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5.3 Friction and Normal Force

5.3 Friction and Normal Force. pp. 119 - 123 Mr. Richter. Agenda. Warm-Up Review HW Notes: What is Friction? Kinetic vs. Static Friction Friction and the Normal Force Avoiding and Using Friction. Objectives: We Will Be Able To…. Distinguish between kinetic and static friction

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5.3 Friction and Normal Force

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  1. 5.3 Friction and Normal Force pp. 119 - 123 Mr. Richter

  2. Agenda • Warm-Up • Review HW • Notes: • What is Friction? • Kinetic vs. Static Friction • Friction and the Normal Force • Avoiding and Using Friction

  3. Objectives: We Will Be Able To… • Distinguish between kinetic and static friction • Explain the cause of friction • Discuss reasons to increase or decrease friction.

  4. Warm-Up: • Timmy and Tommy are holding a box that weighs 700 N. If Timmy can lift with a force of 450 N, how much force does Tommy have to lift with to keep the box at the same height?

  5. Friction

  6. Friction • Friction is a force that resists the motion of objects. • Friction is caused by the microscopic hills and valleys between two surfaces.

  7. The Direction of Friction • Friction is a force. • On a free-body diagram, friction always points in the direction opposite the motion of the object. Even if the object doesn’t actually move! Force of Friction

  8. Two Types of Friction • Kinetic Friction (“sliding” friction) is present when two objects or surfaces slide across each other. • Static Friction is present when forces are acting to cause an object to move but friction is keeping the object from moving.

  9. Two Types of Friction • The maximum static friction is always greater than the kinetic friction between two surfaces. • This is because the little hills and valleys get stuck together when an object is stopped, but they can skim across each other when an object is sliding. • Think about running your fingers along a chain link fence.

  10. What does Friction Depend on?

  11. Friction Depends On… • The normal force is the force of a surface pushing back on an object. • The word “normal” means perpendicular. The normal force is perpendicular (makes a 90 degree angle) to the surface. • If a piece of paper rests on a table, the table exerts a normal force on the piece of paper.

  12. Friction Depends On… • Friction is proportional to the normal force. Meaning… • The greater the force squeezing two surfaces together, the greater the friction force.

  13. Friction Depends on… • Friction also depends on the two surfaces that are trying to slide against each other. • A hockey puck sliding on ice will work better than a hockey puck sliding on sand paper.

  14. Avoiding and Using Friction

  15. Avoiding Friction • Friction exists any time two surfaces rub against each other. • Friction slows motion, causes wear and tear, and turns useful energy into less useful forms, like heat. • Humans use many things to decrease the effect of friction: • ball bearings • oil • magnets • etc.

  16. Using Friction • Sometimes we want surface to resist motion. • We want tires to stick to the road. • We want nails to stick into the wood.

  17. Wrap-Up: Did we meet our objectives? • Distinguish between kinetic and static friction • Explain the cause of friction • Discuss reasons to increase or decrease friction.

  18. Homework • p123 #1-4 • p116 #a,b

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