Physics Question

[Mike L.]

Gravity doesn't have a speed. There is an acceleration due to gravity which is why the bullet is always accelerating. If you define your coordinate system so that down is positive, the acceleration is positive and there is no deceleration.

When you are driving your truck and come up to a stop sign, are you accelerating? If so, then taking off from that stop sign that you accelerated to a halt is the same thing? I am talking about real life here and not classroom stuff.
When someone tells you to accelerate, what does that mean to you? Stay the speed, or go faster? If someone tells you to accelerate will you slow down, because as you guys describe it it means the same thing. Do we need more word in the english language? Seems so to me.

 

[SBLC]

Answer this question, A skydiver with his parachute open is falling straight down at a constant speed, What is the skydivers accleration?

 

[JoshH]

When you are driving your truck and come up to a stop sign, are you accelerating? If so, then taking off from that stop sign that you accelerated to a halt is the same thing? I am talking about real life here and not classroom stuff.
When someone tells you to accelerate, what does that mean to you? Stay the speed, or go faster? If someone tells you to accelerate will you slow down, because as you guys describe it it means the same thing. Do we need more word in the english language? Seems so to me.

I'm pretty sure the title of this thread is "Physics Question".

 

[Poltergeist]

Here is an example.


You hold a pen in your hand 6 feet off the ground. When you let go it falls to the ground.

If the pen has no acceleration, it will hover in mid air and never fall back to the ground.

Sorry using a bad example doesn't make you right. Take a ball or your pen and toss it up in the air, even a foot high will do. For a split second it has no upward or downward movement. Go try and report back on your findings.

 

[JoshH]

Answer this question, A skydiver with his parachute open is falling straight down at a constant speed, What is the skydivers accleration?

Are you asking me?

 

[WolfLMM]

Sorry using a bad example doesn't make you right. Take a ball or your pen and toss it up in the air, even a foot high will do. For a split second it has no upward or downward movement. Go try and report back on your findings.

It has no velocity. Acceleration is not velocity. It's right. Consult a text if you need to.

 

[WolfLMM]

Answer this question, A skydiver with his parachute open is falling straight down at a constant speed, What is the skydivers accleration?

9.81. His chute is providing air resistance (a force) to counter gravity.

 

[Poltergeist]

It has no velocity. Acceleration is not velocity. It's right. Consult a text if you need to.

So you're saying a ball with no velocity is accelerating? If the ball has no movement (up or down) no velocity or acceleration.

 

[SBLC]

Are you asking me?

Anyone, feel free to respond.

I am with Poltergeist on this one, that split yocto second the accleration is zero the bullet is not going up nor down but still feels the force due to gravity.

 

[WolfLMM]

So you're saying a ball with no velocity is accelerating? If the ball has no movement (up or down) no velocity or acceleration.

Yes, consult a text book if you need to.

 

[JoshH]

9.81. His chute is providing air resistance (a force) to counter gravity.

If his velocity is constant, his acceleration is zero.

 

[JoshH]

So you're saying a ball with no velocity is accelerating? If the ball has no movement (up or down) no velocity or acceleration.

Accelration requires a change in time to calculate it. The velocity is only zero for a split second. The instance it stops it immediately starts to fall again. The position function is a parabola with the slope of the line tangent to the curve being the velocity. The velocity function is a straight line with the slope of that line being the acceleration.

 

[DAVe3283]

Please tell me that schools are really teaching that the force of gravity is 32 ft/s/s and you made the conversion yourself.....

Oh screw that! I HATE the American system of measurement. We were taught both, and it took me all of 1 second to decide Metric was the way to go. The math is easier to do in your head, the conversions are nicer, and it makes more sense. I do all my tuning in Metric for the same reason. But, some people learned the old way, and they are faster that way. Just not me.

How did we get to velocity? Acceleration? At the point of return when velocity stopped, the bullet falls, do we still call this acceleration when in fact the governing force is totaly different. Energy caused the climb to velocity and when energy stopped, gravity pulled it down at a controlled force. The bullet did not accelerate to the ground.

When you are driving your truck and come up to a stop sign, are you accelerating? If so, then taking off from that stop sign that you accelerated to a halt is the same thing? I am talking about real life here and not classroom stuff.
When someone tells you to accelerate, what does that mean to you? Stay the speed, or go faster? If someone tells you to accelerate will you slow down, because as you guys describe it it means the same thing. Do we need more word in the english language? Seems so to me.

Mike, you are right, we need a different word. In real life, acceleration implies an INCREASE in velocity, but in physics it means literally a CHANGE in velocity, either more or less. Confusing, I know. Why they didn't pick another word, who knows. This is how I look at it: when you are decelerating at a stop sign, you are accelerating in a negative direction. Imagine you put your truck in reverse and slip the clutch (in a manual). You aren't using your brakes, you are accelerating--backwards! Which happens to decelerate you.

