Unit 2c: Friction and Free Body diagrams

The Normal Force

  • When an object with mass m is resting on the ground, it has the force of gravity (Fg = mg) pulling it down.
  • Opposing this force (Newton’s 3rd law) is the ground pushing up on the object. This force is the normal force (FN)
  • If the forces are balanced (object is resting on level surface), Fg = FN

Frictional Force:

  • force opposing the sliding motion of two surfaces that are in contact with each other
  • Cause:
  • bumps on surface from one object touch those on another, causing micro-welds, or regions of sticking together.
  • A force is required to break these interactions
  • The smoother the surfaces, the fewer the areas of contact, the less friction
  • The frictional forces only act in opposition tomovement and “disappear” when the object is at rest
  • kinds of friction
  • Static friction: initially prevents 2 surfaces from sliding past each other
  • The initial force to get an object resting on a surface to move
  • Example: you push on a heavy box and it doesn’t slide (static force > your applied force)
  • Example: a heavy box is on an inclined plank but it doesn’t slide down (static force > the vector component of gravitational force parallel to the plank)
  • Sliding (kinetic) friction: opposes the motion of two surfaces already sliding past each other
  • Almost always less than the static friction force
  • Example: you slide a box across the floor and let go and the box slows down to a stop
  • Example: Low sliding friction causes drivers to hydroplane or lose control on ice or wet concrete.
  • Rolling friction: special static friction between surface of rolling wheel and ground. Required for rolling. Also called “traction”
  • Example: wheels on ice have low rolling friction so the wheels spin in place
  • Example: “burning rubber”: the force of the tires pushing the car forward is greater than the rolling frictional force, causing spinning in place
  • Calculating frictional force
  • Frictional force = Ff = µ x FN
  • µ is the frictional coefficient. It is dimensionless.
  • µs is the static frictional coefficient and µk is the sliding frictional coefficient
  • The rougher the surface, the greater the value of µ
  • The heavier the object, the greater the Ff

Calculated force needed to accelerate a box at rest for the following scenarios:

Box Mass / Static frictional coefficient (µ) / Force to move
10 kg / Box on rubber: 0.90
10 kg / Box on tile: 0.30
10 kg / Box on ice: 0.025
30 kg / Box on rubber: 0.90
30 kg / Box on tile: 0.30
30 kg / Box on ice: 0.025

Conclusions on effects of mass and friction coefficients:

  • Summary of forces we can write on a free body diagram
  • Fa = applied force
  • Fg = gravitational force
  • FN = normal force
  • Fsf = static frictional force
  • Fkf = sliding/kinetic frictional force
  • Fair = force of air resistance
  • Ft = force of tension (as in on a rope, string, or cable)
  • How to complete a free body diagram:
  • Draw your box/object
  • Add the Fg (down)
  • If the object is on a surface, add FN perpendicular to the surface
  • Add the Fa (applied force) if necessary
  • If the object is moving along a surface, or the Fa is parallel to the surface, add the Fkf or Fsf.
  • If the object is hanging on a rope/string/cable, add Ft (tension) pulling the object up
  • Remember: an object with balanced forces…
  • Will NOT accelerate in any direction
  • May still be moving or may be at rest. If moving, constant velocity
  • If the forces are unbalanced, object will accelerate in direction of net force

Practice:

Draw the free body diagrams for the following scenarios. Use arrows of similar length to illustrate balanced forces and arrows of noticeably different length to illustrate unbalanced forces.

1. While a player kicks a soccer ball at an upward angle, a force is applied in both the horizontal and vertical directions. Ignore air resistance.

2. Draw the free body diagram for the soccer ball once it has left the foot of the kicker.

3. Draw the free body diagram for the ball once it has landed and is rolling to a stop across a field.

4. Draw the free body diagram of the soccer ball if it has been attached to a rope and is hanging. The ball has a mass of 0.43 kg.

5. Draw the free body diagram of the soccer ball on a rope if a boy is in the middle of yanking the rope up to accelerate the ball upward.

6. Draw the free body diagram of a cat slipping on ice, moving at constant velocity (assume friction is negligible).

Name______Date ______

Free Body/Forces Diagram Practice

1. A 30 kg man

A box full of books with a mass of 50.0 kg is pushed at a constant velocity across a floor. The sliding frictional coefficient is 0.45. Draw a free body diagram and quantify the forces acting on the box.

2. A