# How do you imagine momentum?

If you want to explain it to a layman (who belongs to the science stream ofc!!) how do you explain it? Also, what is momentum transfer when it comes to fluid mechanics?

• Imagine a very large rugby player running into you at a low speed - would likely knock you over. Imagine a child running into you at double the speed, would probably bounce off. The combination of weight and speed are what make up momentum. Aug 27 '21 at 6:33
• Ever played snooker, or pool? Aug 27 '21 at 8:13
• "I like to picture Momentum as a mischievious badger," Cal Naughton Jr. Aug 27 '21 at 15:20

@JonathonRSwift provided a good analogy for momentum using a rugby player or a child running at and into you. The rugby player would likely bowl you over while a child even moving at twice the speed would likely bounce off.

Another example would be a child hitting your hand with a (blow molded/hollow) plastic hammer compared to that of a real hammer being swung with the same velocity.

A simple answer for the momentum transfer of moving fluids could be that of incorporating an waterwheel at the surface of a river to pump water.

Where water is held back by a dam, a Pelton wheel (below) may turn a generator for electricity, also imagine Windmills for grinding grain.

Now imagine reversing the application and you have an outboard engine propelling a boat on the water, a submarine under the waves, or an airplane / jet airliner traveling high overhead.

Addendum: Thank you @Trioxidane for making the point about a Pelton wheel. The scoops are specifically designed to redirect the impacting water "backwards" as much as possible to maximize energy extraction from the moving fluid.

• Good answer. Only suggestion for improving the answer is that a Pelton wheel is a specific type of waterwheel. It has special scoops to project incoming water back, maximising the energy transfer. The one in the picture is a normal waterwheel. Despite that, the idea stays the same. Aug 27 '21 at 14:27

The mass has a fundamental property and that is like a battery that can be filled with the charge and become a container of energy, mass by virtue of its inertia can accept the work of a force F moving it and store that work by accelerating and becoming a container of energy.

Say a ball with mass m is moving horizontally in space with a velocity v. what do we mean by recognizing its momentum $$\ P=mv$$.

We mean this ball in motion has energy and can deliver work. How much work? Let's say we try to stop this ball by hitting a spring.

It will gradually slow down losing its momentum till it loses all its velocity.

If we add up the amount of decreasing momentum over the span of v going to zero (calculating the are of the triangle in the diagram) or integrating with respect to v we get $$E =1/2 mv^2$$

Meanwhile, all the energy of the ball has been transferred to the spring.

The same applies if the ball is a droplet of water.

Momentum is a way we can measure mass that is in motion. Any object that is moving has momentum.

The momentum $$p$$ of an object is equal to the mass $$m$$ times the velocity $$v$$.

$$\vec{p} = m \cdot \vec{v}$$

(As a consequence of Newton's laws,) Momentum is directly proportional to the force required to stop a moving object (or in general change its velocity).

Momentum transfer in a fluid is the study of :

• the motion of fluids (how do particles interact with each other) and
• the forces that produce these motions. (and also what are the forces are produced from those motions)