Problem 1:

When sliding a heavy box across the floor, it is often times helpful to lift up on it slightly because
A) You are lessening the friction force by reducing the contact force between the box and the floor
B) You are reducing the weight of the box.
C) You are working against friction by lifting up.
D) You are working against gravity, even if the box does not move upwards.
 

• Solution:
  A) You are lessening the friction force by reducing the contact force between the box and the floor
  The maximum force of static friction between two surfaces is roughly proportional to the magnitude of the normal force N pressing the two surfaces together.  Lifting the box up slightly reduces N.