Calling a method when a slider is moved

As already pointed out in solution 1, simply move the relevant portions of the code to another method and call that.

Few other observations based on the snippet you posted
- You correctly use TryParse to convert the values from text boxes. However, you don't investigate if the parsing fails. TryParse returns a boolean so you should check if it is true or false and act based on that
- It looks like you have a class level variable (potentialEnergy) that you use to transfer data from the method (Calculate). If that is correct, it's considered as a bad habit. Instead, define the Calculate method so that it returns the answer as a return value

Pseudo example
==============

public void btnCalculate_Click(object sender, EventArgs e) {
    DoCalculation();
}

private void trackBar1_Scroll(object sender, EventArgs e)
{
    txtHeight.Text=trackBar1.Value;
    DoCalculation();
}

public void DoCalculation() {
    double mass;
    double height;
    double gravity;
    bool ok = true;

    ok &= double.TryParse(txtMass.Text, out mass);//mass in kg
    ok &= double.TryParse(txtHeight.Text, out height);//height in meters
    ok &= double.TryParse(txtGravity.Text, out gravity);//gravity 9.81 default

    if (!ok) {
       // some error message goes here
       return;
    }
    txtAns.Text = Calculate(mass, height, gravity);
}

public double Calculate(double mass, double height, double gravity) {
   // do the calculation
   return result;
}

Interesting that you chose the Scroll event, rather than the ValueChanged event; but, they both do the same thing ... Scroll will be called first if you handle both events (but, why would you ?). Why MS defined two identical events; who knows :)

One suggestion; if you call your calculation functions continuously ... every time the slider moves ... you are going to get a lot of calls which may bot be what the user wanted, and may lead to refresh issues.

Consider calling your calculation code when the Leave event of the trackbar occurs ? Note that just clicking on the Form will not trigger the Leave event: you have to select some other Control.

Sample code:

private void trackBar1_ValueChanged(object sender, EventArgs e)
{
    // deliberately not implemented
}

// see how many slider moves you are NOT handling ?
private void trackBar1_Scroll(object sender, EventArgs e)
{
    Console.WriteLine($"scrolling: {trackBar1.Value}");
}

// value on entry
int entryValue;

private void trackBar1_Enter(object sender, EventArgs e)
{
    entryValue = trackBar1.Value;

    Console.WriteLine($"enter: {trackBar1.Value}");
}

// Leave event
private void trackBar1_Leave(object sender, EventArgs e)
{
    Console.WriteLine($"leave: {trackBar1.Value}");

    if (trackBar1.Value == entryValue) return;

    // do calculations
    // Calculate(trackbar1.Value);
}

// if the user presses Enter, force Leave
// you must set Focus to another Control on the Form
private void trackBar1_KeyDown(object sender, KeyEventArgs e)
{
    if (e.KeyCode == Keys.Enter) ActiveControl = button1;
}