# Kinematics problem help

## Homework Statement

A ball is thrown vertically upward from the ground with an initial velocity of 30 m/s. Assume the acceleration due to gravity is g = 10 m s-2.

How long will it take for the ball to rise to the highest point on its trajectory? (AF:no decimals)

i got v1 = 30m/s
a= 10m/s2
x1=0

but i cant think of where to start given this information can someone point me in the right direction

## The Attempt at a Solution

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SteamKing
Staff Emeritus
Homework Helper
What will be the velocity of the ball when it reaches the highest point of its trajectory?

0 is the final velocity. is the answer 3s? i think i may have got it

30=10t
t=3

anyone am i right?

also the height it reaches using 3?

would i use d=30*3 + 1/2(10)(3^2)

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anyone?

Doc Al
Mentor
0 is the final velocity. is the answer 3s? i think i may have got it

30=10t
t=3

anyone am i right?
Right!

also the height it reaches using 3?

would i use d=30*3 + 1/2(10)(3^2)
Almost. Careful with signs.

would it be d= 30*3 + 1/2(-10)(3^2)?

giving me 45?

Doc Al
Mentor
would it be d= 30*3 + 1/2(-10)(3^2)?

giving me 45?
Yep!

You can also solve it using the concept of average speed. What's the average speed of the ball as it rises?

average speed 15 m/s?

also i need help with this part... On the way up, at what height is the speed of the ball equal to half its speed of projection?

i get that it wants me to find at what distance v = 15 but im having trouble munipulating the equations to solve this

Doc Al
Mentor
average speed 15 m/s?
Right. So you can find the distance using average speed X time.

also i need help with this part... On the way up, at what height is the speed of the ball equal to half its speed of projection?

i get that it wants me to find at what distance v = 15 but im having trouble munipulating the equations to solve this
Solve it exactly as you solved the first part. First figure out the time.

okay thank you ill try and edit this

t=1.5

d=(15)(1.5)+(1/2)(-10)(1.5^2)= 11.25?

or (30)(1.5)+(1/2)(-10)(1.5^2)= 33.75?

i think its the second one

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Doc Al
Mentor
okay thank you ill try and edit this

t=1.5

d=(15)(1.5)+(1/2)(-10)(1.5^2)= 11.25?

or (30)(1.5)+(1/2)(-10)(1.5^2)= 33.75?

i think its the second one
Right. The initial velocity is 30 m/s, not 15. (That's the final velocity.)