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Sliding Friction: State before Subtraction

Use an invented box on a stationary level floor. In each separate trial it is already sliding right. The only horizontal forces are a rightward push and sliding friction.

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Watch the lesson | Review: friction before sliding starts

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In these trials sliding friction is 6 N left. Use that value while the box slides right, then subtract opposing forces. Net force right speeds this rightward slide up; net force left slows it. Zero net force keeps velocity unchanged.

Worked example

In a different example, an already-sliding box has push 8 N right and sliding friction 3 N left. Net force is 5 N right. If the push were instead 3 N right, it would keep the same velocity. These are horizontal forces only.

An already-sliding box has eight newtons pushing right and three newtons of friction left, giving five newtons net right.
Worked example uses 3 N sliding friction. The questions use a different value: 6 N.
Question 1 Already sliding right: push 15 N right, sliding friction 6 N left. What is the net horizontal force?
Question 2 Already sliding right: push 6 N right, sliding friction 6 N left. What happens to its velocity?
Question 3 Already sliding right: the push is removed and sliding friction is 6 N left. What happens while it continues sliding right?
Question 4 The box has now stopped, with no horizontal push or other horizontal force. Which rule is appropriate next?
Question 5 A person starts a forward step without the planted foot slipping. Can static friction from the ground push that foot forward?

Further inquiry

Draw a new already-sliding-right trial with sliding friction 4 N left. Choose a push that speeds it up, one that keeps velocity unchanged, and one that slows it. Label force direction separately from motion direction.

Review criteria

  • State that the box is already sliding right.
  • Choose pushes above, equal to, and below 4 N and calculate each net force.
  • Explain why a leftward net force first slows a rightward slide rather than immediately reversing it.