Answer: Kinematics Free Fall Homework Solution

[madah12]

Homework Statement

. A ball is thrown straight up at t=0. It is found to be falling with a speed of 8 m/s at t= 6 sec. Find the displacement of the ball.

Homework Equations

The Attempt at a Solution

v=v₀-gt
8=v₀-60
v₀=68
deltax =v₀t - .5gt²
delta x = (68*6)- 5*36=228
but it says the answer is -132 am i wrong or is it wrong?

Edit:
nvm I got it it was supposed to be -8
-8=v₀-60
52=v₀
deltax=52*6 - .5*10*36 = 132

uhm it seems I messed up again this time I got a plus why isn't it a minus?

 

[madah12]

Also I have another question is delta y = v(t)t+.5 gt^2 considered one of the kinematics equations I mean can I use it without having to derive it every time?

 

[physx_420]

"Find the displacement of the ball."

Since you throw the ball straight up and it comes back down to where you initially threw it from, then I believe the displacement should be zero. It would be like running a lap around the track. Sure you have ran a 1/4 of a mile but your total displacement would be zero because you end up back to where you originally started.

"Also I have another question is delta y = v(t)t+.5 gt^2 considered one of the kinematics equations I mean can I use it without having to derive it every time?"

Yes that is one of the kinematic equations that you can memorize. It only applies, however, to constant accelerations.

 

[madah12]

i am pretty sure it meant the displacement at t=6 ...

 

[physx_420]

i am pretty sure it meant the displacement at t=6 ...

Did you use a positive acceleration or a negative acceleration? Always remember which way you set up your coordinate system (i.e. down is negative and up is positive.) If you used (-8)m/s for the velocity than we assume down is negative.

 

[madah12]

I used negative acceleration. - 10

 

[physx_420]

displacement = x(final)-x(initial)
s = 0-132
When you used 52 m/s (which was your initial velocity) for your motion equation, you get an initial position.

 

[madah12]

x final is 132 and x initial is 0 why did you reverse it ?