Physics: Forces and Newton’s Laws 1/1/2019 Objective: I can determine the effect that friction has on a moving object and solve problems involving friction

Normal Force The normal force is a contact force exerted by one object on another It always acts perpendicular to the plane of contact

Normal force The normal force is a general term for forces from several different sources For example, gravity pulls me toward the floor; the floor provides a normal force that holds me in place.

Tension If I pull on one end of a rope with a 5 N force, the other end of the rope pulls on whatever it’s attached to with an equal force Tension is another force we often encounter in physics problems – It’s the force exerted by a string or rope From Newton’s Laws, we know that tension must be equal at both ends of a rope

Tension Even if the ends of the rope are in different directions, they still exert equal forces Tension always pulls an object toward the rope or string.

Friction Friction is always a contact force that opposes motion between two objects. We have two types of friction: Static Friction and Kinetic Friction

Friction Static Friction Kinetic Friction Force exerted by one object on another when there is no motion between them Example: You try to push a heavy couch across a carpeted floor. Because of the static friction, you are unable to get the couch to move. Force exerted by one object on another when they rub against each other because one or both is moving. Example: You push your book across your desk, and feel the force of kinetic friction resisting your movement

Friction and the Normal Force The force of friction, either static or kinetic, is always related to the normal force The greater the normal force, the greater the force of friction.

Coefficient of Friction The coefficient of friction is a property of materials. It tells you how much friction there will be when two objects are in contact. Notice that the coefficient is different for static and kinetic friction

Coefficient and Forces We represent the coefficient of friction with the Greek letter μ. We use subscripts to define which type of friction we’re dealing with: μk = kinetic, μs = static We find the force of friction by multiplying the coefficient by the normal force: Ff = μFN

Force of Friction So, if we want to find the force of static friction on a 2 kg box with μs = .75:

Homework Problems P. 128, # 17- 21 These will be due Friday. That means you’ll have a couple days to work on them and some time tomorrow to ask questions about them. P. 130, # 22-26 Some of them are tough, so I recommend starting them tonight so you can get help tomorrow.