# Philosophy in Physics

1. Mar 16, 2004

### cristina

What forces act on a thrown baseball?

(Assuming the baseball motion is an upside down parabola). What forces act on a thrown baseball? 1) After the first fraction of second after it leaves your hands? 2) After it leaves your hands and at the top of it motion (top of the parabola). Some people think that the force of the hand continues to act throughout the motion. Other think that the only force acting throughout the motion is force of gravity. Who is correct from an experiment point of view?

I think after the few fractions of the second when the ball leaves the hand, there is the ball’s weight acting on it, and the force of the hand. At the middle of the parabola there is only the weight = mass x (9.81 m/s^2) acting on it. what about the air?. From an experience point of view I think that they are both wrong, that hand force act on the ball at the beginning of its motion and after the hands force fades the gravity force takes over.
Am I correct?

Last edited: Mar 16, 2004
2. Mar 16, 2004

### EL

I'm sorry but you aren't.
After the ball has left your hand, only gravity and air resistance will act on it (although for the trajectory to take the form of "an upside down parabola" the air resistance must be negligeble).

3. Mar 16, 2004

### cristina

What about the froce of the hand?

4. Mar 16, 2004

### Chi Meson

If your hand is not touching the ball, how can it be applying a force on it? YOu are confusing "force" with "momentum." Your hand applied force in order to chage the ball's momentum. After it leaves your hand, the ball continues with its momentum, but your hand no linger has any influence on it.

Well, technically, there is a gravitational force from your hand, but its way too small to measure.

5. Mar 16, 2004

### cristina

What is happening at the top of the baseball's motion for the baseball to fall down? Can someone give me a complete answer instead of frangments please, how should I understand the concept otherwise?

Last edited: Mar 16, 2004
6. Mar 16, 2004

### Staff: Mentor

Its pretty simple: two forces act on the baseball at all times after it has left your hand and until it hits the ground: gravity and aerodynamic. Gravity is a constant force and aerodynamic changes with changes in speed, rotation, and the properties of the air during the flight. Nothing at all special (new) happens at the top of the arc to make the ball start coming back down.

The others are correct: if your hand is not touching the ball, it is not imparting any forces on it.

Sometimes its tough for us to answer questions because we need to understand what you are saying first and we're not speaking the same language. Science has its own language. Perhaps if you elaborated on the specific nature of your question it would help us help you.

7. Mar 17, 2004

### EL

Assume for simplicity that the air resistance is negligeble.
After the ball has left your hand only the gravitational force will act on it, trying to pull it back against the earth. Since the ball prior has been given an upward motion (by the force from your hand!) the effect of the gravitational force is to slow that motion down until the ball turns, and then pull it back to the earth.
It is exactly the same force acting on the way up as on the way down!

What I think you may not have understood is that a force is NOT needed to make an object move with constant velocity. A force changes the objects velocity (i.e it accelerates the object)!
For example if you are throwing a ball somewhere out in space far away from any large masses, the ball will keep on going with the same velocity as it had when it left your hand since no forces will act on it...

If you want to understand this better I suggest you read about Newton´s laws. Anyone who has a good link?