Abeka Physics Section 8.2 Flashcards
For most systems, Uk is what value? Explain.
For most systems, Uk is less than 1, showing that when an object moves over a horizontal surface, kinetic friction is less than the objects weight. Thus, the object can be slid more easily than it can be lifted.
Define and Describe Friction
> “the resistance arising when an object moves through a medium or across a surface”
friction arises from the interlocking of these irregular surface features
since electromagnetic forces prevent one atom from entering the space of another atom, motion is resisted
Consider a large block of wood on a flat table. If a small horizontal towing force is exerted on the block, the block will not move. Increasing the amount of towing force causes the block to suddenly start sliding. As long as the block is kept moving, less towing force is required to keep it moving than was required to start it moving originally. This simple example bears out what principle?
This simple example bears out the principle that “resistance to sliding is usually less than resistance to starting.” Resistance to sliding is called kinetic friction. Resistance to start sliding is called static friction. Thus, the principle could be restated as “Kinetic friction is usually less than static friction.”
List the four laws of kinetic friction
- Kinetic friction is independent of the area of contact between the surfaces.
- Kinetic friction is independent of velocity.
- Kinetic friction on a horizontal surface is proportional to the weight of the moving object.
- Generally, rough surfaces produce more friction than smooth surfaces.
If you attach a spring balance to a brick and pull the brick at constant speed across a smooth surface, the reading on the balance shows the force required to overcome kinetic friction. Kinetic friction is found to be the same whether the brick is resting on its broad side, on its long edge, or on its end. This law applies primarily to hard surfaces. Tell me which law of kinetic friction is being displayed.
“kinetic friction is independent of the area of contact between the surfaces”
Explain “kinetic friction is independent of velocity”
Kinetic friction is about the same whether the brick is pulled rapidly or slowly across the surface. To push a box or a chair across the floor requires about the same force whether you creep along or break into a trot. This law is only an approximation because kinetic friction actually increases slightly in the low range of velocities before leveling off in the high range of velocities.
Explain “kinetic friction on a horizontal surface is proportional to the weight of the moving object”
> If you pull a stack of two bricks across the table simultaneously, kinetic friction is doubled. With three bricks in the stack, you must pull three times as hard.
What factor affects the amount of friction between two surfaces?
The amount of force the two objects exert on each other affects the amount of friction between two surfaces.
Define and describe normal force
> normal forces is the force of the surface against the weight of the object
normal force always acts perpendicularly to the surface
If a surface is not horizontal, the perpendicular force applied by the surface against the object is reduced.
As the surface is tilted increasingly away from the horizontal, the normal force is a diminishing fraction of the object’s weight.
When the surface is vertical, the normal force is zero, and kinetic friction is then also zero. Explain why.
Because normal force acts perpendicularly to a horizontal surface, as the surface tilts toward the vertical, the surface supports less and less of the object’s weight. When the surface is vertical, the normal force is zero, and kinetic friction is then also zero. For example, a brick held against the wall and then released has no friction from the wall to overcome in its downward fall.
