Can someone answer this question for me about Newton’s third law?

some of the force is absorbed by the movement of grass and dirt.

You are correct. Grassy surface like golf course is much softer than pavement, the force back peaks at a lower amount but is over a longer period. It is difficult to bounce off grass as quickly as you can on a harder surface. You may be trying to push hard on it, but it mushes down under your foot.

donairs wrote:

You are correct. Grassy surface like golf course is much softer than pavement, the force back peaks at a lower amount but is over a longer period. It is difficult to bounce off grass as quickly as you can on a harder surface. You may be trying to push hard on it, but it mushes down under your foot.

Thank you.

So the opposite force is equal because force is absorbed on initial contact?

Like the guy said the force of impact is spread over a longer time period than it would be on concrete.

Freedumb wrote:

Like the guy said the force of impact is spread over a longer time period than it would be on concrete.

I do understand the nature of the force and how that differs as the foot strikes the ground.

I might seem dumb in asking the question, but I am trying to clear this in my head that an equal force will be delivered back from the object (in this case the ground).

In my head the only way that works is the force is absorbed in both directions. I think that’s the answer but I am clarifying it with the physics people on this board. I want to make sure there is nothing else going on?

newton confused wrote:

I have a question. Maybe I know the answer already but I am not sure?

If someone lands with a force on pavement when running and an opposite equal force is made back then how does that relate to running on grass?

Is there less force being applied into the grass compared to pavement and that is why there less force applied back making the athlete travel slower?

It's not really less Force being applied to the grass it's the fact that the grass and the dirt and the ground moves a little bit more in that situation. Basically making the whole experience less efficient

Newton's third law is for every action there is an equal and opposite reaction

When you are running on a hard surface like concrete the concrete does not give very much.

When you are running on soft surfaces like grass the grass and the mud and the dirt might give quite a bit

So when you are running on concrete most of the force is absorbed back by your body

Butt on grass some of the force is going to be put into moving the grass and displacing some of the surface

That efficiency will in turn Tire the runner out and they'll probably run with less Force but I think it's more about the efficiency of the running. And the fact that some of the forces being wasted in some sense, by displacing the grass in the mud and moving all that stuff around

Newton's 3rd law applies imperfectly for real world running because it's not a "perfectly elastic" impact. If your body was perfectly rigid and hit a perfectly rigid surface, then the law would work. We're not billiard balls bouncing off of slate.

If we were running in mushy waterlogged sand, all our impact force would be spread out in different directions, and the portion that we end up pushing on to pop back up would be, maybe, only 5% of the force we impressed into the ground at 'impact'.

This all comes into play when using performance enhancing shoes. If your weight is too heavy, or your bracing strength and neuromuscular coordination aren't just right, you won't get the performance boost that others get. Training with those shoes is important. Your coordination has to be trained and adjusted to the type of spring-back action offered by the shoes ...actually any shoe. Likewise, the running surface.

Maybe I am not asking for a dumb clarification because most are not even understanding what I am even asking. Maybe it’s my fault for not being clear about it.

I am looking for a response from a physics vantage point. You can say that Newton’s 3rd law does not apply. It has to. It is not avoidable. You can be wearing two cinder blocks on your feet and this would happen.

When you jump up in the air and land, even if you come to a dead stop you still have to deal with the opposite equal force.

So what I am attempting to clarify from someone that understands physics (not from a biomechanic view) since force applied has to equal the returned force.

Please tell me about the returned force from the ground and why that is the same as the force applied when running in different surfaces. Like grass, sand, pavement, concrete, etc...?

Your mass and the acceleration of gravity remain constant. That's why the force returned from any given running surface (assuming flat terrain) remains the same as the force applied at toe off.

newton confused wrote:

I have a question. Maybe I know the answer already but I am not sure?

If someone lands with a force on pavement when running and an opposite equal force is made back then how does that relate to running on grass?

Is there less force being applied into the grass compared to pavement and that is why there less force applied back making the athlete travel slower?

Softer surfaces distribute the force in a more spread out manner meaning the force is being spread out further through the ground. On pavement and a harder surface it's usually more focused on one point providing you more of a boost. That and the friction of the grass.

On a softer surface, you are running slower, so the force is less.

Newtons 3rd law was abolished about 6 years ago as it was too difficult to enforce.

Newton's 3rd law states that the amount of force you put into the ground is the same as the ground puts on you. You can split it into two vectors for the thought experiment.

The normal force, perpendicular to the earth, is close to the same on concrete or dirt.

The parallel force, friction, is lower on dirt than on concrete, meaning you get a little less forward motion each step.

You spend less energy on forward motion on dirt, but about the same in up and down motion, so it's less efficient.

Look at it this way: ground can hit you only as hard as you hit the ground. It does not matter hw hard you try, it is all about result at the moment of impact.

are you running at the same pace? or same effort?

look at it from the law of conservation of momentum.

newton confused wrote:

Please tell me about the returned force from the ground and why that is the same as the force applied when running in different surfaces. Like grass, sand, pavement, concrete, etc...?

Perhaps its easiest to think of force as a vector with both a magnitude and direction, further complicated by realizing that force is not just one single force as such, but the integrated directed sum of many forces, not all of which remain kinetic.

In the case of a rigid surface like concrete, there is a large force in the downward direction, and a nearly equal force in the opposite direction returning back to the foot.

In a less rigid surface like grass or sand, some of the force applied by the foot changes direction as it displaces some of the dirt or sand under the foot, and the direction of the equal and opposite reactive forces are the opposite of all these newly redirected forces, some of them now missing the foot, but instead pushing up the dirt that was outside the foot.

This is further complicated by other forces which exists, such as frictional forces (e.g. energy is lost when you exceed the coefficient of static friction and slip on wet grass or even smooth but wet concrete), and the conversion of some of the kinetic forces into heat.

When your shoe hits the ground with the typical heel strike, the impact force spikes from zero to a peak at 50 milliseconds, drops slightly, and then climbs to a new peak between 100 and 250 milliseconds. In order to provide a rebound effect, the landing surface must return the energy before the shoe leaves the ground AND before the shoe leaves the optimal position to receive the energy.

When you run on solid rock, steel, or concrete, there is virtually no give, so almost all of that energy is immediately transferred back into your shoe.

When you land on grass or sand or soft dirt, the landing surface compresses and absorbs the energy, leaving a divot in the sand and/or pushing the sand outwards so the energy is not returned to the shoe. Instead, the energy is used to move the sand.

When you land on hard dirt, the energy is mostly returned, but a little slower, which results in the shoe being out of the optimal position to capture the return of available energy.

In a perfect performance shoe, the energy from the impact on concrete would be captured in the foam/carbon plate and transferred back to your foot at the optimal time to provide a rebound effect.

In most training shoes, the foam doesn't expand fast enough after the impact to provide a rebound effect. Your foot is already off the ground or past the point where it can get a rebound effect.

tl;dr Part of the energy is absorbed by the dirt/grass. The energy that's returned comes back too slow to provide a rebound effect because the shoe has moved out of position to receive it.

The action creates an equal and opposite reaction but that doesn't mean it's identical in how it plays out.

When your heel (if you run this way) hits the ground there is an immediate force felt back even if it does not propel you. There is symmetry between two colliding objects so this can’t be it.