# Derive a general formula for finding range with a spring launcher

## Homework Statement

So I made a spring launcher that will fire marbles. I need to derive a general formula for finding range for any given angle and x value.

## Homework Equations

2) Ek=Es
3) 1/2mv2=1/2kx2
4) d=v*t
This projectile was shot from 1.1m off of the ground.

## The Attempt at a Solution

I have no idea where to start.

Last edited:

Cyosis
Homework Helper
This is an experiment you're doing at home? The equations you've noted down do not take resistances into account. Marbles are relatively heavy though so you should be alright with these equations to some extend.

Using equation 3) you can calculate the velocity with which the marble leaves the spring, you can then split v up in its x and y components. When it leaves the spring it's trajectory will roughly be a parabola. The x-distance is given by your equation 4. The equation that is missing is the one that describes the motion in the y-direction. Can you come up with an equation for the y-direction.

hint: what is the only force on the object and in which direction does it point?

Yes it is an experiment I am doing at home and I am not too worried about resistance.
Ok, I am going to assume it is
v2=v1+at because I need time, and I don't have distance.
t=v1sinθ/a because v2 is 0 halfway up
so
t=2(vsinθ/a) still am not accounting for the 1.1m off the ground (not sure how to do that)

1/2mv2=1/2kx2
v=((kx2)/m)^1/2 but I only want the x speed so
v=(((kx2)/m)^1/2)cosθ

So
d=v*t
d= (((kx2)/m)^1/2)cosθ * 2(vsinθ/a)

Still does not accounting for the 1.1m off the ground

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rock.freak667
Homework Helper
For you to account for the 1.1m you will need to use the formula y=y0+uyt-1/2gt2

then to get the time for the entire motion, set y=0 and use the quadratic equation formula for t.

Cyosis
Homework Helper
To clear somethings up what is this range you want to find?

t=v1sinθ/a because v2 is 0 halfway up

Not true since you start at 1.1 meters and the projectile lands at 0 meters. The general kinematic equation you're looking for is $s=s_0+v_0t+\frac{1}{2}a t^2$.

Edit:kept the post open for way too long, not adding anything to rock's explanation.

I am looking for horizontal range.
My equation sheet has the formula
dy=v2t-1/2at2

what does y0/s0 stand for?

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rock.freak667
Homework Helper
I am looking for horizontal range.
My equation sheet has the formula
dy=v2t-1/2at2

what does y0/s0 stand for?

displacement.

There should also be a t in the formula for dy.

BUT your equation assumes you started with 0 distance (vertically since you said dy)

if s0 stands for displacement (it should be -1.1 then?), then I set s = 0 and do the quadratic?

EDIT: Also why is yours +1/2at^2 where as mine and cyosis is -1/2at^2 ?

rock.freak667
Homework Helper

if s0 stands for displacement (it should be -1.1 then?), then I set s = 0 and do the quadratic?

well it can be -1.1 but we took the origin to be where the marble will land, 1.1m below where it was launched.

EDIT: Also why is yours +1/2at^2 where as mine and cyosis is -1/2at^2 ?

well the general form is

s=s0+ut+1/2at2

Cyosis put a - sign because the acceleration is due to gravity which acts downwards, which is usually taken as -ve.

Ok so I did:
dy=v1yt+1/2ayt2
-1.1=v1yt-4.905t2 9.81 for g here.
4.905t2-v1yt-1.1

t=-v1y+/-(((-v1y)2-4(4.905)(-1.1))^.5)/2(4.905)
t=-vsinθ+(((-vsinθ)2+21528)^.5)/9.81 (has to be positive or time will be negative)

then from before:
1/2mv2=1/2kx2
v=((kx2)/m)^1/2 but I only want the x speed so
vx=(((kx2)/m)^1/2)cosθ

now put those both in dx=vx*t

dx=(((kx2)/m)^1/2)cosθ * (-vsinθ+(((vsinθ)2+21.582)^.5)/9.81)

dx=(((kx2)/m)^1/2)cosθ * (-(((kx2)/m)^1/2)sinθ+(((((kx2)/m)sinθ)+21.582)^.5)/9.81)

not sure if I can simplify further or if I made a mistake.

Last edited:
rock.freak667
Homework Helper
You should get

$$t= \frac{vsin\theta+\sqrt{v^2 sin^2 \theta-4(\frac{1}{2}g)(-1.1)}}{g}$$

then put that into dx=(vcosθ)t

(sin2θ=2sinθcosθ)

Can you show all your work on how you got t, because I keep getting a negative in front.

also fixed my final t line.

rock.freak667
Homework Helper
Can you show all your work on how you got t, because I keep getting a negative in front.

also fixed my final t line.

$$-1.1=(vsin\theta)t-\frac{1}{2}gt^2$$

$$\Rightarrow \frac{1}{2}gt^2-(vsin\theta)t-1.1=0$$

$$a= \frac{1}{2}gt^2 \ b= -vsin\theta \ c=-1.1$$

$$t = \frac{-b \pm \sqrt{b^2-4ac}}{2a}$$

Thanks
I am going to assume here g = 9.81 not -9.81 or else the inside of the root could fail.