A15.0 kg crate sits on a 20.0 kg table, which sits on the floor. what is the normal force acting on the table?

Both the crate and the floor exert normal forces on the table.

Explanation:

The weight of the crate is equal to

.

That should also be the magnitude of the gravitational force that points downwards. The crate isn't moving relative to the table. By Newton's First Law of Motion, the table must exert an upward normal force of 147.15 N on the crate to balance the gravitational force. However, by Newton's Third Law of Motion, this upward normal force (from the table to the crate) comes with a reaction force of equal size (from the crate to the table) in the opposite direction. That is: the crate must also exert a downward normal force of 147.15 N on the table.

Similarly, the table-crate system sits on the floor without moving. The magnitude of gravitational force on the table-crate system will be equal to

.

Again, by Newton's First Law of Motion, the floor must exert an upward force of 343.35 N on the table to balance the downward gravitational force on the system.

c. newton's third law of motion