Newton’s laws and application of Newton’s laws
(This section must be read in conjunction with the CAPS, p. 62–66.)
Different kinds of forces: weight, normal force, frictional force, applied force (push, pull), tension (strings or cables)
• Define normal force, N, as the force or the component of a force which a surface exerts on an object with which it is in contact, and which is perpendicular to the surface.
• Define frictional force, f, as the force that opposes the motion of an object and which acts parallel to the surface.
Define static frictional force, fs, as the force that opposes the tendency of motion of a stationary object relative to a surface.
Define kinetic frictional force, fk, as the force that opposes the motion of a moving object relative to a surface.
Know that a frictional force:
o Is proportional to the normal force
o Is independent of the area of contact
o Is independent of the velocity of motion
• Solve problems using fmaxs = μsN where fmaxs is the maximum static frictional force and μs is the coefficient of static friction.
o If a force, F, applied to a body parallel to the surface does not cause the object to move, F is equal in magnitude to the static frictional force.
o The static frictional force is a maximum (fmaxs) just before the object starts to move across the surface.
o If the applied force exceeds fmaxs, a resultant/net force accelerates the object.
• Solve problems using fk = μkN, where fk is the kinetic frictional force and μk the coefficient of kinetic friction.


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