# Action Reaction on a discus thrower.

• imanator
In summary, the conversation discusses the physics involved in a discus throw, specifically the action-reaction forces on the thrower's feet and the simplified problem of a spinning arm on a pivot with a mass at the end. It is mentioned that in the real discus throw, the thrower runs in a circle to increase their angular velocity before throwing. The conversation also explores the concept of momentum conservation in a simplified scenario where two spinning arms with masses at the end are released in the same direction, and whether the platform they are attached to will move as a result.

#### imanator

Hi there,
I am trying to understand the physics acting on a discus thrower, namely at their feet when they throw a discuss.
first the player spins to pick up momentum for the discus, he or she does this by pushing on the ground but when the actual discus leaves the hand, is there an action reaction? isn't the discus just leaving their hand? if you were to simplify the problem to just a spinning arm on a pivot with a mass at the end, when the mass leaves the arm, does it push back on the arm, does the pivot feel the tangential force back?
In other words if the discus arm was in space or on water, assuming you could spin it up, when you release it, would the arm be flung back? would the displacement of the pivot change? not in a circle but linearly?

imanator said:
if you were to simplify the problem to just a spinning arm on a pivot with a mass at the end, when the mass leaves the arm, does it push back on the arm, does the pivot feel the tangential force back?

For that simplified problem, there is no tangential force. But you have made the problem too simple. You don't just "let go" of the discus, you "throw" it.

Think about some simpler situations first:

1. What are the action reaction forces if you stand still, and throw the discus?

2. What if you run forward in a straight line, and throw the discus while you are running?

In the real discus throw, you "run" in a circle to increase your angular velocity before you throw.

Thanks Sci Advisor. I am interested in this simplification.
Suppose I had a platform in space, with this apparatus(spinning arm on a pivot and a mass on the end) bolted on to the platform, and that I put another similar apparatus above it spinning in the opposite direction so that the platform has no net rotation in space. Now if I release the masses so that they both travel in the same direction, will the platform move when the balls are released? is there a reaction that the platform experiences?

when I say does the platform move, I mean a net movement, clearly the platform will oscillate while it is spinning.

I don't think the spinning matters. When you "throw" from a platform in space, whether by fling, by compressed air, or by a spring, momentum is conserved. m*v of projectiles and m*v of platform are equal and opposite. The same basic thing is true for the discuss thrower in spite of all the pre and post throw motion.

Thanks, so are you saying the platform will move back, or the momentum is conserved in the spinning, and the platform stays where it is?

## 1. How does the action of the discus thrower affect the reaction of the discus?

The action of the discus thrower, or the force applied to the discus, directly affects the reaction of the discus. The force creates a change in motion and direction of the discus, causing it to move through the air and eventually land in a desired location.

## 2. What factors influence the action and reaction in a discus throw?

The action and reaction in a discus throw are influenced by several factors including the force applied by the thrower, the angle at which the discus is released, the weight and shape of the discus, and external factors such as wind and air resistance.

## 3. How does Newton's Third Law of Motion apply to a discus throw?

Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction. This applies to a discus throw as the force applied by the thrower creates an equal and opposite force on the discus, causing it to move in the opposite direction.

## 4. What are the potential energy transformations involved in a discus throw?

The potential energy transformations involved in a discus throw include the conversion of chemical energy from the thrower's muscles into kinetic energy as they release the discus. There is also a transformation of potential energy into kinetic energy as the discus moves through the air.

## 5. How does the discus' diameter and weight affect the action and reaction in a discus throw?

The diameter and weight of the discus play a significant role in the action and reaction of a discus throw. A larger diameter and heavier discus will require more force to be thrown and will also create a greater reaction force. This is because the discus has a greater mass, so it has more inertia and is harder to accelerate.