Two balls thrown from bridge one upwards, other downwards?

[ecoimpact]

I would have thought the force of a persons arm being thrown downwards (with gravity) would essentially increase a balls initial downward acceleration immediately following the balls release. As compared to an upwards thrown balls initial acceleration; in which the persons upwards arm movement is working against gravity. Of course, in my mind, the question is metaphorically suggesting two people who could hypothetically both throw a ball (simultaneously - in the same direction) at exactly the same speed as each other...which I'm sure the odds are pretty low. For my question I have answered B and wanted to check on the question and my answer.

Q) Two people are standing on a bridge with a tennis ball each. They throw balls at the same time and the same speed. But one person throws the ball upwards and the other throws it downwards. Which ball has the greatest acceleration at the instant of release (just after they are let go)

a. The ball thrown upwards.

b. The ball thrown downwards.

c. The acceleration for both is the same.

Regards,

Jesse

 

[theodoros.mihos]

What is acceleration definition?

 

[Thewindyfan]

Well in both cases the acceleration will be due to gravity, so it's really just a matter of looking at the kinematic equations for the two situations and looking at it more in-depth.
This is kind of tricky to think about because we're dealing with this in general terms, but the key part is that it's asking about the acceleration immediately at the moment of release, or t = 0. Your intuition is not wrong in the case where after the initial second, the ball being thrown down will experience a greater speed than the ball thrown up earlier, but the question is asking about acceleration. Think about what acceleration is going to be for the two balls in this scenario, and if it is dependent on time.

 

[CWatters]

I would have thought the force of a persons arm being thrown downwards (with gravity) would essentially increase a balls initial downward acceleration immediately following the balls release.

After release how does the arm apply a force on the ball?

 

[A.T.]

For my question I have answered B and wanted to check on the question and my answer.

That's the only one of the 3 answers which doesn't make sense to me.

 

[theodoros.mihos]

Which ball has the greatest acceleration at the instant of release (just after they are let go)

Just after they let go, there is no interaction between people and ball, there is only g.

 

[sophiecentaur]

would essentially increase a balls initial downward acceleration immediately following the balls release.

Newton tells us that the acceleration is only due to forces being applied. When it's been launched, the only force / acceleration is due to gravity. But the direction of its initial velocity is important. When you throw it down, it's already got a downwards velocity (you were accelerating it with the force of your arm) but, when you throw it up, the velocity has the other sign, g will slow the speed to zero (at the top if its flight) and then its downward velocity will increase all the way to the ground.
What I have described is, of course, 'obvious' and it used to be explained in the terms you use - till Newton spotted what is actually going on. There are a set of "Equations of Motion" which describe motion under constant acceleration. They do make sense and will give you the right answer to all these problems. They are based on Newton's laws of motion. You really have to do some example questions yourself to get familiar with how it works.

 

[HallsofIvy]

I would have thought the force of a persons arm being thrown downwards (with gravity) would essentially increase a balls initial downward acceleration immediately following the balls release.

Then you would be wrong. Throwing downward increases the initial velocity, not the acceleration.

 

[A.T.]

Throwing downward increases the initial velocity

As compared to just dropping. But the two cases to consider here are both assumed to have the same initial speed.

 

[mrspeedybob]

Which ball has the greatest acceleration at the instant of release (just after they are let go)

Highlighted key words...
F=ma
The mass of the balls is the same.
What is the F on each ball after release?
Once you know about F and m, you can say something definite about a.

 

[Thewindyfan]

Don't confuse acceleration with velocity.

 

[HallsofIvy]

As compared to just dropping. But the two cases to consider here are bath assumed to have the same initial speed.

Same initial speed, yes, but opposite directions.

 

[CWatters]

Same initial speed, yes, but opposite directions.

How does this relevant to the question?

 

[jbriggs444]

How does this relevant to the question?

It could be relevant if we wanted to consider some velocity-dependent force. There is one handy.

 

[ecoimpact]

Thankyou all very much. I have a lot to learn, I need to increase the velocity of my studies acceleration.

 

[zanick]

once the ball is released downward, it is accelerating at 1g. the ball released upward is not accelerating once it leaves your hand. like a bullet fired from a level gun. once it leaves muzzle, it instantly is decelerating

 

[zanick]

Which ball has the greatest acceleration at the instant of release (just after they are let go)

Just after they let go, there is no interaction between people and ball, there is only g.

yes, so relaltive to the person throwing the ball.. the downward ball is accelerating toward earth. the tossed up ball is decelerating relative to the person at a with a force of 1g. after it slows to 0 velocity relative to the person, it will start accelerating toward the Earth with a force equal to 1g.

 

[CWatters]

Both balls are accelerating downwards due to gravity. The only difference is the initial velocity.

 

[Satonam]

Acceleration is defined as the change in velocity per unit time, for example, meter/second/second or velocity/second. When the ball is thrown regardless of its direction and velocity, they are both affected by a single force at the time of release; the acceleration due to gravity g (9.8m/s²). In other words, the ball thrown downward is accelerating by 9.8m/s every second while the ball thrown upwards is decelerating -9.8m/s every second until it reaches its highest altitude and descends.

 

[A.T.]

In other words, the ball thrown downward is accelerating by 9.8m/s every second while the ball thrown upwards is decelerating -9.8m/s every second until it reaches its highest altitude and descends.

Using "accelerating" and "decelerating", together with opposite signs is just confusing. The acceleration vector has the same direction and magnitude in both cases.

 

[NihalRi]

I think that the key point is :

Which ball has the greatest acceleration at the instant of release (just after they are let go)

The moment the ball leaves the hand the force from the hand ceases to act on it (contact force). The resulting upwards motion is due to the change in velocity induced by the hand's force. At release the only force acting on the balls becomes gravitational. This force is equal to the mass times the constant of acceleration due to gravity.

 

[Salman Rayeen]

Acceleration of both is same

I would have thought the force of a persons arm being thrown downwards (with gravity) would essentially increase a balls initial downward acceleration immediately following the balls release. As compared to an upwards thrown balls initial acceleration; in which the persons upwards arm movement is working against gravity. Of course, in my mind, the question is metaphorically suggesting two people who could hypothetically both throw a ball (simultaneously - in the same direction) at exactly the same speed as each other...which I'm sure the odds are pretty low. For my question I have answered B and wanted to check on the question and my answer.

Q) Two people are standing on a bridge with a tennis ball each. They throw balls at the same time and the same speed. But one person throws the ball upwards and the other throws it downwards. Which ball has the greatest acceleration at the instant of release (just after they are let go)

a. The ball thrown upwards.

b. The ball thrown downwards.

c. The acceleration for both is the same.

Regards,

Jesse