The Physics of a Kick

CD,

I have been thinking about how a team might model the mechanics of kicking a ball using highschool physics (or physics that could be taught quickly to FIRST students). If you want to take a shot at this problem on your own DO NOT CONTINUE READING POTENTIAL SPOILER ALERT. All I can come up with is the following:

  1. Using Conservation of Energy, determine speed of kicker on contact with ball. (Given the likely use of rotating components, Kinetic rotational energy must be understood)
  2. Using Conservation of Angular Momentum, determine the velocity* of the ball at the end of the collision. (Angular Momentum must be understood)
    *One problem I notice is accounting for the end velocity of the kicker (Would this be treated as some form of elastic collision?)
  3. Using Projectile Motion, figure out the characteristics of the balls flight.

This is definitely outside the bounds of the Ontario Grade 12 Physics Cirriculum, so my question is: can you come up with an easier model? I can also see some interesting other effects going on related to conservation of energy after the ball is released. This is a true test for young aspiring engineers to grapple with, so have fun! :smiley:

Modeling the physics of a kick is going to depend a lot on modeling the spin (or lack thereof) of a soccer ball. For those who have played soccer, think of the toe ball versus striking the ball with the laces of your shoe.

It seems to me that even those who have the physics experience to work this type of problem may not be able to do so until they know more of the variables. For instance, what is the weight and size of the soccer ball? How much is it inflated, and therefore what would be its coefficient of constitution? What is the friction between the soccer balls and the carpet? Of course someone with some serious skills in this area could set up the equation in terms of those variables, but we would still need to know this information eventually.

I’d take a crack at this, but this problem is too much for someone who hasn’t taken their first college physics class yet.

you could get extremely technical with this. First, you need to find out what type of gear motor ratio you will have. the motor specs will give you the max rpm, and from there, you would have to calculate the acceleration of the “foot” that kicks the ball. (I apologize i don’t know the formula). Once you have the acceleration, you could probably somehow measure the angle of acceleration (how high the ball flies as you kick it). From there, all you have to do (neglecting the air pressure of the ball, and the air resistance) is find out how heavy the soccer ball is and you could measure everything else from there. Sorry I don’t know the equations for the acceleration of the motor and angle of acceleration, but I’m only in high school myself :slight_smile: haha

No kidding haha I’m trying to calculate the drag force of a model rocket, and it’s kickin my butt lol :slight_smile:

I’m sure someone has found the coefficient of restitution for a soccer ball. That might help you out for a back of the napkin approach to the physics if you want to account for an inelastic collision between the ball and the kicker.

In a conservation of momentum equation, be sure to consider the momentum from the kicker that isn’t imparted to the ball, instead being released into a collision with the robot frame.

From my rough calculations, the kicker would need to operate at 110 ft/s :eek: and would require 188.4 lbs of force, this could be done with a 1.7 foot long cylinder at 60 psi and would shoot the ball 60 feet (about the diagonal length of the court) for a corner kick. Can someone please check these numbers?

We are not familiar with pneumatics. What and where can we get a pneumatic device for our kicker?

Team 1713

The simplest model I know is the ballistics model of parabolic flight. For a non-spiining ball, this has been pretty accurate (especially to the apex) for the foam poof balls, and for these soccer balls. It was quite inaccurate for the Moon-rocks as they had a ton more internal friction and much higher drag.

For ballasitics, you only neead a couple pieces of information.

Mass of the soccer-ball (410-450g)

Speed, Angle, distance, Apex (maximum).


I did the math, but cut it out as I am not sure if the Original Poster wants the Math or not. If there is demand, I will post an example.

Here’s a good article on the trajectory analysis of a soccer ball:

I found it helpful.

This post is long, I apologize. If you are interested in this topic, I believe my post is worth the time it will take to read it.

Something I haven’t seen posted on here so far: Breaking the forces up into components. For this problem, it is much easier to understand if it is broken into vertical and horizontal components. If you can get an accurate reading of its horizontal speed, this speed is constant throughout the flight.(we are neglecting air resistance) Also, if you know the height the ball reaches you can find the initial vertical speed of the ball. This vertical speed won’t be affected by the carpet because the carpet only affects the horizontal component.(directly) Now, once we have this…we can model its flight from the moment it leaves the ground to the moment it lands and the energy within the ball during this time. However, to model the actual kick…there is more to it. It really depends on how the kicker is designed, but I’m going with a leg with one joint. If you can find the speed it is rotating with, you can find the energy within the kicker upon impact. However, to do this math you must understand rotational inertia and angular speed. To explain angular speed, I’d suggest a merry go round. Explain how the linear speed changes with distance from the middle, but the angular speed is constant. As for rotational inertia, that is a bit harder. Try getting an item with high rotational inertia and an item with low rotational inertia. Put them both on a wheel or something and let them spin them. If the inertias are different enough, they will feel the difference in resistance. Ideally, they should have the same mass so they don’t mistake the mass to being the sole cause of the increase in difficulty. Other then that, I’d just use a formula to calculate the actual rotational inertia of the kicker. Now, you can use the above data to find the energy in the kicker before the kick. This allows us to see the energy before the kick. We can now use the difference of energies(the ball after the kick, and the kicker before the kick) and we will see the energy lost due to the actual kick. Some of these losses were explained above in other posts. I’d go over them briefly with the student’s but not go into too much depth on the actual math behind them.

Now, with the above explanation they get a thorough understanding of explaining what you can’t understand by using what you can. Also, it teaches them rotational inertia by experiment(something my college never really did properly and is largely the reason it took so long for people to understand it). It also teaches them angular velocity which also confuses some if you don’t get a good visual. After they understand all of this, they might be able to come up with a way to increase the distance of the ball by increasing the energy within the kicker prior to the kick. For instance, do we want a kicker with high rotational inertia? If so, how can we increase it? If not, how can we decrease it?

I know my explanation above explains alot of concepts that they don’t go into fully in high school level physics. However, they are concepts that could be grasped by their age group with proper explanation and visuals. I chose these concepts because they are the concepts college kids sometimes don’t understand, and ensures that your students will benefit greatly from the experience. If nothing else, exposing them to these ideas will have a positive effect.

If anyone has questions, feel free to PM me. Also, if some part of this is wrong(I’m pretty sure all the concepts are there) please respond immediately.

The Pneumatics Manual will answer both of these questions and more. Find it listed under Section 8 of the Game Manual on the FIRST web site.

http://www.usfirst.org/roboticsprograms/frc/content.aspx?id=452