Horizontal projectile motion lab

[VitaminK]

Homework Statement

The assigment is to calculate the inital velocity of a marble rolling off a table. I'm not supposed to perform the lab itself, so all the values presented here are made up.

Relevant Equations

y=Voy×t-(gt^2)/2

Vox=x/t

This is how I think the set up of the lab should look like.

I've decided the height to be y=1,5m.
The length x is assumed to be 0.32m.

where Voy =0m/s

the time it takes for the marble to drop 1,5m is 0,55s

the inital velocity of the marble is around 0,47m/s
I Think I have the calculations correct. My problem is the discussion part. Since all my values are made up how credible are my results? What could errors in a visual lab be?

 

[jbriggs444]

My problem is the discussion part. Since all my values are made up how credible are my results? What could errors in a visual lab be?

You could start with the obvious. What thing did you directly measure? What quantity were you trying to calculate based on that raw measurement? What things would you need to assume to make the calculation? How reliable are those assumptions? How accurate was your raw measurement? How might you go about testing any of it?

 

[gmax137]

What are the forces acting on the marble?

 

[VitaminK]

What are the forces acting on the marble?

Friction from the table and air. And also gravity

 

[etotheipi]

Friction from the table

That also begs the question of when the motion is supposed to start. Your diagram has a $15\text{cm}$ length labelled onto it; perhaps you are supposed to let $t=0$ when the particle is $15\text{cm}$ from the edge of the table, invent a suitable coefficient of friction and use the reduced velocity once it reaches the edge for your $V_{ox}$.

 

[VitaminK]

That also begs the question of when the motion is supposed to start. Your diagram has a $15\text{cm}$ length labelled onto it; perhaps you are supposed to let $t=0$ when the particle is $15\text{cm}$ from the edge of the table, invent a suitable coefficient of friction and use the reduced velocity once it reaches the edge for your $V_{ox}$.

Think that is a bit advanced for me (doing high school physics :) )

 

[etotheipi]

Think that is a bit advanced for me (doing high school physics :) )

Fair enough if you haven't covered the material yet, though in terms of difficulty it's nothing harder than the projectile motion part you've already done. If you're interested, you might just say the frictional force has a magnitude $F$ and acts to the left, and that the work done by friction over the $15\text{cm}$ equals the change in kinetic energy over that $15\text{cm}$.

How about some of the suggestions @jbriggs444 mentioned?

 

[VitaminK]

Fair enough if you haven't covered the material yet, though in terms of difficulty it's nothing harder than the projectile motion part you've already done. If you're interested, you might just say the frictional force has a magnitude $F$ and acts to the left, and that the work done by friction over the $15\text{cm}$ equals the change in kinetic energy over that $15\text{cm}$.

How about some of the suggestions @jbriggs444 mentioned?

Well I haven't measured anything directly, everything is assumed. So I have to evaluate the assumed values. For instance I assumed that the length x is 0.32m. But this length could be longer or shorter depending on:
1. friction
2. how fast/hard the marble is set in motion, 15cm from the edge.
Could this be a valid discussion Point?

 

[etotheipi]

Well I haven't measured anything directly, everything is assumed. So I have to evaluate the assumed values. For instance I assumed that the length x is 0.32m. But this length could be longer or shorter depending on:
1. friction
2. how fast/hard the marble is set in motion, 15cm from the edge.
Could this be a valid discussion Point?

I suspect the whole aim of the experiment would be to verify that the equations of motion you have deduced are valid. To do this, you would take a set of measurements (what would you vary/what would you measure?), maybe think about plotting a graph of some sort...

Your model is valid only under certain assumptions. Like you mentioned a few posts back, the results you compute from this model will deviate from reality in that it doesn't take into account things like dissipative forces. What effect might these have on $x$, or your graph? Also, for theoretical calculations it is assumed that the "measuring apparatus" is totally exact; this is evidently not true in practice. Have a look at the measurements you need to take, think about how you might take those measurements experimentally and perhaps try and determine some ballpark estimates of uncertainties in those measurements.

 

[VitaminK]

I suspect the whole aim of the experiment would be to verify that the equations of motion you have deduced are valid. To do this, you would take a set of measurements (what would you vary/what would you measure?), maybe think about plotting a graph of some sort...

Your model is valid only under certain assumptions. Like you mentioned a few posts back, the results you compute from this model will deviate from reality in that it doesn't take into account things like dissipative forces. What effect might these have on $x$, or your graph? Also, for theoretical calculations it is assumed that the "measuring apparatus" is totally exact; this is evidently not true in practice. Have a look at the measurements you need to take, think about how you might take those measurements experimentally and perhaps try and determine some ballpark estimates of uncertainties in those measurements.

Thanks!

 

[jbriggs444]

Well I haven't measured anything directly, everything is assumed. So I have to evaluate the assumed values. For instance I assumed that the length x is 0.32m. But this length could be longer or shorter depending on:
1. friction
2. how fast/hard the marble is set in motion, 15cm from the edge.
Could this be a valid discussion Point?

Possibly the problem is that the experiment is not specific enough for you to analyze. So make it specific. That's a good way to get a discussion going.

That's from a little tidbit creative writing that I once stumbled across. A student is tasked with writing an essay about downtown <somewhere>. He is stuck. Cannot think of a thing to write about. So his teacher suggests that he write about the theater. Still stuck. So the teacher suggests that he write about one of the bricks in the theater. The student gets unstuck.

How does the experiment run? What mechanism fires the ball? If you do not know, make something up. It's your thought experiment. You get to make it work the way you want. What is the purpose of the 15 cm start ramp? Do we get a ball timing across that distance to assess starting horizontal velocity? Is it just there to assure a horizontal launch? When the ball lands, is there carbon paper so that it makes a mark on the ground? Do you use a tape measure to see how far it went?

Do you have make-believe video equipment? Did you make a make-believe paper backdrop with ruled lines to analyze the video? How stable is the frame rate? How far is the camera from the ball? How far is the ball from the lines on the paper?

Describe the experiment in detail. Then analyze the experiment in detail. There are lots of ways to go with this.

 

[gmax137]

If you neglect the friction, what can you say about Xm, Ym, and V0x?

 

[VitaminK]

Possibly the problem is that the experiment is not specific enough for you to analyze. So make it specific. That's a good way to get a discussion going.

That's from a little tidbit creative writing that I once stumbled across. A student is tasked with writing an essay about downtown <somewhere>. He is stuck. Cannot think of a thing to write about. So his teacher suggests that he write about the theater. Still stuck. So the teacher suggests that he write about one of the bricks in the theater. The student gets unstuck.

How does the experiment run? What mechanism fires the ball? If you do not know, make something up. It's your thought experiment. You get to make it work the way you want. What is the purpose of the 15 cm start ramp? Do we get a ball timing across that distance to assess starting horizontal velocity? Is it just there to assure a horizontal launch? When the ball lands, is there carbon paper so that it makes a mark on the ground? Do you use a tape measure to see how far it went?

Do you have make-believe video equipment? Did you make a make-believe paper backdrop with ruled lines to analyze the video? How stable is the frame rate? How far is the camera from the ball? How far is the ball from the lines on the paper?

Describe the experiment in detail. Then analyze the experiment in detail. There are lots of ways to go with this.

Thanks!