http://www.ck12.org Chapter 3. Forces
Review
Questions
- A 52 N sled is pulled across a cement sidewalk at constant speed. A horizontal force of 36 N is exerted. What
is the coefficient of sliding friction between the sidewalk and the metal runners of the sled? - If the coefficient of sliding friction between a 25 kg crate and the floor is 0.45, how much force is required to
move the crate at a constant velocity across the floor? - A smooth wooden 40.0 N block is placed on a smooth wooden table. A force of 14.0 N is required to keep the
block moving at constant velocity.
(a) What is the coefficient of sliding friction between the block and the table top?
(b) If a 20.0 N brick is placed on top of the wooden block, what force will be required to keep the block and
brick moving at constant velocity?
- friction:A force that resists the relative motion or tendency to such motion of two bodies or substances in
contact. - coefficient of friction:The ratio of the force that maintains contact between an object and a surface (i.e. the
normal force) and the frictional force that resists the motion of the object. - normal force:The perpendicular force one surface exerts on another surface when the surfaces are in contact.
To understand physics, it is necessary to understand how objects move. Acted on by forces, objects are subject to
Newton’s Three Laws of Motion. The most widespread force is gravity, which has the same effect on all objects all
over the world; it is the force of gravity that gives an object of some mass its weight. Finally, a significant force often
overlooked in elementary physics courses is friction, but understanding friction equations and friction’s influence on
an object’s motion is necessary to properly calculate its trajectories.