Friction: force that opposes start of motion between 2 surfaces *Box will move when the force you use is greater then the maximum value of static friction between the floor and box. Box moves, force of friction decreases symbol: µ s

Friction There are two forms of friction, kinetic and static. If you try to slide two objects past each other, a small amount of force will result in no motion. The force of friction is greater than the applied force. This is static friction. If you apply a little more force, the object "breaks free" and slides, although you still need to apply force to keep the object sliding. This is kinetic (sliding) friction. You do not need to apply quite as much force to keep the object sliding as you needed to originally break free of static friction.

Friction What if you want to keep an object moving at a constant velocity? You must exert a force that is just the same size as the frictional force If an object is moving at a constant velocity, then the force applied to the object WILL BE EQUAL TO the frictional force

Forces on an Object The force you exert: Applied force F A Force of friction: F f F f depends primarily on the force pushing the surfaces together F N - Normal force - perpendicular to F f, opposite direction of Weight force W - Weight - force of gravity on object

Friction F f = µ F N orF = µN Equation to find Frictional Force: *ratio of frictional force and normal force between 2 objects If a book is resting on a table, F N = weight of book Two Sets of Forces: 1) Parallel to the surfaces they are touching. - Force that moves the object and the force that opposes 2) Perpendicular to the two surfaces. - downward force, object’s weight, and the other is equal but upward force