Understand the Physics of Throwing a Ball Upward

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In summary, the conversation discusses the concept of acceleration in relation to a ball being thrown upwards. The key point is that once the ball is thrown, the only force acting on it is gravity, causing it to decelerate. This explains why the motion diagram shows the acceleration being opposite to the velocity. The conversation also touches on the importance of understanding acceleration as the change in velocity per unit time. Overall, the conversation emphasizes the correct understanding of acceleration in relation to projectile motion.
  • #1
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Hi everyone,

I was just reading a physics textbook when I noticed something. The motion diagram shows that if a ball is thrown upward into the air, the acceleration is always the opposite of the velocity. Wouldn't the acceleration initially have to be in the same direction as the velocity? How else could the ball go upward if it's never accelerating?
 
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  • #2
The key here is the phrase, "...if a ball is thrown upward...," in which I have emphasized the past tense. If the ball is being thrown, then yes, it is accelerating upward. However, once the act of "throwing" is complete, only gravity accelerates the ball (ignoring wind resistance, of course).
 
  • #3
LURCH said:
The key here is the phrase, "...if a ball is thrown upward...," in which I have emphasized the past tense. If the ball is being thrown, then yes, it is accelerating upward. However, once the act of "throwing" is complete, only gravity accelerates the ball (ignoring wind resistance, of course).

What do you mean "act of throwing is complete"? If you mean when the ball is on its way back down, then the acceleration is still positive, it's just in another direction. In a motion diagram the arrows for velocity and acceleraton will all point in the same direction for the ball on its way down.

What I am referring to is the few moments just after the ball has left the persons hand. At this point, the acceleration arrows for the motion diagram would HAVE to be in the same direction as the velocity arrows, indicating positive acceleration. Then, while the ball is still traveling upward, deceleration would kick in and then the motion diagram arrows would have to point in the opposite direction of the velocity arrows.

It's not really the concept I'm worried about, it's the diagram they made. All the arrows for velocity on the balls upward movement are pointing in opposing directions relative to the acceleration arrows. This seems incorrect.
 
  • #4
Just a nitpick. "Is thrown" can hardly be argued to be the past tense. That having been said, LURCH is exactly right about how we are meant to interpret what has been said. I suppose that saying, "Once a ball *has been thrown* upward..." might be slightly clearer. I'm not sure exactly what your book said, since you have only paraphrased it.
 
  • #5
fischook said:
What I am referring to is the few moments just after the ball has left the persons hand. At this point, the acceleration arrows for the motion diagram would HAVE to be in the same direction as the velocity arrows, indicating positive acceleration. Then, while the ball is still traveling upward, deceleration would kick in and then the motion diagram arrows would have to point in the opposite direction of the velocity arrows.

NO. The upward force (and therefore acceleration) occurs only when the ball is in contact with the thrower's hand. As soon as he or she releases it, the ball's motion becomes projectile motion (motion under the influence of gravity only). Its initial velocity (upon being released) is upward, and its initial acceleration (upon being released) is downward.

Yes, the ball really does begin decelerating as soon as the thrower let's go of it. That is what you seem to be having trouble with.
 
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  • #6
fischook said:
What do you mean "act of throwing is complete"? If you mean when the ball is on its way back down, then the acceleration is still positive, it's just in another direction. In a motion diagram the arrows for velocity and acceleraton will all point in the same direction for the ball on its way down.

What I am referring to is the few moments just after the ball has left the persons hand. At this point, the acceleration arrows for the motion diagram would HAVE to be in the same direction as the velocity arrows, indicating positive acceleration. Then, while the ball is still traveling upward, deceleration would kick in and then the motion diagram arrows would have to point in the opposite direction of the velocity arrows.

It's not really the concept I'm worried about, it's the diagram they made. All the arrows for velocity on the balls upward movement are pointing in opposing directions relative to the acceleration arrows. This seems incorrect.

The instant the ball is no longer in contact with the hand, it is no longer being accelerated upwards and at that point the only force is the gravitational acceleration downward. Even though the ball is still moving upwards due to its initial positive velocity, it is slowing down to zero velocity at its maximum height, and then negative as it falls back down.
 
