Register to reply

Projectile motion off a cliff at an angle (help )

by Wraith09
Tags: angle, cliff, motion, projectile
Share this thread:
Wraith09
#1
Feb11-10, 02:07 AM
P: 3
Hi all,

I am new to the forum, 1st year chemical engineering student. I am struggling with this problem:

1. The problem statement, all variables and given/known data

A stone is thrown at an angle of 37' from the top of a cliff which is 40m above the surface of a lake. The stone hits the lake's surface at a point which is 75m horizontally from the base of the cliff. Find the total time of flight of the stone:

MCQ:

a) 4.44s
b) 4.00s
c) 3.80s
d) 3.60s
e) 3.40s


2. Relevant equations

sy = syo + vyot + 0.5ayt2


3. The attempt at a solution

I drew a free body diagram for the statement incl. a triangle with an angle of 37'. The hypotenuse of the triangle is speed (vo) and its two sides are vox and voy. I also calculated Sy using Cosө = adj / hyp and then sinө = opp / hyp to get 56.516m

To find voy: sin 37' = voy / vo
voy = vo sin 37'

To find vox: cos 37' = vox / vo
vox = vo cos 37'

I relate vox with dislpacement x over a certain period through the eq:

vox = x / t
x = (vox) t

I relate motion along the y-axis to the displacement in this direction over a certain period via the eq:

y = (v0y)t - 0.5at2

Sustituting eqs: vo = x / t
vo cos 37' = x / t
vo = x / (cos 37') t

Substituting eqs: sy = soy + voyt + 0.5(a)t2

sy = soy +(x / (cos 37')t) (sin37') t + 0.5(a)t2

57.516m = 40 + 75 tan37' +0.5(-9.8m.s-2)t2

t2 = 8.16
t = 2.85s

That is the answer I get... which is wrong as its not listed in the MC's
Phys.Org News Partner Science news on Phys.org
Experts defend operational earthquake forecasting, counter critiques
EU urged to convert TV frequencies to mobile broadband
Sierra Nevada freshwater runoff could drop 26 percent by 2100
rl.bhat
#2
Feb11-10, 03:13 AM
HW Helper
P: 4,433
Hi Wraith09, welcome to PF.
Measure the displacement from the starting point. So so(y) = 0 and s(y) = -40 m.
Wraith09
#3
Feb11-10, 03:27 AM
P: 3
Thank you, thats the one thing I missed!


Register to reply

Related Discussions
Projectile Motion of Car and Cliff Introductory Physics Homework 1
Projectile Motion off a cliff Introductory Physics Homework 8
Projectile Motion of a grasshopper off a cliff Introductory Physics Homework 5
The Cliff Diver - Projectile motion Introductory Physics Homework 3
Projectile motion at top of cliff with angle... Introductory Physics Homework 1