physics help .. URGENT (1 Viewer)

[shantu1992]

okay felllas..

a stone is thrown horiztonatally out to sea from the top of a cliff with a velocity of 15 ms-2 and reaches the water below after 3 seconds

a) how high is the cliff?
b) how far from the base of the cliff is the stone when it reaches the water?
c) what is the velocity of the stone just prior to striking the water?

thanks haha, this is prejectile motion by the way
i think u have to use suvat eqts
have no idea tho
thanks thanks thanks thanks so much

 

[gonutsmate]

Yeah use suvat for this shiz.

a) Find Delta s for y
b) Find delta s for x
c) use the final velocity equation

 

[12o9]

What's a suvat =/? anyways, enjoy.

 

[shantu1992]

What's a suvat =/? anyways, enjoy.

haha dude thats awesome

 

[shantu1992]

haha dude thats awesome

are u joking when u said what is suvat loll
and can u do another q or meeeeeee pleasssee

okkkk

A ball of mass 0.5 ks rolls offf a horiztonal ledge 4.0 m above the ground with a speed of 4 ms-1

a) how long does it take the ball to reach the ground below the lodge?
b) how far, horiztonally, from the ledge will the ball strike teh ground?
c) what is the horiztontal component of the balls velocity when it strikes the ground?
d) what is the vertical component of the balls velocity when it strikes the ground?
e) what is the kinetic energy of the ball just before it strikes the ground?
f) what is the speed of teh ball before it strikes the ground?

 

[minijumbuk]

What's a suvat =/? anyways, enjoy.

That's incorrect.
v=u+at does not apply to projectile motion.

Recall that projectile motion is split up into two components: horizontal, and vertical. Each component is completely independent of the others. Here, we need to use the formulas (provided on the formula sheet), or we could also use calculus (although you only learn this in Maths extension 1).

a)
Using the formula: Δy=u_yt + ½ a_yt²
Initially, the time is 0, hence t=0, and a_y is a constant of 9.8 (when going down, or -9.8 when going up)
Also, we know that u_y = 0 because there is no initial vertical velocity component in that motion: It's not inclined at an angle, thus only horizontal.
So, Δy = (0)(3) + ½ (9.8)(3)²
= 44.1 m

b) Δx=u_xt
= 15 x 3
= 45 m

c) Okay, so unless there is an external force applied to accelerate the projectile horizontally, u_x would always be constant throughout the motion. Don't worry about the acceleration bit, since this would never happen in the projectile motion question that they'd give you.
So in this case, u_x= 15 m/s horizontal
Then, you use the formula for vsub]y[/sub].
v_y²= u_y² + 2a_yΔy
= root (0 + 2 x 9.8 x 44.1)
= 29.4 m/s down

Now, since they asked for the velocity, and not for individual components, we use pythagoras' rule to do a little vector addition.

v² = 29.4² + 15²
v = 33 m/s
We also need to find the direction, since it is a vector.
Using basic trigonometry, we'd find the angle to be tan^-1(29.4/15)
= 63^o (to the nearest degree)

So v = 33 m/s 63^o to the horizontal

Also, Shantu, remember that velocity is given in units of m/s, not m/s²

 

[Aerath]

are u joking when u said what is suvat loll
and can u do another q or meeeeeee pleasssee

Dude, if you're going to diss and laugh at a guy who just helped you, then people in future won't really help you will they. But I'm just going to give you the benefit of the doubt and hope that you were going to explain to him what SUVAT is (I have no idea myself).

That's incorrect.
v=u+at does not apply to projectile motion.

Why doesn't it? He's just taking the vertical components. The initial vertical velocity is 0m/s, initial vertical acceleration is 9.8m/s^2 (downwards), and time is 3s. Why can't he use v=u+at. His answer is only wrong because he made a calculation error, no idea where the fuck he got 1.5m from, but if you punch his numbers into the calculator, you will get 44.1m, which is exactly the same as what you got. And as for everything else, he got what you got.

A ball of mass 0.5 ks rolls offf a horiztonal ledge 4.0 m above the ground with a speed of 4 ms-1

a) how long does it take the ball to reach the ground below the lodge?
b) how far, horiztonally, from the ledge will the ball strike teh ground?
c) what is the horiztontal component of the balls velocity when it strikes the ground?
d) what is the vertical component of the balls velocity when it strikes the ground?
e) what is the kinetic energy of the ball just before it strikes the ground?
f) what is the speed of teh ball before it strikes the ground?

Not sure it's all right, but here goes.

 

[Shoom]

Is there a left hand palm rule,

Can Lenz law be used to answer question 7 from the 2005 hsc?

 

[Aerath]

WTF has that got to do with projectile motion?
And you'd have to tell me what question 7 from 2005 HSC is.

 

[12o9]

Why doesn't it? He's just taking the vertical components. The initial vertical velocity is 0m/s, initial vertical acceleration is 9.8m/s^2 (downwards), and time is 3s. Why can't he use v=u+at. His answer is only wrong because he made a calculation error, no idea where the fuck he got 1.5m from, but if you punch his numbers into the calculator, you will get 44.1m, which is exactly the same as what you got. And as for everything else, he got what you got.

WHOOPS ROFLS.

 

[shantu1992]

Dude, if you're going to diss and laugh at a guy who just helped you, then people in future won't really help you will they. But I'm just going to give you the benefit of the doubt and hope that you were going to explain to him what SUVAT is (I have no idea myself).


Why doesn't it? He's just taking the vertical components. The initial vertical velocity is 0m/s, initial vertical acceleration is 9.8m/s^2 (downwards), and time is 3s. Why can't he use v=u+at. His answer is only wrong because he made a calculation error, no idea where the fuck he got 1.5m from, but if you punch his numbers into the calculator, you will get 44.1m, which is exactly the same as what you got. And as for everything else, he got what you got.



Not sure it's all right, but here goes.

wasnt dissing him haha
he used suvat thats why i was like whatt :|
sydney boys huh
tahmeed does physics outside school with me

 

[independantz]

What's a suvat =/? anyways, enjoy.

I assume it means the equation of motions that are given on the data sheet. suvat
s-displacement
u-initial velocity
v-velocity
a-acceleration
t-time

That's my guess lol.

 

[shantu1992]

spot on

 

[minijumbuk]

Yeah, v=u+at works.

Personally, I only ever work with the formulas with the subscripts, since it's so much clearer. Since the v and u value doesn't have any direction, I always tried to stay away from using that formula in projectile motions.

And as for the second set of solutions:

a) Δy=uyt + ½ ayt2

4= 0 + ½ x 9.8 x t2
t = root (8/9.8)
= 0.9 s

b) Δx = uxt
= 0.9 x 4
= 3.6 m

c) vx4 m/s horizontal (like I said earlier, horizontal vel. doesn't change)

d) vy2 = uy2 + 2ayΔy
= 0 + 2 x 9.8 x 4
vy = 8.85 m/s down

e) v = root (vy2 + vu2
v = 9.72 m/s (I won't bother with the angle here, since KE isn't a vector.

So KE= ½mv2
= ½ x 0.5 x 9.722
= 23.6 J

f) 9.72 m/s (it says speed, so no direction required)