Projectile Motion Q (1 Viewer)

[shaon0]

1. A body is projected with an initial velocity of 45 m/s and reaches its max. height at 4 seconds. Find the angle of projection.
Answer: 60 degrees, 35 minutes.

My working:
u= 45sinA, a=-9.8, t=4.

v^2=u^2+2as
0 = [45sinA]^2 - 19.6*h

s=ut+0.5*at^2
h=180sinA-4.9(16)
h=180sinA-78.4

0= [45sinA]^2 - 19.6*[180sinA-78.4]
2025.sin^2(A)-3528+1536.64=0
Then i just used the quadratic formula, but didn't get the correct answer.

 

[lyounamu]

1. A body is projected with an initial velocity of 45 m/s and reaches its max. height at 4 seconds. Find the angle of projection.
Answer: 60 degrees, 35 minutes.

My working:
u= 45sinA, a=-9.8, t=4.

v^2=u^2+2as
0 = [45sinA]^2 - 19.6*h

s=ut+0.5*at^2
h=180sinA-4.9(16)
h=180sinA-78.4

0= [45sinA]^2 - 19.6*[180sinA-78.4]
2025.sin^2(A)-3528+1536.64=0
Then i just used the quadratic formula, but didn't get the correct answer.

y = 45t sin@ + 1/2 gt^2
y' = 45 sin@ + gt
max height is when y'=0
-gt = 45sin@
4 x 9.8 = 45sin@
sin@ = 196/225
@ = 60.5880156... degrees

why the hell do you have the time (in your answer) for?

 

[Miss Jaba]

heya.... i dont know if this will be any help but i just simply put the info u gave me into the following equation: (time of flight equation):

total tme of flight= 8 sec
im neglecting the negative for the gravitational constant

t= 2vsinθ/g
8=2(45)sinθ/9.8
78.4=90sinθ
θ=60°35
do u think that this is right

 

[shaon0]

y = 45t sin@ + 1/2 gt^2
y' = 45 sin@ + gt
max height is when y'=0
-gt = 45sin@
4 x 9.8 = 45sin@
sin@ = 196/225
@ = 60.5880156... degrees

why the hell do you have the time (in your answer) for?

I don't know. I just followed the procedure of a question similar to the question above.
I thought it would work...Is there another book where i could do projectile motion Q's?

 

[lyounamu]

I don't know. I just followed the procedure of a question similar to the question above.
I thought it would work...Is there another book where i could do projectile motion Q's?

Ah, ok. Why not try fitzy? You have that book? There are good questions in that book.

 

[lyounamu]

heya.... i dont know if this will be any help but i just simply put the info u gave me into the following equation: (time of flight equation):

total tme of flight= 8 sec
im neglecting the negative for the gravitational constant

t= 2vsinθ/g
fficeffice" /><O>8=2(45)[FONT='Calibri','sans-serif']sinθ/9.8</O>[/font]
<O>[FONT='Calibri','sans-serif']78.4=90[FONT='Calibri','sans-serif']sinθ[/font]</O>[/font]
<O>[FONT='Calibri','sans-serif'][FONT='Calibri','sans-serif'][FONT='Calibri','sans-serif']θ=60°35’[/font][/font][/font]</O>
<O>[FONT='Calibri','sans-serif'][FONT='Calibri','sans-serif'][FONT='Calibri','sans-serif'][/font][/font][/font]</O>
<O>[FONT='Calibri','sans-serif'][FONT='Calibri','sans-serif'][FONT='Calibri','sans-serif']do u think that this is right?
[/font][/font][/font]</O>

lol?

 

[shaon0]

Ah, ok. Why not try fitzy? You have that book? There are good questions in that book.

I only have the 4unit book. And i can't do the mechanics in that because i haven't learnt most of the projectile motion yet.

 

[shaon0]

lol?

she's trying to say;
t=2vsinA/9.8 [where t=time of light, v=45m/s and g=9.8]
8=90sinA/9.8
196/225=sinA
A=60 degrees 35 minutes.

 

[lyounamu]

I only have the 4unit book. And i can't do the mechanics in that because i haven't learnt most of the projectile motion yet.

fitzy for 4 unit is rubbish apprently. 3 unit is decent. get 3 unit fitzy - pretty damn good for question purposes.

get cambridge too - 3 unit.

 

[shaon0]

fitzy for 4 unit is rubbish apprently. 3 unit is decent. get 3 unit fitzy - pretty damn good for question purposes.

get cambridge too - 3 unit.

I have the cambridge 3unit for year 11. I'm yet to buy my Year 12 textbooks.
The 4unit fitzpatrick is pretty good and everyone recommends that you use it with 4unit cambridge.

 

[Miss Jaba]

lol...the kid is rite!
that is what i meant.
sorry my lame typin...too tired from studyin!
lol

 

[lyounamu]

I have the cambridge 3unit for year 11. I'm yet to buy my Year 12 textbooks.

so where did you get the questions from? couchman?

 

[shaon0]

so where did you get the questions from? couchman?

Yeah but lately couchman has sucked.

 

[u-borat]

I only have the 4unit book. And i can't do the mechanics in that because i haven't learnt most of the projectile motion yet.

uh theres nothing new in 4unit projectile motion in terms of theory.

 

[lyounamu]

Yeah but lately couchman has sucked.

nah, you can still find some good questions from there. Go to page 177 of couchman or exercise 26.4

good questions!

 

[shaon0]

nah, you can still find some good questions from there. Go to page 177 of couchman or exercise 26.4

good questions!

I'm doing them right now. but they are more like physics then math. They don't require you to use calculus but instead use formulae.

 

[lyounamu]

I'm doing them right now. but they are more like physics then math. They don't require you to use calculus but instead use formulae.

rly? don't you go:

x'' = 0
x' = vcos@
x = vtcos@

y'' = -9.8 (ot-10 or whatever)
y' = -9.8t + vsin@
y = -4.9t^2 + vtsin@

and do questions this way?

 

[shaon0]

rly? don't you go:

x'' = 0
x' = vcos@
x = vtcos@

y'' = -9.8 (ot-10 or whatever)
y' = -9.8t + vsin@
y = -4.9t^2 + vtsin@

and do questions this way?

No. I've just been using formulae this whole time.
I'll try your method from now on. Thanks.

 

[shaon0]

uh theres nothing new in 4unit projectile motion in terms of theory.

Isn't there? I'll finish projectile motion in 4unit then try mechanics.

 

[Rampager]

The reason you didn't get this question is because you don't know the equations of the paths of motion.

Look in your textbook and follow the exercises which teach you:

y'' = -g
y' = -gt + Vsin[a]
y = -(1/2)gt^2 + Vt sin[a]

x'' = 0
x' = V cos[a]
x = Vtcos[a]

Once you know these equations of motion, you can solve these problems quite easily.

Edit: Didn't realise there was a second page, everything I said has already been said, but it bears repeating that you should be able to prove these equations of motion to yourself, and to the examiner.