• Support PF! Buy your school textbooks, materials and every day products Here!

Need help projectile motion

  • Thread starter F.B
  • Start date
  • #1
F.B
83
0
I need help on projectile motion questions because i had to miss a whole lesson for some reason and my teacher said to try some of the questions.
Anyways im stuck on this question.

A cannon is set at an angle of 45 degrees above the horizontal. A cannon ball leaves the muzzle with a speed of 220 m/s. Air resistance is negligible. Determine the cannonball's
a) maximum height
b) time of flight
c) horizontal range (to the same vertical level)

This is what i did. Im not going to do step by step on the part i got.

Before these i started i found VIx which is 155.6 m/s and Viy is the same
a) At max height Vfy=0( i dont get why but i know thats how you do it)
2ad=Vi^2 + Vf^2
d= (155.6)^2/-2(-9.8)
d= 1240m

b)i have no idea how to get this one i hope you can help me.

c)i started but i couldnt finish. This is what i did

d=Vit + 1/2a(t^2)
1240=155.6t - 4.9t^2
4.9t^2 - 155.6t + 1240 = 0

I know i need to do the quadratic formula after this but i get a negative with the radical so i dont know what to do i think i made a mistake somewhere.

Can you please help me?
 

Answers and Replies

  • #2
Integral
Staff Emeritus
Science Advisor
Gold Member
7,198
55
To understand why the vy=0 at the peak of motion you need to think about how gravity is effecting the motion. Gravity is always acting toward the earth, so as long as the projectile has a upward directed velocity (Vy>0 it will be slowed by gravity. Now consider what happens when it has been slowed by gravity to the point it has lost all of its upward motion. It will stop, so Vy=0, now it will begin to fall, so now Vy<0 . Can you figure out from this why Vy=0 is at the highest point?
 
  • #3
hotvette
Homework Helper
988
3
For b, you need to think about what the velocity components are when the flight is finished.

For c, if you have an equation for distance vs time, the answer to b will be useful.

hotvette
 

Related Threads for: Need help projectile motion

  • Last Post
Replies
2
Views
2K
  • Last Post
Replies
1
Views
1K
  • Last Post
Replies
1
Views
823
  • Last Post
Replies
1
Views
1K
  • Last Post
Replies
5
Views
2K
  • Last Post
Replies
2
Views
1K
  • Last Post
Replies
1
Views
968
  • Last Post
Replies
20
Views
3K
Top