How Does Projectile Motion Calculate Distance and Height?

[tdusffx]

--------------------------------------------------------------------------------

A ball is tossed from an upper-story window of a building. The ball is given an initial velocity of 8m/s at an angle of 20 below the horizontal. It strikes the ground 3.00s later. The horizontal distance at which the balls strike the ground from the base of the building and the height from which the ball was thrown are respectively given by:

Xf = Xi + Vxi*t

Change of X = Vxi*T

Xf = 8cos(20)(3)

Xf = 22.5 m

and Yi

Y = Vyi - 1/2g*t^2

0 = Yi + 8sin(20) - 1/2(9.81)(3)^2

Yi= 36m

are my answers right?

 

[Gokul43201]

--------------------------------------------------------------------------------

A ball is tossed from an upper-story window of a building. The ball is given an initial velocity of 8m/s at an angle of 20 below the horizontal. It strikes the ground 3.00s later. The horizontal distance at which the balls strike the ground from the base of the building and the height from which the ball was thrown are respectively given by:

Xf = Xi + Vxi*t

Change of X = Vxi*T

Xf = 8cos(20)(3)

Xf = 22.5 m

This is correct.

and Yi

Y = Vyi - 1/2g*t^2

0 = Yi + 8sin(20) - 1/2(9.81)(3)^2

Yi= 36m

are my answers right?

There are 2 errors in the bolded lines:
1. Check the equation - something's missing;
2. Check the signs of all terms - which way is the positive y-axis pointing?

 

[tdusffx]

hmmm...

Yf = Yi + Vyi - 1/2gt^2

0 = Yi + 8sin(20) - 1/2(9.81)(3)^2

-Yi = -36

yi = 36...I can't think of anything mistake here but my book is saying t hat I got the wrong answer...

 

[learningphysics]

hmmm...

Yf = Yi + Vyi - 1/2gt^2

0 = Yi + 8sin(20) - 1/2(9.81)(3)^2

-Yi = -36

yi = 36...I can't think of anything mistake here but my book is saying t hat I got the wrong answer...

As Gokul mentioned there are 2 things wrong with your formula... first off, basically you've got the formula wrong... check what the formula is... second a sign is wrong...

 

[tdusffx]

Yf = Yi + Vyi - 1/2gt^2

0 = Yi + -8sin(20)(3) - 1/2(9.81)(3)^2

-Yi = -52.30

yi = 52.30 m?

now?

 

[learningphysics]

Yf = Yi + Vyi - 1/2gt^2

0 = Yi + -8sin(20)(3) - 1/2(9.81)(3)^2

-Yi = -52.30

yi = 52.30 m?

now?

Yes. Looks right.

 

[Gokul43201]

Yf = Yi + Vyi - 1/2gt^2

0 = Yi + -8sin(20)(3) - 1/2(9.81)(3)^2

-Yi = -52.30

yi = 52.30 m?

now?

Although you've plugged in correctly, the first line should read: Yf = Yi + Vyit - 1/2gt^2