# Minimizing Speed to Reach a Distance H: A Math Challenge

• tomfrank
In summary, the goal is to hit a target in the x-direction, starting from a height in the y-direction, and minimize the speed. The equations for dx and dy are set equal to each other and solved for time to find the angle and speed. If you want to calculate your process you have to change your equations for dx and dy a bit and then set them to your final coordinates. To get the smallest and largest angle you have to solve your resulting equation for v in terms of theta. Then you will be able to read of values of theta beyond which no solution exists. To minimize the speed you should differentiate w.r.t. theta and set it to 0. So i correct the equation
tomfrank

## Homework Statement

So the goal is to reach a distance H in the x-direction, starting from a height H in the y-direction, and you need to minimize the speed, and find the smalest and angle.

## Homework Equations

i did:

dx = V cos(theta) *t and
y = y_0 +V sin(theta)*t-1/2*g*t^2

Vx = Vcos(theta) Vy = Vsin(theta)

## The Attempt at a Solution

i set them equal to each other and solve for time

but I am not quite sure and the minimization part.
do I have to take the derivative with respect of theta and set it equal to 0?

Last edited:
There is a t missing in your formula for dx.

If you equate the two you will get the angle and speed when you hit the target as you are considering the reverse process this way.

If you want to calculate your process you have to change your equations for dx and dy a bit and then set them to your final coordinates.

To get the smallest and largest angle you have to solve your resulting equation for v in terms of theta. Then you will be able to read of values of theta beyond which no solution exists.
To minimize the speed you should differentiate w.r.t. theta and set it to 0.

so i correct the equation put it in the t.

By solving for V i got

V = (1/2)*g*t/(cos(theta)-sin(theta))

if I differentiate w.r.t. theta and set it = to 0 I got theta = -pi/4

is that right, how do i get the speed?

You have two equations. Use one to eliminate t and solve the second for v. There should be only one v in your first formulas. You have
$$v_x=v\cos\theta$$ so you included the cos doubly.

Last edited:
so just to understand, I solve for t and for the x equation and plug into the other one, so that I can eliminate t and I got.

v = sqrt((1/2)*sqrt(2)*sqrt((2*sin(theta)^4-2+3*cos(theta)^2)*cos(theta)*h*g*(sin(theta)+cos(theta)))/(2*sin(theta)^4-2+3*cos(theta)^2))

now I take the derivative w.r.t. theta and set it = to zero.

I try it but how don't get a value for V?

Your formula for y is still not correct. With increasing time your projectile would go higher and higer, a bit unrealistic.
At t=0. You should have y=H. This is the initial condition.

Yes. Now you can set for y and dx the coordinates of your target. This will give you two equations.
Then you can use one to eliminate t and the second to solve for v.
More from my side tomorrow if you still need help.

i try but i don't really know how to get the value of V out.

What are your two equations? Can you solve them for t?
Do you get an expression for v depending on theta?

If you post your calculations I can check for errors and tell you what to try next.

## 1. How do I calculate the minimum speed needed to reach a distance H?

To calculate the minimum speed needed to reach a distance H, you can use the formula v = √(2ad), where v is the velocity, a is the acceleration, and d is the distance. Plug in the values for a and d, and solve for v.

## 2. What units should I use for the distance and speed?

The distance should be in meters (m) and the speed should be in meters per second (m/s). It is important to use consistent units throughout your calculations.

## 3. Can I use this formula for any distance?

Yes, this formula can be used for any distance. However, it is important to note that the distance and speed should be realistic and relevant to the situation you are trying to solve.

## 4. How accurate is this calculation?

The accuracy of the calculation depends on the accuracy of the values used for acceleration and distance. If these values are precise and relevant to the situation, the calculation should be accurate.

## 5. Are there any other factors that should be considered when minimizing speed to reach a distance H?

Yes, there are other factors that may affect the minimum speed needed to reach a distance H, such as air resistance, friction, and the mass of the object. These factors may require more complex calculations and should be taken into account for a more accurate result.

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