# Free body diagram - Weight and normal reaction

1. Jun 7, 2013

Let's consider a stationary ball lying on the floor.

The weight of the ball acts downwards and the normal reaction acts upwards. They are of the same magnitude but not of the same type.

So normal reaction is the reaction to what, if not weight ?

2. Jun 7, 2013

### jbriggs444

The "normal reaction" force of the floor on the ball is the reaction force to the normal force of the ball on the floor.

The force of the ball on the floor is not the weight of the ball. However, for a ball that is stationary and supported only by the floor, it will be equal to the weight of the ball.

3. Jun 7, 2013

Okay.

So if the ball is stationary, the force of the ball on the floor is equal to the weight of it but isn't that of weight; if I understood what you said. Then, what is the force ?

What if the ball is moving horizontally with a speed, u m/s on the floor ?

4. Jun 7, 2013

### Staff: Mentor

There are two meanings of the term "reaction" force:

(1) Since the ball does not fall through the floor, we can deduce that the floor must be exerting an upward normal force. This is often called a constraint or a "reaction" force. This term is used in engineering, not in physics! In physics it's usually just called the normal force.

(2) Per Newton's 3rd law, any two objects exert equal and opposite forces on each other. These are often called "action/reaction" pairs. That's something of an old-fashioned term; better to call those force pairs just "3rd law pairs".

Do not mix up the two meanings, or you'll just get confused.

If you're asking what's the 3rd law pair to the upward normal force exerted on the ball by the ground, then that's the downward normal force exerted on the ground by the ball.

The 3rd law pair to the weight (the gravitational pull of the earth on the ball) is the gravitational pull of the ball on the earth.

5. Jun 7, 2013

### Staff: Mentor

What matters is whether the ball is vertically accelerating or not. Since it's just rolling along the horizontal floor, we know that the floor must be exerting an upward force enough to balance the weight of the ball.

Weight is a gravitational force; the normal force is a contact force (largely electromagnetic).

6. Jun 7, 2013