Momentum

If something heavy and fast hits you, it's really gonna suck.

Momentum can be defined as "mass in motion." Under Newtonian mechanics, you'll see the formula:
momentum = mass * velocity

To put it simply:

• If something heavy hits you, it's gonna suck.
• If something fast hits you, it's gonna suck.
• If something heavy and fast hits you, it's really gonna suck.

If you've ever had a heavyweight attempt a flying scissor takedown on you, you understand the danger of momentum.

Momentum is related to inertia. An object at rest must overcome inertia to achieve momentum. Similarly, you must overcome inertia to stop an object with momentum.

Momentum and force:

The formula for force is a bit different:
force = mass * acceleration

In other words:

• momentum is about how much mass is in motion, and
• force is about how fast that mass is picking up speed.

To stop momentum, you need to generate force against it. The more force you can generate against the momentum, the faster you can stop it.

Momentum and Jiu-Jitsu:

Momentum is an important consideration in Jiu-Jitsu because it illustrates the advantages that come from size. If two people are moving the same speed, the bigger person will have more momentum. And if someone a lot bigger than you has momentum, it's probably a bad idea to try stopping their momentum directly.

That said, momentum also teaches us that speed matters too! Being fast can be just as dangerous as being big.

If you're unable to generate enough force to stop someone's momentum directly, the best strategy is to get out of the way and redirect them. Redirection of momentum is the key idea behind most Judo throws.

It's important to note that momentum causes injuries. Too much mass at motion can lead to a trip to the emergency room. Unless you're experienced and have very good body control, relying on momentum is probably dangerous.

Further study:

• Momentum — Wikipedia
• Momentum — The Physics Classroom
• Inertia vs. Momentum: Which Keeps You Moving? — Dictionary.com
• Inertia