FRICTION

Friction

An opposing force to an outside force that tries to move a body surface past another body surface.

The direction of the frictional force is opposite the direction of motion.

Why?

Types of Friction

1. Static Friction (ƒs) – Friction present in bodies at rest ƒs = µsN

2. Kinetic or Sliding Friction (ƒk) – Friction present in moving bodies ƒk = µkN

Static & Kinetic Friction

Factors Affecting Friction (1)

1. Quality or Nature of the Surfaces in Contact

Rough surfaces mean greater friction, smooth surfaces mean lesser friction

Basis of the value of coefficients of static and kinetic friction

Independent of the magnitude of the area in contact

Quality of surfaces in contact affect friction.

Coefficients of Friction*

MATERIALS STATIC FRICTION

KINETIC FRICTION

Rubber on dry ice 1.0 0.7Rubber on wet concrete 0.7 0.5Wood on wood 0.5 0.3Waxed wood on wet snow 0.14 0.1Metal on wood 0.5 0.3Steel on steel (dry) 0.6 0.3Steel on steel (oiled) 0.05 0.03Teflon on steel 0.04 0.04Bone lubricated with synovial fluid 0.016 0.015Shoes on wood 0.9 0.7Shoes on ice 0.1 0.05Ice on ice 0.1 0.03Steel on ice 0.4 0.02

*Values are approximate.Source: Physics With Health Science Applications by Paul Peter Urone

Factors Affecting Friction (2)

2. Load or Normal Force

Force with which the two surfaces are pressed together

Acts perpendicular to the surfaces in contact

Normal Force

F

θθ

θ

W = mgF

W = mg

N = W N = W + F sin θ N = W - F sin θN = W cos θ

More about Friction

Friction is proportional to the normal force.

The force of kinetic friction acts when the object is in motion.

The force of static friction is generally greater than the force of kinetic friction.

Static friction acts to keep the object from moving.

If the shoe fits…

Why rubber?

Push back or threaded?

Problem 1

. 1. A) Find the normal force on a block of wood of mass 1 kg placed on a horizontal wooden surface. Determine friction.

B) If the block of wood is pressed against a vertical wooden wall with a force of 5 N, what is the normal force?

Problems 2 & 3

.

2. A mass of 50 kg is kept in uniform motion along a horizontal surface by a horizontal force. If the coefficient of friction between the surface and the mass is 0.5, how great is the friction force?

3. How much friction will there be in a knee joint of a standing person of weight 500 N? How about in your knee joint?

Problem 4

4. An object of mass 10 kg is on a rough floor and being pulled by force F = 15 N to the right. If the coefficient of static friction between the object and the floor is 0.2 and kinetic coefficient is 0.1, a. is friction static or kinetic? Give the

magnitude.b. Should the force cease to pull, how much

friction is present?c. If the pull is doubled, determine friction.d. If the floor is inclined at 10°, will the

object slide down?

A gem cannot be polished without friction,

nor man perfected without trials.

Lucius Annaeus Seneca