HTML and CSS Reference
In-Depth Information
Mass
The earlier chapters of the topic covered several aspects of motion: velocity, acceleration, vectors, friction,
bouncing, easing, springing, and gravity. Until now, we have ignored the concept of an object's mass when
being moved. Now, scientifically speaking, mass should have been in the equation, but we've generally
concentrated on doing things
mostly
correctly, and kept the emphasis on making sure it
looks
right. Most
important, the final result must be efficient enough so that the web browser can run smoothly in the
process. However, mass is so tied up in the subject of momentum that we can no longer ignore it.
So just what is mass? Here on Earth, we usually think of mass as how much something weighs. And that's
pretty close, as weight is proportional to mass. The more mass something has, the more it weighs. In fact,
we use the same terms to measure mass and weight: kilograms, pounds, and so on. But technically
speaking, mass is the measurement of how much an object resists change in velocity. Thus, the more
mass an object has, the harder it is to move that object or to change how that object moves (slow it down,
speed it up, or change its direction).
This also relates to acceleration and force. The more mass something has, the more force you need to
apply to it to produce a given acceleration. This is expressed in the equation:
F = m × a
For example, the engine in a compact car is designed to produce enough force to provide reasonable
acceleration on the mass of a compact car; but it's not going to produce enough force to accelerate a large
truck. The engine needs a lot more force, because the truck has a lot more mass.
Momentum
Now we move on to momentum, which is the product of an object's mass and velocity. In other words,
mass
times
velocity. Momentum is indicated by the letter p, and mass by m, and is expressed as:
p = m × v
This means that an object with a small mass and high velocity can have similar momentum to an object
with a large mass and low velocity. If the aforementioned truck moving at a mere 20 miles an hour, or a
bullet with a tiny mass but a much higher velocity, collided with you, they'd both ruin your day. Here, you
can see how two objects with a different mass and velocity can have an equal momentum (where m/s is
meters per second):
4 kg × 15 m/s = 20 kg × 3 m/s = 60 kg m/s
Using the formula, a 4 kg ball rolling down a hill at 15 m/s has a momentum of 60 kg m/s. Because velocity
is a vector (direction and magnitude), momentum must also be a vector. The direction of the momentum
vector is the same as the direction of the velocity vector. Thus, to fully describe momentum, you express it
like this:
4 kg × 15 m/s at 23 degrees
With this background knowledge, you see next how you can apply this to collisions.
