Physics 5-5 Friction quiz

If the kinetic friction force is greater than the applied force

the object decelerates (second law)

If the kinetic friction force equals the applied force

the object moved with constant velocity (first law)

Formula for Friction

-Ff= μFn -For an object on a level surface, where applied forces are parallel to the surface -Remember the magnitude of FN is equal or the magnitude of Fg (Fg=mg) -depends on the surface area in contact -depends on the normal force

Kinetic Friction

-Kinetic frictional forces acts on a moving body. It is sometimes called sliding friction -Kinetic friction is a constant value while acting on the moving body *used only for sliding for objects *pulled along a surface *skidding

Static Friction

-Static frictional forces act when a force is applied to a body but the body does NOT move -The applied force can continue to be increased with the body until the applied force equals the maximum calculated value -Once the applied force equals this maximum static frictional force, the bodies will break away and the resting body will accelerate *used for any non-sliding objects *not moving *rolling"

Friction is

-a force that opposes motion -symbol Ff -acts parallel to surface and opposite the direction of motion

The Coefficient of Friction

-depends upon the nature of the surfaces that are in contact with each other -measures the attraction of the two surfaces -the more that the surface molecules adhere, the greater the coefficient value and the greater the friction force -does NOT depend on the surface area of contact -may be decreased by altering/lubricating the surface -Symbol: μ (Greek letter- mu)

Friction depends on

-the surface in contact -the normal force

Values of μ given on the reference table:

0 ≤ μ ≤ 1

Four identical rubber blocks are pushed along four different surfaces at a constant speed (first law). Which of the following surfaces will require the greatest force to move the block? A. Ice B. Dry concrete C. Wet asphalt D. Dry asphalt

B. Dry concrete (Use reference table)

If the kinetic friction is less than the applied force

the object accelerates (second law)

A wooden crate is pushed to the left across a wooden floor. The force of friction acting between the crate and the floor is directed _______________.

right

Two types of friction

Static friction- acts on a stationary object Kinetic friction- acts on a moving object *The maximum amount of static friction is always greater than kinetic friction

Normal Force

The "support force" for an object on a surface or leaning against a surface; perpendicular to and away from the contact surface