Math Is Fun Forum

headhurts

Member

Registered: 2014-05-04

Posts: 8

why do people keep throwing the ball up?

Q. If a ball is thrown vertically upwards with velocity 28 m/sec, its height (in metres) after t seconds is given by y=28t−4.9t^2. Find the velocity at t = 2.

I do know how to solve this problem... as an initial velocity problem, using integration.

However, this question was covered near the beginning of the semester so long before we got to integration so there's obviously some way to figure out the answer without integration.

gAr

Member

Registered: 2011-01-09

Posts: 3,482

Re: why do people keep throwing the ball up?

Since velocity is (displacement) / (time taken), we need to use differentiation, not integration.

Suppose we don't know differentiation too, then we need to obtain the answer by putting in actual numerics.

Then we can make out that the answer approaches 8.4 m/s.

Last edited by gAr (2014-06-01 17:11:05)

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"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

Bob

Administrator

Registered: 2010-06-20

Posts: 10,621

Re: why do people keep throwing the ball up?

Must you use that formula?

It comes out easily with

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

gAr

Member

Registered: 2011-01-09

Posts: 3,482

Re: why do people keep throwing the ball up?

Yes!
And I thought the initial velocity given was redundant there.
I think the equation was given to obtain the value of 'a', to indirectly say that the ball is thrown on earth-like planet!

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"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

Bob

Administrator

Registered: 2010-06-20

Posts: 10,621

Re: why do people keep throwing the ball up?

Well, it is true that it doesn't actually say it's on the Earth.    But the 4.9 is a give away.

Bob

why do people keep throwing the ball up?

Because it keeps coming down again. 

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob