Question

A ball is thrown down vertically with an initial speed of 24 ft/s from a height of 47 ft. (a) What is its speed (in ft/s) just before it strikes the ground? (b) How long does the ball take to reach the ground? What would be the answers to (c) part a and (d) part b if the ball were thrown upward from the same height and with the same initial speed ? Before solving any equations, decide whether the answers to (c) and (d) should be greater than, less than, or the same as in (a) and (b).

Answer 1

the speed is still 24 ft/s because speed doesn't change! for part b) you would use the formula d= at^2/2 where d= 47, a= 9.81 and t is what you are finding

Answer 2

I thought for (a) you would have to use a constant acceleration formula where you know the initial velocity is -24 ft/s, the displacement is -47 ft. ??? This is my first physics class, and this is chapter 2 of our book. So...I am trying to understand it from the book's view, but is your way easier?

Answer 3

D:

Answer 4

a= -32 ft btw

Answer 5

yes you would have to use the equation \(v_{f}^2=v_{i}^2+2as\) for the first one.
For (b), you use
v=u + at.

Answer 6

What are the notations s and u

also, should i ignore that persons post? i think they were wrong but i felt bad to just say it xD

Answer 7

yeah you should.
so do you want the downward direction to be positive?

Answer 8

no i meant s is time and u is initial velocity by the way you labeled it right?

Answer 9

s is displacement, t is time

Answer 10

0=(-24)^2+(2)(-32)(-47)t

Answer 11

aghhh sorry

Answer 12

no t lol and no 0 haha

Answer 13

Say you want the downward direction to be positive, then 'g' is positive, s is positive (take the point at which the object is thrown as the origin). For (a) and (b), u is positive because it's downward. For (c) and (d), it's negative cuz it's upward.

Answer 14

im running on 3hrs sleep

final velocity is supposed to be 54.6 ft/s? o.e

Answer 15

&I thought downward was supposed to be negative. im confused

Answer 16

i got 59.86 ft/s.
and it's your choice. you can choose whichever direction you want as positive/negative.
The thing is, once you've made your choice, stick with it for the rest of problem.
So, downward-negative is equally correct.

Answer 17

did you get this equation?
\[v^{2}=(-24)^{2}+2(-32)(-47)\;?\]

Answer 18

oooh i see. no wonder. I was doing this problem 538470465 times, i got 59 as an answer too. yes i did get that eq

Answer 19

good :)

Answer 20

can you do the rest of 'em by yourself or do you want help?

Answer 21

59= -24 + (-32)t

Answer 22

Id like help please :3 the first part was easy

Answer 23

if downward is negative, the final velocity would be -59.86

Answer 24

ok. time is 1.9 s

Answer 25

*1.09 s

Answer 26

i notice that you're using 59 instead of 59.86. It's better to use 59.86.
But if you want to approximate it, use 60 because that's closer.
and yes, your answer is right.

Answer 27

would c) just be positive 59.86 since its going upwards and a was -59.86

&thank you for the tip

& how should i interpret what d) is saying?

Answer 28

The initial velocity is 24 ft/s. Therefore, for (c) and (d), you take +24 for u.
note that you'll get the same -59.86 ft/s for the final velocity. Minus because the final velocity is still downward.
and the great thing about these equations is that you can just use the same equation you used for (b) for (d). All you have to do is take +24 for 'u' and the equation will take care of the rest.

Answer 29

c)&a) Right because it's squared etc. okay.

d) t= 2.62 s

Answer 30

That's absolutely right! :)

Answer 31

Thanks so much for explaining and staying around. (:

Answer 32

yw :)