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# projectile

## Question

A body is projected downward at an angle of 30 degrees to the horizontal from the top of a building 170m high. It's initial speed is 40m/s. a) How long will it take before striking the ground? How far from the foot of the building will it strike? c) At what angle with the horizontal will it strike?

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I'd start by making a table of information with the initial velocity, final velocity, displacement, acceleration, and time for both the vertical and horizontal components of motion.  Once you have the information in the table, your kinematic equations will be of help.  If you're not sure how to do that, the following video might be a great place to start:

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It not an assignment am just putting the things I learnt from your video on projectile to work. I have tried every method possible but am confused with the case of downward projection. Please kindly put me through.

Thanks.

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Glad to help -- why don't you show what you've done so far -- i.e., show the tables of information for vertical and horizontal motion that you've put together.  That'll give me an idea how to guide you to a solution most efficiently.

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Vertical competent

Vi= 40sin30

Vf

d=170m

a= 9.8ms-2

t=

Horizontal component

V avg= 34.6ms-1

d=

t=

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For the vertical component, let's assume down is the negative direction...  You can solve for time in the air using the quadratic:

$d=v_{i}t+\frac{1}{2}at^{2}$

and the quadratic equation, or make your life simpler and solve for vf first, then time.

Once you know how long the projectile is in the air, use that time in your horizontal equation to find how far it travels horizontally.

As for the angle with which it strikes, use the final vertical velocity component and the horizontal velocity component (along with a little trig) to solve for the impact angle.

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