Homework Help: Time of a projectile

MrRandom66 · Nov 1, 2011

Problem solved, many thanks

grzz · Nov 1, 2011

Write down some equations that may be used. The we can discuss which of them can apply.

obafgkmrns · Nov 1, 2011

You might want to check your units.

MrRandom66 · Nov 1, 2011

I'm really not sure what equations to use. I have a list of some equations but they ask for the initial velocity.

grzz · Nov 1, 2011

You can find the initial speed in the vertical direction.

MrRandom66 · Nov 1, 2011

grzz said: ↑

You can find the initial speed in the vertical direction.

Could you explain that please?

grzz · Nov 1, 2011

Try to find what is known in the vertical direction.

MrRandom66 · Nov 1, 2011

grzz said: ↑

Try to find what is known in the vertical direction.

Vertical displacement is 15m, and Gravity due to acceleration is 9.8 m s^2

grzz · Nov 1, 2011

initial speed
final speed
distance travelled
acceleration
time
Which of the above are known in the vertical direction?

grzz · Nov 1, 2011

What is the speed in the vertical at the maximum height?

MrRandom66 · Nov 1, 2011

Distance Travelled, and Acceleration?

Feel like I'm missing something here...

grzz · Nov 1, 2011

MrRandom66 said: ↑

Vertical displacement is 15m, and Gravity due to acceleration is 9.8 m s^2

Correct but acceleration is negative if we choose upward direction as positive.

MrRandom66 · Nov 1, 2011

Oh zero. I was missing something

grzz · Nov 1, 2011

Now you CAN find the initial speed in the vertical direction.

MrRandom66 · Nov 1, 2011

Before I calculate the final speed, I have to get t, correct? So, am I write in believing this is 15m/-9.8 m s^2

grzz · Nov 1, 2011

MrRandom66 said: ↑

Before I calculate the final speed, I have to get t, correct?

I think that you mean the initial (vertical) speed.

MrRandom66 · Nov 1, 2011

grzz said: ↑

I think that you mean the initial (vertical) speed.

Yes, I did.

grzz · Nov 1, 2011

If you know the initial and final vertical speeds, and the vertical distance you can find the time which is the quantity that you originally wanted to find.

MrRandom66 · Nov 1, 2011

grzz said: ↑

If you know the initial and final vertical speeds, and the vertical distance you can find the time which is the quantity that you originally wanted to find.

Sorry, I'm having a hard time getting my head round this.
I'm not sure what equation to use to find final velocity.

grzz · Nov 1, 2011

You will only learn how to do physics only if you try to pick the equation yourself.
Try to find - from notes or internet - the equations you can use.

MrRandom66 · Nov 1, 2011

OK, I've got the equation for final velocity, give me a minute and i'll work out the time.

MrRandom66 · Nov 1, 2011

Ok, so now I have the time, am I right in believe the total time for the projectile will be the time i've just calculated, x2?

grzz · Nov 1, 2011

Yes.

MrRandom66 · Nov 1, 2011

Brilliant! Thanks, Now i've got to work out the launch speed.... :-/

I'm going to double check my notes, but i'm sure i'll need help sooner or later.