Do Now: A 40 N chair is pushed across a room with an acceleration of 2 m/s 2. Steven pushes with a force of 15N. What is the force of friction acting on.

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Presentation transcript:

Do Now: A 40 N chair is pushed across a room with an acceleration of 2 m/s 2. Steven pushes with a force of 15N. What is the force of friction acting on the chair?

Static – not moving Kinetic - moving

Maximum Force of Static Friction When the max force is applied to an object before the object begins to move, force of static friction is at a maximum

Force of Kinetic Friction friction when object is moving

Friction is dependent upon: static or kinetic normal force (which is equivalent to weight on a flat surface) texture of surface

Which is Greater: Max Force of Static Friction or Kinetic Friction? Do an experiment to find out which is greater. Use spring scales and pull a cork tray. Increase the mass of the tray and repeat to verify.

Which is Greater: Static or Kinetic Friction? > When a force is applied to an object at rest it does not move initially because the force of static friction balances with it. As the applied force increases so does the static friction… UNTIL it reaches the maximum force of static friction. If the applied force continues to increase the object moves and kinetic friction begins to act on the object.

Relationship Between Friction and Normal Force Ratio of friction to normal force = a constant called the coefficient of friction Rearrange:

Coefficient of friction (μ) Number that describes the amount of friction between 2 surfaces has no units, cannot be negative, usually (but not always) less than 1 2 surfaces have a max coefficient of static friction ( ) and coefficient of kinetic friction ( ) >

Example: ice on ice, = 0.1, = 0.03 rubber on concrete, = 1.0, = 0.8

Example Problem 1 A 4 kg box is pushed with a force of 15 N to the right. Determine the coefficient of friction between the box and floor if the acceleration of the box is 2 m/s 2.

Example Problem 2 A N box is pushed with a force of 25 N to the right. What is the acceleration of the box if the coefficient of friction between the box and floor is 0.4?