Help me understand this one?

[thickmax]

Can someone please break this down for me?

0 = 9t-4.9t^2

t= 9/4.9


Learning about motion but this jump is really confusing me

 

[Cubist]

0 = 9t-4.9t^2

You could write this as...

0=t ( ? )

can you fill in the "?"

 

[pka]

Well $0=9t-4.9t^2$ is the same as $4.9t^2-9t=0$ or $t(4.9t-9)=0$.
Can you solve the last?

 

[thickmax]

You could write this as...

0=t ( ? )

can you fill in the "?"

0 = t(9-4.9t)

But I don't see how that helps sorry...

 

[thickmax]

Well $0=9t-4.9t^2$ is the same as $4.9t^2-9t=0$ or $t(4.9t-9)=0$.
Can you solve the last?

I'm sorry, I can't. I do not know what you are asking...

 

[Cubist]

0 = t(9-4.9t)

But I don't see how that helps sorry...

Well, if a*b = 0 then a=0 and/or b=0

So, t=0 is a solution. To find the other solution what value must t have to make (9-4.9t)=0 ?

EDIT: This idea can be extended, so if you have ( x-2 )( 2x-3 )( x-5 ) = 0, then the possible solutions are x=2, x=? and x=?

 

[thickmax]

Well, if a*b = 0 then a=0 and/or b=0

So, t=0 is a solution. To find the other solution what value must t have to make (9-4.9t)=0 ?

EDIT: This idea can be extended, so if you have ( x-2 )( 2x-3 )( x-5 ) = 0, then the possible solutions are x=2, x=? and x=?

Again, I really do not know how to solve (9-4.9t)=0...

Your edit, x could also equal 1.5 and 5 right?

 

[Cubist]

Again, I really do not know how to solve (9-4.9t)=0...

Your edit, x could also equal 1.5 and 5 right?

If you can solve for x, then would it help if I write...

x ( 9 - 4.9x) = 0

( x - 0 )( 9 - 4.9x ) = 0

- ( x - 0 )( 4.9x - 9 ) = 0

( x - 0 )( 4.9x - 9 ) = 0

Could you solve the last line? What value of x would make the second bracket zero?

EDIT: Think about how you got the (correct) 1.5 answer

 

[Cubist]

I assume that you're still struggling. How would you start to simplify the following...

( 4.9t - 9 ) = 0

4.9t - 9 = 0

Hint: now add something to both sides so that the "4.9t" will be on its own

 

[thickmax]

I assume that you're still struggling. How would you start to simplify the following...

( 4.9t - 9 ) = 0

4.9t - 9 = 0

Hint: now add something to both sides so that the "4.9t" will be on its own

I get this now. To resolve,

4.9t = 9

t = 9/4.9.

But how where does the square root go?

 

[Cubist]

But how where does the square root go?

Do you mean square rather than square root. Are you talking about the $t^2$ from the original equation $0 = 9t-4.9t^2$ ?

 

[thickmax]

Do you mean square rather than square root. Are you talking about the $t^2$ from the original equation $0 = 9t-4.9t^2$ ?

sorry, yes, you are quite right, the square.

 

[Cubist]

The initial equation is a quadratic. It can be rewritten in a form that you might find more familiar...

$4.9t^2-9t+0=0$

But there's no need to use the quadratic equation here. It's much easier to write the above as...

$t ( 4.9t-9 ) = 0$

The $t^2$ is still there if you expand this. BUT in factored form you can (with practice!) directly see the TWO solutions t=0 and t=9/4.9. If you want to convince yourself, then you could use the quadratic equation on the top equation and you should obtain the same two answers. But I think you'll agree it's much easier, and quicker, to factor in this case!