Physics Questions on Thrown Up Ball & Bullet

[A.I. BOT]

[Solved] Thrown up ...

I am stupid when it comes to Physics so please guide me :). I have 2 questions on a worksheet I got today and I am stuck.

A ball is thrown up with a speed of 30 m/s. What is the maximum height reached by the ball?
a) 23m b) 46m c) 92m d) 132m

Is it b? .. (30 m/s) / 9.8 m/s^2 = 3.06s ... 30 x 1/2(3.06) = 46m?

A bullet is shot stright up and returns to its starting point in 10 s. What is the initial speed of the bullet
a) 9.8 m/s b) 25 m/s c) 49 m/s d) 98 m/s

Is it b? .. Vi = Vf - a*1/2(t) ... Vi = 0, Vf = 0, a = 9.8, t = 1/2(10) = 5 ... Vi = 0 - 9.8*1/2(10) ... 25m?

Thanks :)

 

[berkeman]

The first answer looks to be correct (not sure what the 2nd part is you used to find it), but I get something different on your second question.

"Vi = Vf - a*1/2(t)"

Are you sure there's a 1/2 there? http://en.wikipedia.org/wiki/Kinematic

 

[A.I. BOT]

Hmm your right, so it should be d) 98 m/s I am assuming ?

 

[OlderDan]

Hmm your right, so it should be d) 98 m/s I am assuming ?

How does the final velocity at the end of 10 seconds compare to the initial velocity?

 

[A.I. BOT]

I was told in school that when something is thrown up ... the speed it went up, will be the same speed coming down. The bullet is going to return to it's start point it states, thus making its final velocity 0m/s ? so ... If I just plug the numbers into Vi = Vf - at I get 98m/s, so I will go with that and see what it says when I get the assignment back.

Thanks for the help guys :)

 

[OlderDan]

I was told in school that when something is thrown up ... the speed it went up, will be the same speed coming down. The bullet is going to return to it's start point it states, thus making its final velocity 0m/s ? so ... If I just plug the numbers into Vi = Vf - at I get 98m/s, so I will go with that and see what it says when I get the assignment back.

Thanks for the help guys :)

Your statement is contradictory. The speed is the same when it gets back to the starting point that it had when thrown. That is not zero. If it were zero, it would never have gone up.