Why Does an Accelerometer Show Zero During a Ball's Flight?

[Sam Smith]

I have a cricket ball and I place a dual axis accelerometer on it and throw is up in the air and catch it repeatedly. What would I expect its acceleration trace to look like. I am getting myself confused as I think there is an error in my trace.

 

[Nugatory]

You would expect acceleration upwards during both the throwing phases. What are you seeing?

 

[Sam Smith]

I was expecting acceleration to increase as i throw the ball upwards, that then to slowly decrease and come to zero when it stops at maximum height before acceleration increases in the opposite direction then slows down and come to zero when it stops in my hand so like two camel humps is that correct. My trace instead gives just one big hump which is confusing me. Also what would acceleration trace look like if I put my sensor on a pendulum?

 

[PeroK]

I was expecting acceleration to increase as i throw the ball upwards, that then to slowly decrease and come to zero when it stops at maximum height before acceleration increases in the opposite direction then slows down and come to zero when it stops in my hand so like two camel humps is that correct. My trace instead gives just one big hump which is confusing me. Also what would acceleration trace look like if I put my sensor on a pendulum?

Are you sure you're not confusing velocity and acceleration?

 

[Sam Smith]

Possibly. How would you expect the acceleration curve to look for a ball being thrown and a pendulum? I would like to take a look at a graph so I can work through it but I can't find a simple graph of it. For example on my dual axis how would the vertical and horizontal graphs look?

 

[PeroK]

Possibly. How would you expect the acceleration curve to look for a ball being thrown and a pendulum? I would like to take a look at a graph so I can work through it but I can't find a simple graph of it. For example on my dual axis how would the vertical and horizontal graphs look?

Okay, let me rephrase what I said. When you say "acceleration", you are thinking about velocity. You are confusing these two concepts. The acceleration due to gravity is constant.

A pendulum is more complicated, because it has two forces acting on it: gravity and tension in the string.

 

[Sam Smith]

Yes I think you may be write my accelerometer measures acceleration in terms of g. If that's the case have I described hwo I would expect the graph to look correctly.,

 

[Nugatory]

I was expecting acceleration to increase as i throw the ball upwards, that then to slowly decrease and come to zero when it stops at maximum height before acceleration increases in the opposite direction then slows down and come to zero when it stops in my hand

Any time that the ball is not being accelerated by your hand it is in free fall and the acceleration (as measured by an accelerometer) is zero.

 

[Sam Smith]

In terms of the pendulum oi I ignore the tension of the arm and consider simply the motion if for example I had a robotic arm and it was moving as a pendulum how would I expect it to look

 

[russ_watters]

In terms of the pendulum oi I ignore the tension of the arm and consider simply the motion if for example I had a robotic arm and it was moving as a pendulum how would I expect it to look

You can't ignore the tension in the arm (you just replaced the simple arm with a more complicated looking one that does exactly the same thing), but in either case, the acceleration detected oscillates from g to a little below g to a little above g over and over again.

 

[A.T.]

The acceleration due to gravity is constant.

An acclerometer doesn't measure the acceleration due to gravity, just proper acceleration due to other forces. As Nugatory said, it is zero during flight (ignoring air resistance), and more than 1g upwards during throwing up and catching.