2 Physics Questions Gr 11 physics
1.) An astronaut fell off of a cliff on the moon and hit the surface of the moon at 60m/s. Assuming acceleration is 1.6m/s^2 (DOWN), what is the height of the cliff? How long did it take the astronaut to hit the ground?
I keep getting the cliff is -1125 meters for the height of the cliff...
September 30, 2012
16 Comments • Newest first
@Ickest: What? In exams. You'll only get marks for your final answer. At least that's how it works in Aus. Using calculus generally takes longer, anyway.
Vf = Vo +at
solve for t
t= (Vf-Vo)/a
t = (60 - 0)/1.6
t = 37.5s
then find D using D = Vot +0.5at^2
D = 0*37.5 +0.5*-1.6*37.5^2
D=-1125
height = 1125m
you are right, since you went down, the distance is negative. just make it positive & that is the height of the cliff.
[quote=nsdarknessx]Our teacher said whenever you have free fall questions, you should automatically say up is positive.[/quote]
It's just a convention, doesn't really matter which you use. You just pick one and stick with it.
Your distance is negative because remember, using the formula it's [b]Delta d[/b], your final distance is at a [b]lower[/b] height than your [b]initial[/b] height.
Hence your [delta d] would be negative.
[quote=nsdarknessx]Our teacher said whenever you have free fall questions, you should automatically say up is positive.[/quote]
Ok it doesn't matter as long as you know how to interpret your answers. A negative displacement where up is positive means your displacement is down.
@Ickest: You can, but you don't get marks for the working out.
[quote=simaini]it doesn't make sense that UP is positive if you're going down. dropping down should be positive since gravity is making you go faster. when you're going up, you're going against gravity so it should be negative o.o
i believe that when you have negative velocity you're going backwards? but i'm not quite sure.[/quote]
Our teacher said whenever you have free fall questions, you should automatically say up is positive.
[quote=nsdarknessx]Alright, I'll try to make it as organized as possible for you guys:
Velocity 1 is 0. Velocity 2 is -60m/s. Acceleration is -1.6m/s^2. I solved for time and got 37.5 seconds, which is right I know. Then, to figure out displacement, I use the formula:
displacement=(V1+V2/2)(t). I plug everything in and ensure I have the correct numbers, and I get -1125m...
Our teacher told is in free fall questions, to state that anything going UP is a positive number, and anything going down is a negative number.[/quote]
it doesn't make sense that UP is positive if you're going down. dropping down should be positive since gravity is making you go faster. when you're going up, you're going against gravity so it should be negative o.o
i believe that when you have negative velocity you're going backwards? but i'm not quite sure.
@nsdarknessx: d = (-60)(37.5) - (1/2)(-1.6)(37.5)^2
= -1125m
Yes, the answer is right. You move 1125m down, so the height of the cliff is 1125m.
vf^2 = vi^2 + 2ax
vf = 60
vi = 0
a = 1.6
Why does someone always say integrate/differentiate when it gets to Physics projectile motion? That's maths, you use basic formulas in physics.
[quote=ClementZ]Post what it is you're doing and we'll see if and where you're making mistakes.[/quote]
Alright, I'll try to make it as organized as possible for you guys:
Velocity 1 is 0. Velocity 2 is -60m/s. Acceleration is -1.6m/s^2. I solved for time and got 37.5 seconds, which is right I know. Then, to figure out displacement, I use the formula:
displacement=(V1+V2/2)(t). I plug everything in and ensure I have the correct numbers, and I get -1125m...
Our teacher told is in free fall questions, to state that anything going UP is a positive number, and anything going down is a negative number.
[quote=nsdarknessx]How can the cliff's height be a negative number though?O.o
and I know HOW to do it (like using the certain formula for this scenario and everything), but why the hell am I getting a negative number for a height?...[/quote]
Post what it is you're doing and we'll see if and where you're making mistakes.
the answer should be negative because hes falling? also in physics grade 11
EDIT:
Nevermind you probably can't use calculus
I have physics next semester.
[quote=Ickest]That seems right.[/quote]
How can the cliff's height be a negative number though?O.o
and I know HOW to do it (like using the certain formula for this scenario and everything), but why the hell am I getting a negative number for a height?...