Question

Which of the following forces are not equal but are opposite to each other?

1. The force you exert on the Earth when you stand and the force exerted by the floor on you.
2. The force that, a bat exerts on a ball and the force exerted by ball on bat.
3. The force that, a piano player exerts on the piano key and the force by the keys on pianist​.

Answer 1

The force you exert on the Earth when you stand and the force exerted by the floor on you.

1. As the force exerted by you will be a gravitational force that is $mg$, product of mass of the person, $m$ and acceleration due to gravity $g$. Whereas floor exerts an upward force of same magnitude mg on you also.
2. Therefore, both the forces are in the opposite direction and have the same magnitude in this case.

The force a bat exerts on a ball and the force the ball exerts on the bat.

1. The force a bat exerts on a ball and the force by the ball exerted on the bat are equal and opposite directions according to Newton's third law.

The force a piano player exerts on the piano key and the force the key exerts on the pianist.​

1. The force a piano player exerts on the piano key and the force the key exerts on the pianist​ are equal and opposite in direction according to Newton's third law.
2. This is the reason for the pain in the fingers of the pianist after playing piano for a long time.

Therefore, in all three cases, the direction is opposite, and the magnitude is the same.

Hence, none of the given forces are not equal but, are opposite to each other.