Physics 311, Quiz #6

Question 1: A box of mass M = 20 kg sits on the floor. Express as a vector the normal force exerted by the floor on the box.

    There are two forces acting on the box: gravity and the normal force.
    We know the total force must be zero, because the box isn't
    accelerating up or down.
    Using a coordinate system in which "up" is positive y direction
    and "down" is negative y direction, 

             F(total)  =  F(gravity)  +  F(normal)

	        0      =    -Mg       +  F(normal)

    So

            F(normal)  =    +Mg   =   196 N  up

Question 2: Joe presses down on the top of the box with a force of 20 N. What is the normal force exerted on the box by the floor now?

    There are now three forces acting on the box: gravity, the normal force,
    and Joe's push.

             F(total)  =  F(gravity)  +  F(normal)  +  F(Joe)

	        0      =    -Mg       +  F(normal)  +   -20 N

    So

            F(normal)  =    +Mg + 20N   =   216 N  up

Question 3: Joe pulls up on the top of the box with a force of 20 N. What is the normal force exerted on the box by the floor now?

    There are again three forces acting on the box: gravity, the normal force,
    and Joe's push.

             F(total)  =  F(gravity)  +  F(normal)  +  F(Joe)

	        0      =    -Mg       +  F(normal)  +    20 N

    So

            F(normal)  =    +Mg - 20N   =   176 N  up

This page maintained by Michael Richmond. Last modified Jan 6, 2001.