# PHYSICS HELP?

A box of mass 12 kg is at rest on a horizontal surface. What is the magnitude and direction of the gravitational and normal forces acting on the box?(b) Another box of mass 12 kg is also at rest on a horizontal surface, but a student is pushing downward on this box with a force of magnitude 25 N. What is the magnitude and direction of the gravitational and normal forces acting on this 12 kg box?

### 3 Answers

- oubaasLv 75 months ago
a)

gravitational and normal forces acting on the box are exactly the same (12*9.807 = 117.7 N in module) and directed downward

b)

gravitational and normal forces acting on the box are exactly the same (12*9.807 +25 = 142.7 N in module) and directed downward

- 5 months ago
The gravitational force is the same in both cases, W = mg = (12 kg)(9.81 m/s^2) = 117.72 N.

In both cases, the box is not moving or accelerating in the vertical direction, so by Newton's Second Law the sum of the forces in the vertical direction is zero.

Case 1: The only two vertical forces acting on the box are the gravitational and normal forces. Therefore:

N - mg = 0 => N = mg = 117.72 N

Case 2: Now we have a new 25 N force acting downwards on the box, so we have to include that in the sum of the forces:

N - mg - 25 N = 0 => N = mg + 25 N = 117.72 N + 25 N = 142.72 N

I know it looks like a lot more work to do it this way, but when more forces are added in, having an approach to fall back on makes this type of thing much easier to deal with.

Hope this helps!

- AshLv 75 months ago
1st situation

gravitation force = mg = 12*9.8 = 117.6 N vertically downwards

Normal force = 117.6 N vertically upwards

2nd situation

gravitation force = mg + 25 = (12*9.8) + 25 = 142.6 N vertically downwards

Normal force = 142.6 N vertically upwards