Projectile Problem Confusion

[Ehwic]

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So the problem states: What is the range for a projectile that is launched at an angle of 45 degrees with respect to horizontal with an initial speed of 25m//s? This is a BR example problem.

A)50
B) 33.3
C) 72.2
D)62.5

Correct Answer: D
My Answer: C


The way I approached this was that I found the time needed to reach to the top, which could be found through the use of this equation:

t=[Vo👍]/g---> 25m/s*(sqrt(2)/2) which is approximately 18 m/s. 18m/s/10m/s^2 gave 1.8 seconds to reach to the top which means the total time should be 3.6 sec. Knowing this, since 3.6 is close to 4 secs, I used x=vo(t) to find the range which was 18*4= around 72 meters.

What did I do wrong here? was it because I simply rounded wrong?

 

[cloak25]

yep. you rounded wrong...

25sin(45) = 17.6
17.6/10 = 1.76 x 2 = 3.53
3.53 x 18 ~ 63 which is closest to D.

You did everything right. This problem is just not a good problem to round on since C & D are pretty close..

 

[Ehwic]

ok yeah it was just a simple miscalculation then. Also, for anyone who has the TBR book, question number 21 on chapter 1 of the physics section, why couldnt C, traveled with increasing velocity work? The question is a d vs. t graph and thus, I cannot give you any more information besides the fact that if you have the book, please help me out when anyone has the time.

thanks