# Need Help w/ Projectile Motion Problem

• ch3570r
In summary, the conversation discusses a problem involving a water gun being fired while being held horizontally at a height of 1.00 m above ground level and a child sliding down at a 45 degree incline at a constant speed of 2.00 m/s. The question is how far the water will travel horizontally when fired by the child. The known values and equations are listed, but the correct answer of 4.11m is not reached. The conversation then provides advice on using the initial velocity of the gun and sliding to find the answer. The final equation needed is d = Vox * t, where the time t is given.
ch3570r
I've spent the past two hours or so working on some homework problems, and now I'm stuck on one question:

"When a water gun is fired while being held horizontally at a height of 1.00 m above ground level, the water travels a horizontal distance of 5.00 m. A child, who is holding the same gun in a horizontal position, is also sliding down at a 45.0degree incline at a constant speed of 2.00 m/s. If the child fires the gun when it is 1.00 m above the ground and the water takes 0.329 s to reach the ground, how far will the water travel horizontally?"

The answer is 4.11m, but I don't know how to get that.

My knowns:

Y = 1
X = 5
45 degrees
Vx = 2m/s
t = .329(s)
Change in X = ??

Equations (▲ = change in : Ø = degree: G = Gravity 9.8m/s)
▲Y = Vi*sinØ-G*▲t
Vy = Vi*sinØ

▲x = Vi*cosØ-▲t
Vx = Vi*cosØ
Vi^2 = (g*▲x)/(2*sinØ*cosØ)

Anyone have any ideas?

Last edited:
Use what they tell you about the normal firing (squirting, really) of the gun to find the initial velocity given to the water by the gun.

Then notice that the child, and hence the gun, are moving with a velocity of 2m/s not in the x direction, but at a 45 degree angle.

So when the kid squirts the gun, the water gets an initial velocity both from the squirting mechanism of the gun and the sliding that's going on.

Does that help?

hopefully it helps, I'll try ur advice later (im busy at the moment)

thanks

well, that did help w/ better understanding the problem, but I still can't get 4.11m as an answer. It makes sense that it travels less, but I am having trouble w/ what equation(s) to use. Would the initial velocity be 2, because its a constant speed...or is it something else??

ch3570r said:
well, that did help w/ better understanding the problem, but I still can't get 4.11m as an answer. It makes sense that it travels less, but I am having trouble w/ what equation(s) to use. Would the initial velocity be 2, because its a constant speed...or is it something else??

As stated before, the initial velocity consists of two contributions: the initial velocity of the gun and the initial velocity of the sliding. So, the initial velocity in the x direction is Vox = Vox(sliding) + Vox(gun) = 2.0 * cos(45) + Vox(gun). You can calculate the initial velocity of the gun easily, since the height and horizontal distance are given.

So, the only equation you need now is d = Vox * t, where the time t is given.

thanks radou, that really helped me out

## 1. What is projectile motion?

Projectile motion is the motion of an object through the air or space under the influence of gravity. It is a type of motion that occurs when an object is thrown, kicked, or launched into the air. The path of a projectile is a curved trajectory due to the force of gravity acting on it.

## 2. How do you solve a projectile motion problem?

To solve a projectile motion problem, you need to break it down into two separate components: horizontal motion and vertical motion. You can then use the equations of motion, such as the kinematic equations, to determine the projectile's position, velocity, and acceleration at any given time.

## 3. What are the key factors that affect projectile motion?

The key factors that affect projectile motion are the initial velocity, angle of launch, and the effects of air resistance. The initial velocity determines the speed and direction of the projectile, while the angle of launch determines the trajectory. Air resistance can also affect the projectile's motion by slowing it down.

## 4. What are some common mistakes to avoid in projectile motion problems?

Some common mistakes to avoid in projectile motion problems include neglecting air resistance, forgetting to break the motion into horizontal and vertical components, and using the wrong equations. It is also important to pay attention to the units of measurement and use the correct values in your calculations.

## 5. How can projectile motion be applied in real life?

Projectile motion has many real-life applications, such as in sports like baseball, football, and golf. It is also used in engineering and design for objects like rockets, missiles, and projectiles. Understanding projectile motion is essential in fields such as physics, mathematics, and mechanics.

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