What equation to use for projectile motion problems

AI Thread Summary
The discussion revolves around confusion regarding two projectile motion equations that use similar variables but yield different results. The first equation, which relates final and initial velocities over time, was correctly applied to determine the time of flight for a marble rolling off a table. The second equation, which should use the change in vertical position (Δy) instead of time (Δt), was incorrectly utilized, leading to an inaccurate final velocity calculation. Participants clarified that the correct application of the first equation aligns with the textbook's approach, while the second equation's misprint could cause confusion. Understanding the proper context for each equation is crucial for solving projectile motion problems accurately.
pickle37
Messages
3
Reaction score
0
I was given a list of equations for projectile motion and two of the equations have the same variables but give different outputs. I don't understand when to use one equation and when to use the other. The equations are:

ay=(vfy-viy)/(delta t)

and

(vfy)^2= (viy)^2 + 2ay(delta t)

I tried using the second equation to solve the following problem but got it wrong. In the book they used the first equation. I see how using the first equation makes sense now, but why is it wrong to use the second equation?

A marble rolls off a table at the horizontal velocity of 1.93 m/s. The tabletop is 76.5 cm above the floor. If air resistance is negligible, determine the velocity at impact.

I solved for (delta t) and got 0.4s. I rearranged the second equation to find (vfy) and tried to solve using (viy)=0m/s, ay= 9.8 m/s^2, and (delta t)= 0.4s . I found (vfy) to be 2.8 m/s when its supposed to be 3.9 m/s
 
Last edited:
Physics news on Phys.org
Recheck your second equation. Are you sure 'delta t' isn't 'delta y'?
 
They have (delta t) written in the textbook
 
Everything looks good except I think your second equation isn't quite right. 0.4s is correct. What textbook is this?
 
The second equation should definitely have \Delta y instead of \Delta t. If your book has \Delta y there, it's a misprint.
 
I multiplied the values first without the error limit. Got 19.38. rounded it off to 2 significant figures since the given data has 2 significant figures. So = 19. For error I used the above formula. It comes out about 1.48. Now my question is. Should I write the answer as 19±1.5 (rounding 1.48 to 2 significant figures) OR should I write it as 19±1. So in short, should the error have same number of significant figures as the mean value or should it have the same number of decimal places as...
Thread 'A cylinder connected to a hanging mass'
Let's declare that for the cylinder, mass = M = 10 kg Radius = R = 4 m For the wall and the floor, Friction coeff = ##\mu## = 0.5 For the hanging mass, mass = m = 11 kg First, we divide the force according to their respective plane (x and y thing, correct me if I'm wrong) and according to which, cylinder or the hanging mass, they're working on. Force on the hanging mass $$mg - T = ma$$ Force(Cylinder) on y $$N_f + f_w - Mg = 0$$ Force(Cylinder) on x $$T + f_f - N_w = Ma$$ There's also...
Back
Top