Projectile Motion (1 Viewer)

[Michaelmoo]

Sorry everyone. The textbook I'm using has bodgy answers. If anyone could do this question I'd really appreciate it:

A group of lemmings run over the edge of a 200m cliff at 0.6m/s.

a) Calculate their time to fall to the bottom of the cliff

b) Calculate their velocity half way down.

c) Calculate the time their (vertical) speed will be 30.0m/s

d) Calculate the speed at which they hit the ground.

 

[4theHSC]

LOL surfings book hey! I answered that question already, will show you a lil later if u want, or jus add me on msn... cya

 

[Michaelmoo]

LOL surfings book hey! I answered that question already, will show you a lil later if u want, or jus add me on msn... cya

Well. Did you find the answers at the back wrong? Btw.. It doesn't take that long to do.

 

[4theHSC]

Well. Did you find the answers at the back wrong? Btw.. It doesn't take that long to do.

well no they are right... it doesn't take that long to do but jus typing it and all takes like 20 mins... I will give u the working out, hang on a bit

 

[Michaelmoo]

a) Δy = Uyt + ½ay(t^2)

200 = 0t + .5 x 9.8 x t^2

t^2 = 40.81632653...

t = 6.4s

What Have i Done wrong there???

 

[4theHSC]

a) Δy = Uyt + ½ay(t^2)

200 = 0t + .5 x 9.8 x t^2

t^2 = 40.81632653...

t = 6.4s

What Have i Done wrong there???

LOL you havn't done anything wrong, your working out is correct.

 

[4theHSC]

urghhhhhhhhhhhhh i did all the working out for you and then it got deleted!... damn!

for b) start with Vy=Uy+at... and for c start with 30=0+9.8t... for d start with V^2y=U2y+2as... once you found Uy use pythagoras to find the velocity... you should end up with sqrt 62.6^2+0.6^2= 62.613m/s... after that find the angle to the horizontal.

 

[Michaelmoo]

LOL you havn't done anything wrong, your working out is correct.

Exactly. So the answers are wrong. If you look at the answers, for some reason it says 4.52 seconds.

Also, I did all that you said above. Its just that for part a and b the answers are wrong.

Dw about posting. I've got my answer. Thanks.

 

[4theHSC]

Exactly. So the answers are wrong. If you look at the answers, for some reason it says 4.52 seconds.

Also, I did all that you said above. Its just that for part a and b the answers are wrong.

Dw about posting. I've got my answer. Thanks.

The answers are correct, I think your confusing the back answers with other answers... check again because they are perfectly correct and it says 6.4 s as the time. I'm pretty sure they are correct because the answers at the book say so... check again, otherwise the back is incorrect.

 

[Michaelmoo]

The answers are correct, I think your confusing the back answers with other answers... check again because they are perfectly correct and it says 6.4 s as the time. I'm pretty sure they are correct because the answers at the book say so... check again, otherwise the back is incorrect.

Surfing Physics Space - By Brian Shadwick

Answers on page 63
8 Projectile Problems-1

12 a) 4.52s
b)42.3ms-1


Definately wrong. Parts c and d are correct though, everything else was fine. Although there are occasionally answers that pop up that are wrong. If yours is right, then mine must be a different version. Mines Published in 2003.

Also I find that the book has some questions that are ridiculous in that they contradict themselves. e.g.


A projectile needs to be fired with minimum velocity and hit a target 3km away (at the same level) 21s after firing. Find the initial velocity of the projectile.

Clearly, the initial angle must be 45 degrees and horizontal and vertical components must be equal. But consider this:

Change in x = (ux)t

3000 = (ux)21

ux = 142.9ms-1

But now lets consider the vertical component. It should be equal. But its not. Note the time to rise must be 10.5s as the trajectory is symetrical (same level):

(vy) = (uy) + (ay)t

0 = (uy) + (-9.8)(10.5)
(uy) = 102.9ms-1

Answers say both are 142.9 and the angle is 45 which should make sense, but the question contradicts itself. Unless offcoarse the projectile doesn't land at the same level. Although the question clearly says this.

 

[4theHSC]

LOL I think its because I have the revised edition... on page 58 question 12, the answers are a)6.4s, b)103.9m, c)3.06s and d)62.613 m/s

 

[4theHSC]

yeah I agree with your working out as well... The old one has alot of mistakes... should talk to brian about it... he tutors physics