Monday, July 2, 2012

Your First Mechanics Lesson

Now that we’ve gotten through the definition of Force, we can start applying the physics to basic mechanics.
Way back when, when we were young and teachable, our first lesson in mechanics was the ‘block on and incline’.  When you place a block on an incline, three forces act on the block:  gravity, friction, and a normal force.

The basic question is when the block will begin to slide down the incline.  The block will begin to slide when gravity overcomes friction.
note: To simplify the equations, we first define the x-axis and the y-axis in accordance with the incline. (see diagram).

Next we look at the forces acting on the block:
Gravity:  As previously discussed (long ago), the force due to gravity is the weight of the object; the force acting on the block due to gravity.  This force acts directly downward.

Friction: Friction is the resistance to movement of a surface.  If you place your block on a rough surface, it less likely to slide than on a smooth surface.
Normal Force: A normal force is the resistance to gravity.  If you place your block on a table, it doesn’t fall into the table because the table applies a force to the block that equals the force due to gravity.

How does the block slide?
The block will slide when gravity overcomes friction.  In this case this means that the x-portion of the force due to gravity becomes larger than the x-portion of the force due to friction.

Notice that gravity acts on the block at an angle compared to the x and y axis. This means that part of the force acts in the x-axis and part in the y-axis.  The x portion of gravity balances the x portion of friction when the block doesn’t move.
The block’s potential to slide is base only in the x-axis so the normal force has no influence on the block’s sliding and can be ignored.  Only the x portion of gravity (gravity(x) in the diagram) and friction which acts only in the x-axis determine whether the block slides.

When the angle of the incline is sufficient to make gravity(x) exceed friction then the block will slide.

Is this it?
We’re nearing the end of basic physics.  Next we’ll apply the block on the incline principles to rotational movement …. Let’s face it, gymnastics isn’t about standing still, it’s about rotating.

Wednesday, January 18, 2012

Forces - Pushing and Pulling – Weight v. Mass

A force pushes or pulls a mass.  But, what exactly is a mass anyway?

Weight v. Mass

A force pushes or pulls a mass.  But, what exactly is a mass anyway?

Weigh yourself.  The scale tells you that you weigh 190 pounds (lbs).  After wondering how you got so heavy, a brilliant idea dawns on you.  You can lose weight by moving to Denver …  or if you’re already in Denver, try Bolivia, or … the Moon.

Mass is a constant measure of matter within an object.  Your mass remains constant wherever you go.

Weight is a measure of the force created on a mass by gravity.  This is why you lose weight by going to the Moon.


Force is an attribute of physical action or movement measured by the ability of the attribute to move an object.

Force = Mass x Acceleration

F = ma

In the case of weight, this formula can be written as

Weight = Mass x Acceleration due to Gravity

W = mg

Be aware that our discussions from here on will apply use "mass" rather than "weight."