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There are four forces acting on the box: A spring pulls with a force of 40 N. The force of gravity acts downward; and the force of the table is pushing up on the box. What MUST be true of the frictional force if the box moves across the table?

AIt is equal to the effort force.
BIt is less than the effort force.
CIt is more than the effort force.
DIt is equal to the force of gravity.

Verified Answer :

leecanmex

If it MOVES across the table, you are assuming that there is an unbalance between at least two forces, in this case, since gravity and the force normal cancel each other out, only friction and the spring's force remain. 
Friction is always less than the effort force, thus the answer is B)

Answer:

B) It is less than the effort force.

Explanation:

As we know that box is moving across the table

it is having four forces on it

1) Force due to its own weight (W = mg)

This force is vertically downwards

2) Force due to contact force of table. It is vertically upwards and magnitude is same as that of weight.

3) Spring force which is pulling the box by 40 N force.

This is the force due to which box is pulled in the direction of the force

4) Frictional force on the box is opposite to the direction of the motion of the box. So here frictional force must be smaller than the pulling force due to which box will slide on the floor

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