Projectile Motion: Calculating Speed of Ball Thrown from Third Floor Window

[Brainiac11]

<< Thread moved to the HH forums from the technical physics forums, so no HH Templatge is shown >>

So I have this question that says, "Someone at a third floor window (12 m above ground) hurls a ball downward at 45 degrees at a speed of 25.0 m/s. How fast will it be traveling when it strikes the sidewalk?"

I broke it up into horizontal and vertical components, but I don't know where to use them in solving for it. I tried to use the equation Yf=Yi+Vi(t)+(1/2)(a)(t)^2 but then I got confused. I feel like the question is asking me for an acceleration but I thought the acceleration in all projectile motion questions was -9.8 (gravity). So if I use the equation I was wanting to I would have put: Yf=0, Yi= 12, Vi=-25.0, and a=-9.8. However, that would have given me time which I don't think is what I'm being asked to find.

 

[brainpushups]

I think you should post this in the homework help forum, but I'll leave that for a moderator to relocate.

I broke it up into horizontal and vertical components, but I don't know where to use them in solving for it. I tried to use the equation Yf=Yi+Vi(t)+(1/2)(a)(t)^2 but then I got confused. I feel like the question is asking me for an acceleration but I thought the acceleration in all projectile motion questions was -9.8 (gravity).

I guess we should start with what the question is asking. According to the beginning of your post:

How fast will it be traveling when it strikes the sidewalk?"

So, out of the kinematic quantities (time, displacement/distance, velocity/speed, and acceleration) which are you being asked about?

 

[Brainiac11]

I think you should post this in the homework help forum, but I'll leave that for a moderator to relocate.
I guess we should start with what the question is asking. According to the beginning of your post:
So, out of the kinematic quantities (time, displacement/distance, velocity/speed, and acceleration) which are you being asked about?

Speed?

 

[brainpushups]

Indeed. Suppose I told you that an object had a velocity of

$<3m/s, 4m/s>$

(note that I am referring to components of a vector). What would be the speed of this object?

 

[billy_joule]

Speed?

Correct:

How fast will it be traveling when it strikes the sidewalk?"

You know gravity will act in the Y direction which will cause a downward acceleration.
What is the initial velocity in the Y direction? It isn't -25m/s as you've calculated, that would be true if the ball was thrown straight down but it isn't. it depends on the initial angle..
The equation you've used has time as the unknown, but are you interested in finding the time? You have an initial velocity, a distance and an
acceleration and you are looking for the final velocity. Is there is a SUVAT equation to do that?

What can you say about the velocity in the X direction? will it change? (I think it's safe to assume no air resistance acts)

 

[Brainiac11]

Correct:You know gravity will act in the Y direction which will cause a downward acceleration.
What is the initial velocity in the Y direction? It isn't -25m/s as you've calculated, that would be true if the ball was thrown straight down but it isn't. it depends on the initial angle..
The equation you've used has time as the unknown, but are you interested in finding the time? You have an initial velocity, a distance and an
acceleration and you are looking for the final velocity. Is there is a SUVAT equation to do that?

What can you say about the velocity in the X direction? will it change? (I think it's safe to assume no air resistance acts)

Yes! I got it! Thank you all!