If a steel ball hits an iron surface it bounces back due to the Newton's third law of motion. Then why does'nt it bounce back when it hits a water surface? or for that matter even air? The molecules of water / air should apply the force of equal magnitude on the ball as the ball applies to the water molecules. Can anyone please explain this?

# [Physics] Violation of Newton’s third law in case of fluids

collisionconservation-lawsmomentumnewtonian-mechanicswater

## Best Answer

The third law of motion:

Edit after revisit:The basic misunderstanding here is in the notion of force. Acceleration for impacts will define the force, and it is best to think of the

dp/dt definition of force.An incoming ball does not have 10 Newton's force , it has a constant velocity, so no force .Force is generated on impact by dp/dt.Impacts with solids distribute momentum and energy on the whole solid due to the cohesiveness of the electromagnetic interactions, and the total momentum energy balance can be calculated with the impact point , and dp/dt calculated to give the force of impact.Impact on air is coming in tiny scatters and tiny dp/dt. Impact on water is similar if the ball has enough energy, molecules bounce left and right with a small dp/dt slowly dissipating the energy and dispersing the momentum.

end editYou ask :

Bouncing back depends on elasticity of surfaces deformations and dissipative motions of the surfaces which absorb the energy and momentum of impact , and have to be estimated for each individual case.

Have you estimated the force that the steel ball exerts on the water molecules? dp/dt will transfer momentum to all the column of water it penetrates and dissipate the energy until it hits the bottom. At the surface little except ripples because the liquid is displace-able with little energy/momentum needs . Look at the video for a drop falling, which has the same density as the water and small energy/momentum to transfer to see how the third law is obeyed by dissipating the impulse and still a small drop bounces back.

Air is a non thinker: individual molecules have very small mass with respect to the ball and pick up the tiniest fraction from the impact diminishing the motion of the ball very slightly at best.

One has to do the calculations to see that the third law is OK ? It is called a law because innumerable people have done the calculations and found that the law is validly describing data.