Troubleshooting Answers to Easy Physics Questions on LON-CAPA

[Mert Kerem Yavuz]

Hey guys, hello. There is a system belongs in internet called LON-CAPA, and my teacher told that homeworks are done via that system. So I logged in, the questions are not that difficult to solve. But system doesn't accept some of my answers which I'm so sure that being true. So I'm going to write 3 easy questions for you, I'm just wondering if I did a mistake or not. Here we go.

1 - The position of an object as a function of time is given as x = At3 + Bt2 + Ct + D. The constants are A = 2.31 m/s3, B = 1.07 m/s2, C = -4.91 m/s, and D = 3.45 m.
Q-1 What is the acceleration of the object at t = 0.357 s?
(MY ANSWER IS 7,088 m/s^2)

2- A ball is thrown straight upward in the air at a speed of 17.1 m/s. Ignore air resistance.
Q-2.1 What is the maximum height the ball will reach?
(MY ANSWER IS 14,92m)

Q-2.2 How long will it take to reach 2.31 m above its initial position on the way down?
(MY ANSWER IS 3,34s)

Thank you guys for you answers already!

 

[haruspex]

Hey guys, hello. There is a system belongs in internet called LON-CAPA, and my teacher told that homeworks are done via that system. So I logged in, the questions are not that difficult to solve. But system doesn't accept some of my answers which I'm so sure that being true. So I'm going to write 3 easy questions for you, I'm just wondering if I did a mistake or not. Here we go.

1 - The position of an object as a function of time is given as x = At3 + Bt2 + Ct + D. The constants are A = 2.31 m/s3, B = 1.07 m/s2, C = -4.91 m/s, and D = 3.45 m.
Q-1 What is the acceleration of the object at t = 0.357 s?
(MY ANSWER IS 7,088 m/s^2)

2- A ball is thrown straight upward in the air at a speed of 17.1 m/s. Ignore air resistance.
Q-2.1 What is the maximum height the ball will reach?
(MY ANSWER IS 14,92m)

Q-2.2 How long will it take to reach 2.31 m above its initial position on the way down?
(MY ANSWER IS 3,34s)

Thank you guys for you answers already!

I agree with your first two answers but not the third. Please post your working.

 

[Mert Kerem Yavuz]

I agree with your first two answers but not the third. Please post your working.

I solved this in two steps. First, I calculated the landing time.

I calculated g=9.8 m/s^2.
V= Vo+g*t
0=17.1+(9.8)*t
tlanding=1.744 s (landing time)

Then, i calculated the time between the maximum height position to 2,31 m above ground position.maximum height=14,92 m (Found it another question, which you said that you found it too)
14.92-2.31=12.61 m to go down
X=Vo*t+(1/2)*g*t^2
12.61=0+(1/2)*9.8*t^2
t=1.6 s

t1+t2=3.34 s (whole process)

 

[haruspex]

I solved this in two steps. First, I calculated the landing time.

I calculated g=9.8 m/s^2.
V= Vo+g*t
0=17.1+(9.8)*t
tlanding=1.744 s (landing time)

Then, i calculated the time between the maximum height position to 2,31 m above ground position.maximum height=14,92 m (Found it another question, which you said that you found it too)
14.92-2.31=12.61 m to go down
X=Vo*t+(1/2)*g*t^2
12.61=0+(1/2)*9.8*t^2
t=1.6 s

t1+t2=3.34 s (whole process)

Ok, I found my mistake, but it is nearer to 3.35. You may have got a bit of rounding error because you calculated two stages and added them together. That is not necessary. You can solve it in one step with s=ut+1/2 at². You just have to pick the larger root of the quadratic.

 

[Mert Kerem Yavuz]

Ok, I found my mistake, but it is nearer to 3.35. You may have got a bit of rounding error because you calculated two stages and added them together. That is not necessary. You can solve it in one step with s=ut+1/2 at². You just have to pick the larger root of the quadratic.

So in conclusion, you tell me that you found around 3.34,-3.35. So system must accept my answers cause the answer don't have to be the absolute true. In my other calculations, it accepted answers like 7,1 which's true answer is 7,15.

 

[gneill]

[Mod note: Please note that I have changed the thread title to better reflect the thread content. Pleas for help are not descriptive of the physics to be discussed and not allowed as thread titles]

 

[haruspex]

So in conclusion, you tell me that you found around 3.34,-3.35. So system must accept my answers cause the answer don't have to be the absolute true. In my other calculations, it accepted answers like 7,1 which's true answer is 7,15.

Yes.

 

[Mert Kerem Yavuz]

Yes.

Thanks, really appreciate it.