Question

Please help me out. :(

A baseball is thrown vertically upward with a speed, as it leaves the hand, of 30 m/s.

a) how long will it rise?
b) how high will it rise?
c) how long after it leaves the hand will it return to the starting point?
d) when will its speed be 20 m/s?

Answer 1

\[\large v(t) - v(0) = \int \limits_0^t a~ dt\]

Answer 2

yes.. slight.

Answer 3

you know \(\large v(0) = 30\) and \(a\) is constant
can you integrate ?

Answer 4

\[\large v(t) - 30 = at\]

Answer 5

am, we don't use integration for solving this..

Answer 6

Okay, you want to solve it using kinematics equations is it ?

Answer 7

yes. :) you got it. :)

Answer 8

\[\large v = u + at\]

Answer 9

you know \(u = 30\)
and at highest point \(v = 0\)

plug them and solve \(t\)

Answer 10

that gives you the time for which it rises

Answer 11

what is "u" in the equation v = u + at ?

Answer 12

u : initial velocity
v : final velocity


your notes uses different letters is it ?

Answer 13

ahh.. got it. :) ^_^ then, can i plug it now?

Answer 14

yes

Answer 15

okay..
i got 3.058 s for letter a. :)

Answer 16

what value are you using for g ?

Answer 17

9.8 or 10 ?

Answer 18

i use.... -9.81

Answer 19

g = 9.81
a = -g = -9.81

Answer 20

g is a positive constant always.

Answer 21

what? it's direction is upward right?

Answer 22

g is a constant whose value is 9.81 or whatever it is, its always positive.


a = -g

Answer 23

since it is acting down

Answer 24

ahh.. got it.. :) is my answer correct?

Answer 25

Correct! for part b, you may use below :

Maximum height = \(\dfrac{u^2}{2g}\)

Answer 26

just plugin the numbers and evaluate

Answer 27

why?

Answer 28

why ?....you have the values give the machine(formula) its input and it will feed you back with yar desired output

Answer 29

please explain it to me.... why did he use that formula... thankyou.

Answer 30

nitial velocity (u) = 30
velocity at the highest point = 0 ( becayse velocity decreases when something is thrown up and becomes 0 at the maximam point )
acceleration = -9.81 / 9.81 downwards

(a)
V= u+at
0 = 30+ (-9.81)*t
t =3.05s

(b) V^2=u^2+2as
0 = 30^2 +2* (-9.81) * s
s = 45.9m

(c) t*2
3.05*2
=6.1 s

(d) V=u +at
20= 30= (-9.81)*t
t = 1.01s

methods are correct,,but smtimes there might be mistakes in da calculations because i did it in a hurry ! anyway this is the method to do the qstion :) hope it helped

Answer 31

you don't like integral
you don't like formula

what do you want ? :o

Answer 32

have you read about the formula's of the motion if yes ...he used some simple formulas and replaced the variable of acceleration with g .....and got the resulting formula for the hieght ..DO you want to see how it is derived? @itsmeeQUENNIE

Answer 33

no.. it's not what you think @rational .. i just dont get it. sorrry. :(

Answer 34

Do you get this thing posted by @anwaarullah :

(b) V^2=u^2+2as
0 = 30^2 +2* (-9.81) * s
s = 45.9m

in the above formula just make few changes for the acceleration parameter and you can simple derive the the formula for max. height

Answer 35

you have to write down coupl of time the answer and formulas,,,without lookng at the answer,,,then your mind will strat pickng it,,,i must say

Answer 36

@letterb .. i don't get it..

Answer 37

Check this out: http://physics.stackexchange.com/questions/67432/intuitive-meaning-of-factor-2-in-formula-of-vertical-throw-max-height-h-v2-2g

Answer 38

not direct explianation but gives you some direct hints

Answer 39

ahh.. got it.. thanks @Adjax

Answer 40

letter c,,,,its double distance from downward to upward and than donward,,,, thts why multiply with 2* 3.5 i guess in part ,(a)

Answer 41

ahhh.. :))) got it.. then in letter b... 20 m/s will be the final velocity, right?

Answer 42

letter d rather...

Answer 43

yup you are right,,b letter,,d asking about time??? now we have initial and final velocities,,,just put them in formuls,,,,but remeber you have to learn,,,memeorise the formula whtever you do,,,,note down,,,writ down again and again,,,and get the theme of question,,,,, got it,,,,,but writing down is must

Answer 44

yes.. thank you. i'll do it. :) thank you very much.

Answer 45

no problem lol,,,wlecom any time but when available,,,remeber writing down question,,,special math or physcs nomericls,,,,are very helpful to write down,,,,,,got it

Answer 46

aye aye captain. :)

Answer 47

captain,,,,huh,,,,when i used to study,,,i write down alll formulas,,,,,whther math or phy

Answer 48

yeah right..
but wait..
when you will use that formulas?
how will you know that you will use that formulas?

Answer 49

that;s fairly easy you need your intuition..JUst know what variable you currently have at the hand and user the simplex formula on whioch plug-chuggin will get you the answer

Answer 50

from v^= U^2 +2as you cant calculate time???could you you....
similarly likwise

Answer 51

ahh.. okay..
when you will use.... v = u + at ?

Answer 52

let suppose thy are asking about time,,,like in part (a) ,,,you could see

Answer 53

similary you cn nt count distance from abov formula which you mention recently

Answer 54

ahh.. got it.. but, how about s = ut + 1/2 at^2, when you will use that?

Answer 55

wher is (g) and (a )are acceleration if you know!!!! g when height ect mention,,,,,a,,,when horizntal or ground mention