Study for test on forces.
See these sites for review of the topics:
See this page for additional resources.
Practice:
(a) A 10 kg steel box on a steel floor (mus=.74; muk=.57) is at rest. A 500 N force is applied
to the
right and a 450 N force is simultaneously
applied to the left. Draw the FBD.
Find the normal force. Find the force of friction (Will it move?). Find the acceleration.
(b) For the same box, now change part
(a) so that the force to the left is only 300 N
(you still have the 500 N force to the right). Draw the FBD.
Find the normal force. Find the force of friction (Will it move?).
Find the acceleration.
Answers: a) FN = 98 N; Ff = 50 N left. It does not move because the net applied force is less than max static friction. a = 0.
b) Now it moves. The net applied force is 200 N, which is greater than the max static (74.48 N). The kinetic friction now applies and it is 55.86 N.
a = 14.4 m/s2.
Sample question:
Solve for the normal force, the force of friction and the acceleration along the plane in
the following problem:
A 2-kg steel block sits
on a steel surface (same coefficients as above) that is inclined at an angle of 30 degrees. A force
of 20 N is applied to the block up the incline parallel to the surface of the incline.
Find FN,
Ffriction and acceleration along the incline.
Try this problem first. If you have trouble, check here.
Determine what happens to the acceleration of a 2-kg block on a frictionless
floor when the angle of the 15-N applied force increase from 0 degrees (horizontal)
to higher angles above the horizontal.
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