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

• Brainiac11
In summary, the conversation discusses a physics question about determining the final velocity of a ball thrown from a third floor window at a 45 degree angle with an initial speed of 25.0 m/s. The question is moved to a different forum, and the participants discuss breaking the question into horizontal and vertical components and using the equation Yf=Yi+Vi(t)+(1/2)(a)(t)^2. They determine that the question is asking for the speed and not the acceleration, and they also consider the effects of gravity and air resistance on the velocity in the X direction. Eventually, the question is answered correctly.
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.

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

Brainiac11 said:
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).

Brainiac11 said:
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?

brainpushups said:
I think you should post this in the homework help forum, but I'll leave that for a moderator to relocate.
So, out of the kinematic quantities (time, displacement/distance, velocity/speed, and acceleration) which are you being asked about?
Speed?

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?

Brainiac11 said:
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)

billy_joule said:
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!

billy_joule

## What is projectile motion?

Projectile motion is the motion of an object through the air that is subject only to the acceleration of gravity. This type of motion is often observed in objects such as thrown balls, launched rockets, and falling objects.

## What factors affect the trajectory of a thrown ball?

The trajectory of a thrown ball is affected by the initial velocity, the angle at which the ball is thrown, and the force of gravity. Other factors such as air resistance and wind can also have an impact on the trajectory.

## How does the angle at which a ball is thrown affect its range?

The angle at which a ball is thrown can greatly affect its range. For example, throwing a ball at a 45 degree angle will result in the maximum range, while throwing it at a lower or higher angle will result in a shorter range. This is because a 45 degree angle allows for the maximum combination of horizontal and vertical components of motion.

## What is the formula for calculating the range of a projectile?

The formula for calculating the range of a projectile is R = (V^2 * sin(2θ))/g, where R is the range, V is the initial velocity, θ is the angle at which the ball is thrown, and g is the acceleration due to gravity.

## How does air resistance affect the trajectory of a thrown ball?

Air resistance, also known as drag, can have a significant impact on the trajectory of a thrown ball. As the ball moves through the air, it experiences a force in the opposite direction of its motion due to air resistance. This force can cause the ball to slow down and deviate from its expected path. The amount of air resistance depends on the size, shape, and speed of the ball, as well as the density of the air.

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