# Work on a Football: Energy Transfer and Distance

I have some doubts concerning work, here are some examples I wanted to clear out:

So, when I kick a football, I am doing work on it. In this example the ball will be in contact with the ground throughout. It will slide over the floor.

The Energy Transfer: Chemical Energy from my muscles to Kinetic Energy of the ball.

And the distance through which the force is applied: Will be only the very short moment that my foot is in contact with the ball?

But what is the force against the work being done here? Friction at the moment of contact and application of force? And that would also be for a split second right? Because as the ball continued to roll it would still experience friction but no work is being done on it anymore. And can air resistance also be considered a counter acting force?

Now, I know how lifting a book works but I was wondering. What if I kick a ball upwards? Again, I’m assuming the force and the distance application is only during the moment of contact but my doubt is: In order to do work against gravity, am I doing work on the ball? Or did I do work on it, granting the ball Kinetic Energy and the football used its Kinetic Energy to do work against gravity?

And finally, can energy be transferred to different objects but in the same form? For example, when we ski down a mountain the skis deplete snow to the side. We posses kinetic energy and it is used to push the snow over a distance, giving it movement energy, or when we are playing pool and one ball hits the other, kinetic energy to kinetic energy again?

Sorry for the long text and I hope I made my doubts clear.

PeterG

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Delphi51
Homework Helper
But what is the force against the work being done here? friction at the moment of contact and application of force? And that would also be for a split second right?
All true, but this misses the main effect of accelerating the ball.
F=ma. F pushes on the ball causing a; the ball pushes back on the foot with an equal and opposite force.

do work on it, granting the ball Kinetic Energy and the football used its Kinetic Energy to do work against gravity?
Yes to this! Same story with the F = ma.

Yes to the kinetic energy transfer from one object to another.

Ok cool! Thanks again.

And one last thing. During the energy transfers, some energy is lost during the process right?

I thought of two examples to check my understanding:

When we use the energy in food to give Kinetic Energy to a cart as we push it, some energy is lost as heat as energy is released from food.

Or when we are playing pool and one ball hits another. The ball with kinetic energy uses its mechanical energy to do work on the other pool ball but upon contact energy is lost in the form of sound and probably heat.

Thanks,
Peter G

Delphi51
Homework Helper
Yes. Some would say energy is "converted to heat energy" rather than saying it is "lost".

Ok, thanks Delphi

Sorry, I just couldn't get this out of my mind. I'm used to these common examples in books but I just wanted to clear out this last one, I promise

So, when we walk/run, do we do work too? Like against the force of friction and air resistance?

Measuring the distance is easy and we could also find the force using F = ma, but the F is the resultant force so we would need the air resistance to discover the force my legs were exerting I think.

Thanks!

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Delphi51
Homework Helper
These questions are tricky and depend on the wording. We don't "accomplish" any work when moving ourselves or another mass on level ground - the mass has no more energy after the move than it had before. However, work is still done against friction. Even running on the spot converts some biological energy into heat because muscles are not 100% efficient. Even if the person doesn't move, energy is used to keep him living.