  • #7
fischook said:
It's not really the concept I'm worried about, it's the diagram they made. All the arrows for velocity on the balls upward movement are pointing in opposing directions relative to the acceleration arrows. This seems incorrect.

It is correct. Remember, something can be decelerating due to gravity but still have a positive velocity. Once the ball leaves the persons hand, the only force on it is due to gravity which ALWAYS is downward. Think of a car. When you slam on the breaks, you're immediately decelerating but you're stlil going forward with a positive velocity; it's just that the magnitude of that positive velocity is decreasing. In gravity's case, unlike a car, it doesn't stop at 0 but continues to go into the negative, that is to say it starts falling.

It's best to think of acceleration as the change in velocity per unit time . Even though you have a positive velocity, that change in velocity can be positive or negative. In gravity's case, always negative if your positive direction is away from the earth.
 
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  • #8
Pengwuino;2213576 It's best to think of acceleration as the [b said:
change in velocity...[/b].

change in velocity *per unit time*


(I think this is an important addition, even if we are trying to keep things simple)
 
  • #9
Yes that is quite important, thank you, I made the correction.
 
  • #10
fischook said:
Hi everyone,

I was just reading a physics textbook when I noticed something. The motion diagram shows that if a ball is thrown upward into the air, the acceleration is always the opposite of the velocity. Wouldn't the acceleration initially have to be in the same direction as the velocity? How else could the ball go upward if it's never accelerating?

Acceleration is the rate of change of velocity. If the velocity is decreasing, you have a negative acceleration in that direction. But a negative vector in a particular direction is equal to a positive vector in the opposite direction!

Zz.
 
  • #11
cepheid said:
NO. The upward force (and therefore acceleration) occurs only when the ball is in contact with the thrower's hand. As soon as he or she releases it, the ball's motion becomes projectile motion (motion under the influence of gravity only). Its initial velocity (upon being released) is upward, and its initial acceleration (upon being released) is downward.

Yes, the ball really does begin decelerating as soon as the thrower let's go of it. That is what you seem to be having trouble with.


That's the explanation I was looking for. Thanks a lot for clearing that up. It's first year physics, so I wanted to make sure that I was getting the fundamentals right.
 

1. What is the physics behind throwing a ball upward?

The physics behind throwing a ball upward involves several principles, including force, velocity, and acceleration. When you throw a ball upward, you are exerting a force on the ball, which causes it to accelerate in the direction of the throw. As the ball rises, it experiences a downward force due to gravity, which slows its upward velocity until it reaches its peak and then falls back to the ground.

2. How does the angle of the throw affect the ball's trajectory?

The angle of the throw determines the direction of the ball's initial velocity. When you throw a ball straight up, the angle is 90 degrees, and the ball will go straight up and then fall back down in the same spot. However, if you throw the ball at an angle, the initial velocity will have a horizontal component, causing the ball to travel in a curved path.

3. What role does air resistance play in throwing a ball upward?

Air resistance, also known as drag, is a force that opposes the motion of an object through the air. When throwing a ball upward, air resistance can slow down the ball's velocity, making it fall back to the ground faster. This effect is more significant for objects with larger surface areas, such as a beach ball, compared to smaller objects like a baseball.

4. How does the ball's mass affect its motion when thrown upward?

The mass of the ball does not significantly affect its motion when thrown upward. According to Newton's second law of motion, the acceleration of an object is directly proportional to the force applied to it and inversely proportional to its mass. This means that a ball with a larger mass will experience the same acceleration as a ball with a smaller mass when thrown with the same force.

5. What happens to the ball's energy as it travels upward and then falls back down?

As the ball travels upward, it gains potential energy due to its increased height. However, as it falls back down, this potential energy is converted into kinetic energy, which is the energy of motion. At the ball's peak height, it has no kinetic energy, and all of its potential energy is converted back into kinetic energy as it falls back to the ground. This process continues until the ball loses all of its energy and comes to a stop on the ground.

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