Physics : Free-body diagram

Chocolate Cake SG

Hi all,
I need help on this question.

1. The figure below shows a man pushing a box of mass 20kg across the horizontal floor at a constant speed of 0.2m/s. The box slides without tipping. He pushes the box by applying a constant horizontal force of 80N.
( Assume that g = 10N/kg )

(a) Calculate the weight of the box
(b) (i) State the size of the frictional force acting on the box, and explain how you obtain your answer.
(ii) What is the direction of the frictional force?
(c) The box then moves onto the polished section of the floor shown in the figure above. As he continues to apply the same force, the box accelerates at 1.5m/s².
Calculate for the box(on the polished section of the floor).
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How do we find the size of the frictional force?

Thanks.

crimson soldier SG

think about it: when he applies a constant force, the speed is still constant instead of increasing. Why?

freakass SG

think about it: when he applies a constant force, the speed is still constant instead of increasing. Why?

F=ma.

If the man applies a constant force (F), there should be a constant acceleration (a). This means that the speed should be increasing.

However, for this case, speed is constant. This implies that acceleration (a) is ZERO. This is achievable when the friction force is equivalent to the applied force.

F - friction = ma

F - friction = 0

F = friction