A box of mass 10.0 kg is sitting on a table. Assume gravity= 10m/s^2

Question

A box of mass 10.0 kg is sitting on a table. Assume gravity= 10m/s^2

a. What is the weight of the box?

b. What is the normal force on the box?

c. If a force of 30 N downward is placed on the box, what is the normal force?

d. If a force of 60 N upward is placed on the box, what is the normal force?

e. If a force of 134 N downward is placed on the box, what is the normal force?

f. If a force of 230 N upward is placed on the box, what is the normal force?

Answer 1

The basic equation behind "weight" is Newton's second law (F = ma) because weight is a force and since all masses on earth are subject to Earth's force of gravity, and since the force of Earth's gravity provides an acceleration due to that gravity of "g", then Weight = mass times "g" or W = mg.

1) W = (10.0 kg)(10 m/s²) = 100 kgm/s² also known as 100 newtons (100 N)

2) An object experiencing a force will start accelerating unless the force is counteracted by another force. Since the book is not moving (it is "sitting" on the table), then the table must be pushing back on the book the same amount as the force the book is pushing on the table (its weight). The force of the table on the book is called the "normal force". So the normal force must be 100 N

3) 130N (100N from the weight plus the 30 N additional force)

4) 40 N (100N from the weight minus the 60 N offsetting force)

5) 234 N

6) There will be no normal force because the book will start moving up away from the table (won't be touching the table any more). It will have a net force of 130 N upward so it will begin accelerating in accordance to F = ma (or a = F/m = 130/10 = 13m/s² upwards)