Sorry using a bad example doesn't make you right. Take a ball or your pen and toss it up in the air, even a foot high will do. For a split second it has no upward or downward movement. Go try and report back on your findings.

So you're saying a ball with no velocity is accelerating? If the ball has no movement (up or down) no velocity or acceleration.

You are confusing velocity and acceleration. Velocity is simply the speed of the object. You are 100% correct: the pen will have 0 velocity for a split second. However, it is always being pulled down, so there is an acceleration. If there was no acceleration (gravity), the pen would never stop going up, let alone come back down.

And, when an object is at rest, there is no acceleration, because the speed (velocity) is staying at 0 the whole time. There is still the force of gravity pulling down, but there is the force of the floor pushing back up (we rarely think like this in real life though).

 

[SBLC]

9.81. His chute is providing air resistance (a force) to counter gravity.

I am saying its zero because v2=v1 (constant velocity) so v2-v1=0
(0/time=acceleration)

so if the force is counter acting gravity how does he have a acceleration of 9.81 due to gravity?

Just saying:thumb:

 

[DAVe3283]

I am saying its zero because v2=v1 (constant velocity) so v2-v1=0
(0/time=acceleration)

so if the force is counter acting gravity how does he have a acceleration of 9.81 due to gravity?

Just saying:thumb:

Yes, he still has the 9.8 m/s^2, but his parachute is accelerating him the opposite direction at 9.8 m/s^2 as well, for a net 0 acceleration.

This is a special case, since there are TWO forces acting on the guy, not just gravity.

Fun thread, though!

 

[WolfLMM]

If his velocity is constant, his acceleration is zero.

The acceleration of gravity is constant. Close to earth of course.

 

[SBLC]

Accelration requires a change in time to calculate it. The velocity is only zero for a split second. The instance it stops it immediately starts to fall again. The position function is a parabola with the slope of the line tangent to the curve being the velocity. The velocity function is a straight line with the slope of that line being the acceleration.

X2 I agree with that I was off 1 function,
Its good to think sometimes

 

[WolfLMM]

You posted what I wanted to, just didn't have the patience to type it all. Lol

Thanks good post!

Oh screw that! I HATE the American system of measurement. We were taught both, and it took me all of 1 second to decide Metric was the way to go. The math is easier to do in your head, the conversions are nicer, and it makes more sense. I do all my tuning in Metric for the same reason. But, some people learned the old way, and they are faster that way. Just not me.


Mike, you are right, we need a different word. In real life, acceleration implies an INCREASE in velocity, but in physics it means literally a CHANGE in velocity, either more or less. Confusing, I know. Why they didn't pick another word, who knows. This is how I look at it: when you are decelerating at a stop sign, you are accelerating in a negative direction. Imagine you put your truck in reverse and slip the clutch (in a manual). You aren't using your brakes, you are accelerating--backwards! Which happens to decelerate you.


You are confusing velocity and acceleration. Velocity is simply the speed of the object. You are 100% correct: the pen will have 0 velocity for a split second. However, it is always being pulled down, so there is an acceleration. If there was no acceleration (gravity), the pen would never stop going up, let alone come back down.

And, when an object is at rest, there is no acceleration, because the speed (velocity) is staying at 0 the whole time. There is still the force of gravity pulling down, but there is the force of the floor pushing back up (we rarely think like this in real life though).

 

[McRat]

Lordy, and I thought I had no life!! :spit:

It's a calculus problem. Take time slices, and calculate the velocity every millionth of second as the bullet approaches apogee. It goes slower, then slower, then slower, then REALLLLLY FREAKING SLOW, then during next millionth of second, it starts going down.

There is a max altitude, and there is a point in space where the bullet velocity is zero (0.00000000000000000000 fps) but it is not at that velocity for a length of time.

You can calculate out when and where velocity hits zero, but you cannot calculate how long it remains at zero velocity, because it doesn't. The duration is 1/infinity seconds.

But if it hits a bird on the way up, you better have a Fed Waterfowl sticker.

 

[WolfLMM]

Lordy, and I thought I had no life!! :spit:

It's a calculus problem. Take time slices, and calculate the velocity every millionth of second as the bullet approaches apogee. It goes slower, then slower, then slower, then REALLLLLY FREAKING SLOW, then during next millionth of second, it starts going down.

There is a max altitude, and there is a point in space where the bullet velocity is zero (0.00000000000000000000 fps) but it is not at that velocity for a length of time.

You can calculate out when and where velocity hits zero, but you cannot calculate how long it remains at zero velocity, because it doesn't. The duration is 1/infinity seconds.

But if it hits a bird on the way up, you better have a Fed Waterfowl sticker.

Another great post!

This thread is great. Every question and response has been a valid one.