Title: The title should actually read Finding the acceleration of a piston with respect to a cranks angle 1. The problem statement, all variables and given/known data I have found a formula for calculating the acceleration of a piston, with respect to a cranks angle, however I've also found a couple of online calculators that give this result - my problem is that the results are completely different, and I'm not sure which is correct - if either. 2. Relevant equations I am using the formula found on this page: http://www.codecogs.com/reference/physics/kinematics/velocity_and_acceleration_of_a_piston.php On the right hand side I have the formula for acceleration: [itex]a = -rw^2(cos\theta+(cos2\theta/n)[/itex] where: [itex]r[/itex] = crank radius [itex]l[/itex] = rod length [itex]\theta[/itex] = crank angle [itex]w[/itex] = crank angular velocity [itex]n[/itex] = [itex]l/r[/itex] The values I have got are as follows: [itex]r[/itex] = 4 (cm) [itex]l[/itex] = 15 (cm) [itex]\theta[/itex] = 1 (rad) [itex]w[/itex] = 50 (rad/s) [itex]n[/itex] = 3.75 Putting all of this into the formula I should get: [itex]A = cos\theta = 0.5403[/itex] [itex]B = cos2\theta = -0.4161[/itex] [itex]C = -rw^2 = - 4 x (50^2) = -10000 [/itex] [itex]C x ( A + ( B / 3.75 ) ) = -4293.4[/itex] So, the acceleration of the piston at an angle of 50 rads is -4293.4 cm/s, which equates to -4.293 m/s, which I'm guessing means it is going backwards? There are a couple of problems here. It seems after playing with the values within a spreadsheet, the minimum acceleration is approx -12.6666, yet the maximum acceleration is 7.3541, why such a difference, surely the maximum acceleration to be equal in both directions? In comparison, an online calculator I found here (http://www.bigboyzcycles.com/PistonSpeed.htm, when given the following data: Stroke (inches): [itex] ( r x 2 ) / 2.54 = 3.1496 [/itex] Crankshaft Speed (RPMs): [itex] w / 0.104719755 = 477.465 [/itex] Connecting rod length (inches): [itex] l / 2.54 = 5.905511 [/itex] Gives 416 ft/s, which equates to 126m/s... So, for reasons unknown to me I'm getting a huge difference between these calculations. Neither one is giving me much confidence. Thank you.