Kinematics problem help

  • Thread starter tommy2st
  • Start date
  • #1
15
0

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

Homework Equations





The Attempt at a Solution

 

Answers and Replies

  • #2
SteamKing
Staff Emeritus
Science Advisor
Homework Helper
12,796
1,668
What will be the velocity of the ball when it reaches the highest point of its trajectory?
 
  • #3
15
0
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)
 
Last edited:
  • #4
15
0
anyone?
 
  • #5
Doc Al
Mentor
44,994
1,268
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.
 
  • #6
15
0
would it be d= 30*3 + 1/2(-10)(3^2)?

giving me 45?
 
  • #7
Doc Al
Mentor
44,994
1,268
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?
 
  • #8
15
0
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
 
  • #9
Doc Al
Mentor
44,994
1,268
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.
 
  • #10
15
0
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
 
Last edited:
  • #11
Doc Al
Mentor
44,994
1,268
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.)

Verify your answer using the average velocity method.
 
  • #12
15
0
thank you so much for your help doc!
 

Related Threads on Kinematics problem help

  • Last Post
Replies
2
Views
1K
  • Last Post
Replies
2
Views
1K
  • Last Post
Replies
3
Views
1K
  • Last Post
Replies
5
Views
5K
  • Last Post
Replies
3
Views
3K
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
1
Views
3K
  • Last Post
Replies
1
Views
709
  • Last Post
Replies
1
Views
5K
  • Last Post
Replies
4
Views
2K
Top