An object is thrown upwards and after 2 seconds is 78 m above where it was initially thrown. Assume no air resistance, and determine the initial speed the object was thrown at.

Question

An object is thrown upwards and after 2 seconds is 78 m above where it was initially thrown.  Assume no air resistance, and determine the initial speed the object was thrown at.

asnaseer · Answer

you can use this equation of motion to solve this:$$y=ut+{1\over2}at^2$$where:
y = distance travelled
u = initial velocity
a = acceleration (which, in this case, should be equal to -g)
t = time

anonymous · Answer

we are looking for u right?

asnaseer · Answer

yes

anonymous · Answer

-78m=u(2) + 1/2 (-9.8)(2)^2 right?

anonymous · Answer

srry not -78

asnaseer · Answer

78=...

asnaseer · Answer

If you are learning about projectile motion, then you might find this helpful: http://hyperphysics.phy-astr.gsu.edu/hbase/mot.html

anonymous · Answer

i got some weird answer. i got -1372.8

anonymous · Answer

and thank you soo much for helping

asnaseer · Answer

try and work out each part step-by-step.
so, first what is the value of:$$t^2$$when t=2?

anonymous · Answer

y = 78m 
t = 2
a = -9.8

asnaseer · Answer

yes, but your final answer is wrong, so I'm trying to take you through it one step at a time to se where you may have made a mistake.

anonymous · Answer

i think i am making a mistake in the re-arranging the equation

asnaseer · Answer

ok, so first try rearranging the original equation:$$y=ut+{1\over2}at^2$$to get an expression for u. i.e. BEFORE plugging in the values.

anonymous · Answer

y/u= t + 1/2 at^2

asnaseer · Answer

no - that is wrong

anonymous · Answer

then isolate the u? soooo sorry im very terrible at this

asnaseer · Answer

think of it as:$$A=B+C$$how would you rearrange this to get an expression for B?

asnaseer · Answer

you need to subtract C from both sides

anonymous · Answer

a-c

asnaseer · Answer

yes, therefore:$$y=ut+{1\over2}at^2$$first becomes:$$y-{1\over2}at^2=ut$$

asnaseer · Answer

next you need to divide both sides by 't'

anonymous · Answer

OKAYYY!! i was trying to get the easier way out by dividing only. THANK YOU

asnaseer · Answer

np - do you still want me to check your final answer?

anonymous · Answer

yup please. 1 min

anonymous · Answer

48.8 m/s? :s

asnaseer · Answer

yes - you've got it right - well done!

anonymous · Answer

LOL THANK YOU SOOO  MUCH!!! YOU EXPLAIN VERY GOOD

asnaseer · Answer

yw - I'm glad I was able to help :)