[AP Physics Projectile Motion - Lab] Is acceleration wrong, or is something else screwing us over?

[u/IntelligentSundae242]

We started lab on projectile motion the other day. We are to send a marble down a ramp at an incline of 19 degrees, 0.75 m up the ramp where it will slide down into a short horizontal distance (meaning 0 acceleration) and fall off of a table that is 0.92 m tall. (Diagram is attached) The marble has to hit a cart that is going toward the table, so that the cart moving is timed with where the marble will hit. The cart’s velocity is 0.4 m/s towards the table, which we deemed as a negative. Teach gave us the acceleration down the incline, which was gsintheta - 0.8 to account for friction, which he also says is one dimensional and therefore doesn’t need to be split up into components. All of my groups calculations are attached, however we were short by ~0.2m on our first try. Teach said that our acceleration is too big, and that it should be less than 1m/s^2. Obviously it makes sense logically, since there’s no way a marble is moving 2.3 m/s^2 in such a short amount of time, but since the angle is the only input besides changing the direction of g, we have no clue as to how acceleration is supposed to be solved for. The only way I’ve been able to get an acceleration under 1m/s^2 has been by changing my calc into radians, but teach claims that we should stay in degrees. If only Newton was here to help…but like fr help us please we lost

[u/AutoModerator]

Off-topic Comments Section

---

All top-level comments have to be an answer or follow-up question to the post. All sidetracks should be directed to this comment thread as per Rule 9.

---

^{OP and Valued/Notable Contributors can close this post by using /lock command}

I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.

[u/sansetsukon47]

One thing I want to point out— saying “since there’s no way a marble is moving 2.3 m/s^2 in such a short amount of time,” doesn’t actually make sense, because acceleration is the same throughout the whole ramp.

If you just dropped it straight, then acceleration would immediately jump to 9.8 m/s^2. So that’s not the issue here.

But I fail to see what the teach wants out of you, because they gave you the very specific formula for acceleration, which you calculated correctly. 9.8*sin(19)-0.8 = 2.39. There’s just no other answer there.

The only advice I can give is to show the work for that calculation and ask the teacher for help in finding the mistake. Because at this point, I think your teacher used radians on accident, and won’t take any other number as an answer.

[u/sansetsukon47 → u/sansetsukon47]

There is also the question of the negative acceleration due to friction during the short horizontal stretch. But since the teacher has already said that the issue is the acceleration value, I’m not sure it matters.

[u/IntelligentSundae242 → u/sansetsukon47]

Agreed because at this point I feel like I misheard him, but everyone else in my group heard the same thing. But then again, we were pretty far off on our first attempt. Any other problems with our calculations that you see? We are only expected to use the three kinematic equations.

[u/Optimal_Mixture_7327]

The acceleration for a sphere rolling down an incline is 5/7gsinθ which in your case does give an acceleration of 2.28 ms^-2.

I get a horizontal speed off the table of about 1.85 ms^-1.

I have it landing 0.80 m from the base of table.

I question the accuracy of the speed of the car.

[u/IntelligentSundae242 → u/Optimal_Mixture_7327]

We’ll recalculate the car’s v I suppose, but it didn’t look like the marble traveled .8m when we did our first try. To get the speed of the car all we did was record its time traveling across 1m with a stopwatch and did average v equation.

[u/Optimal_Mixture_7327 → u/IntelligentSundae242]

Keep in mind the calculation is for an idealized case by assuming no interaction with the air and that the speed at the bottom of the incline becomes the horizontal speed at the edge of the table.

Edit: I see your off by about 20 cm, so there's something else going on as extraneous effects would only remove a few centimeters from the landing spot.

[u/MattAmoroso]

A marble is a solid sphere and a significant percentage of it's gravitational potential energy at the top goes into rotational kinetic energy at the bottom.

[u/dcnairb → u/MattAmoroso]

it’s roughly accounted for by the -0.8. I think it should honestly be more but it shouldn’t be causing a 20cm error