Section 4 review
The term friction covers a wide range of forces, all of which either oppose motion or oppose the forces that would cause motion. Static friction prevents an applied force from making an object slide. Kinetic friction creates a force that acts opposite to the direction of sliding motion. We model the force of friction as a coefficient times the normal force acting between moving surfaces. The concept of friction also extends to air resistance and viscosity, both of which resist an object’s motion through a fluid. Read the text aloud
static friction, coefficient of static friction, coefficient of kinetic friction, lubrication, sliding friction, rolling friction, coefficient of rolling friction, viscosity, drag coefficient

F f = μ s F N
F f = μ k F N
F f = μ r F N
F f = 1 2 c d ρA v 2

Review problems and questions

A mover pushing on a 40 kg crate
  1. A mover is pushing on a 40 kg crate that rests upon a hardwood floor. Her efforts proceed as follows:
    1. She applies a horizontal 120 N force.
      The crate does not move.
    2. She next applies a 160 N force.
      The crate remains motionless.
    3. She increases her push to 200 N.
      The crate just begins to slide.
    4. She returns to a 160 N push.
      The crate slides at a constant speed.
    1. What is the maximum static friction force (in newtons)?
    2. What is the kinetic friction force (in newtons)?
    3. What is the coefficient of static friction?
    4. What is the coefficient of kinetic friction? Read the text aloud Show
Which of the three diagrams corresponds to the skydiver's terminal velocity?
  1. A skydiver jumps from an airplane and falls toward Earth. His instructor tells him to open his parachute. When he does so, the force of air resistance pushes upon his parachute, and the cords pull up on him. His fall slows down and gradually he reaches terminal velocity. The diagrams show three moments during this process; the lengths of the force vectors indicate how strong the forces are.
    1. Why does the skydiver slow down when the parachute is opened?
    2. Which of the three diagrams corresponds to terminal velocity, and why?
    Read the text aloud Show

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