Quadratic equation ball 72km/h

[LDC1972]

Homework Statement

A ball is thrown at 72km/h speed from top of a building. Building is 125m tall.

Homework Equations

Distance traveled before it hit the ground is as follows:

s = Ut + 0.5 gt^2

The Attempt at a Solution

Using quadratic equation:
x = -b±b^2 - 4ac / 2a

The question is for part one:
Find the time for the ball to drop to fifth of the height of the building.

Now I'm not sure if that means to 25m or 100m?!
For 25m = 1 second
For 100m = 3.3852

I got these from the quadratic equation. Do they seem right?

Thanks!

 

[darkxponent]

Homework Statement

A ball is thrown at 72km/h speed from top of a building. Building is 125m tall.

The question is not clear. Whether the ball is thrown up or thrown down?

Using quadratic equation:
x = -b±b^2 - 4ac / 2a

This is wrong. There is square root missing.

 

[LDC1972]

The question is not clear. Whether the ball is thrown up or thrown down?

Sorry thrown down!

This is wrong. There is square root missing.

Sorry again I clicked the root sign but it couldn't of worked. Here is the equation again:

x = -b ± √b^2 - 4ac / 2a

The question is unclear on the "fifth" part. Do they mean from top or bottom?

Anyway I calculated it from all heights:

At 25m = 1 second
At 100m = 2.899 seconds
At 125m = 3.3852 seconds

Seems right?

 

[darkxponent]

The question is unclear on the "fifth" part. Do they mean from top or bottom?

The question is clear here. They asking from the top that is 25m.

At 25m = 1 second
At 100m = 2.899 seconds
At 125m = 3.3852 seconds

Seems right?

Should i solve it and check it? I am not going to do that. It would better if you tell me what you put the values of U, g and S in this equation. I think this would be enough to enough to tell whether you did it correctly or wrong.

s = Ut + 0.5 gt^2

 

[LDC1972]

The question is clear here. They asking from the top that is 25m.




Should i solve it and check it? I am not going to do that. It would better if you tell me what you put the values of U, g and S in this equation. I think this would be enough to enough to tell whether you did it correctly or wrong.

s = Ut + 0.5 gt^2

OK,
U = 20ms (metres per second) (initial velocity of 72 Km/h I converted to ms)
g = 10ms (this is given in the question (g = acceleration due to gravity 10ms). Without which I'd of put 9.81ms, but I assume they allowed for ball mass?)
s = distance, so 25 metres, 100 metres and 125 metres (although by the sounds of it I can drop the 100 metre one)

 

[LDC1972]

OK,
U = 20ms (metres per second) (initial velocity of 72 Km/h I converted to ms)
g = 10ms (this is given in the question (g = acceleration due to gravity 10ms). Without which I'd of put 9.81ms, but I assume they allowed for ball mass?)
s = distance, so 25 metres, 100 metres and 125 metres (although by the sounds of it I can drop the 100 metre one)

Ah, of course! I got so wrapped up in the quadratics and factoring today I forgot I had the original equation to "plug" my t into and check the solution I got was correct :-)

 

[darkxponent]

OK,
U = 20ms (metres per second) (initial velocity of 72 Km/h I converted to ms)
g = 10ms (this is given in the question (g = acceleration due to gravity 10ms). Without which I'd of put 9.81ms, but I assume they allowed for ball mass?)
s = distance, so 25 metres, 100 metres and 125 metres (although by the sounds of it I can drop the 100 metre one)

Everything looks so damn correct. Good work!

You can always press the thanks button if you find my help useful :)

 

[LDC1972]

Everything looks so damn correct. Good work!

You can always press the thanks button if you find my help useful :)

Done the thanks button. Brilliant mate THANK YOU!

 

[darkxponent]

OK,
U = 20ms (metres per second) (initial velocity of 72 Km/h I converted to ms)
g = 10ms (this is given in the question (g = acceleration due to gravity 10ms). Without which I'd of put 9.81ms, but I assume they allowed for ball mass?)
s = distance, so 25 metres, 100 metres and 125 metres (although by the sounds of it I can drop the 100 metre one)

As the question doesn't state whether the ball is thrown upwards or downwards. You have taken the downward case.

Question: How will the values change if the ball is thrown upwards?

 

[LDC1972]

As the question doesn't state whether the ball is thrown upwards or downwards. You have taken the downward case.

Question: How will the values change if the ball is thrown upwards?

The question does say downwards, sorry for my poor copying!
I'm done for the day, but if it had been upwards g would of worked "against", not "for" the acceleration.

 

[LDC1972]

The question is clear here. They asking from the top that is 25m.




Just had email response from my tutor. It isn't "clear".

They are asking for 100m (a fifth from bottom not top).

So 25m is incorrect...

 

[darkxponent]

Just had email response from my tutor. It isn't "clear".

Doesn't matter. You formed the correct equation which can get correct answer for any 'meter' ;)