Newton’s Apple


Matthew Morrisby: Matthew Morris

Newton was a revolutionary thinker of his time. He is responsible for the three laws of motion that we still use today;

1. Objects that are not in motion remain stationary unless acted upon by another force.

2. There is a direct relationship between the force acted upon the object and the mass of that object times the acceleration the object feels (F=ma).

3. For every action there is an equal and opposite reaction.

NewtonNobody before Newton could explain why objects acted the way they did, but with these three laws he quantified movement in terms everyone could understand.

But there was a problem with his theory; if all motion had to be caused by some force acting on it, then why do objects fall towards the earth when you release them from a fixed position? This free falling object was in fact free, meaning free of outside forces acting upon it (besides wind resistance). There were no visible forces acting upon that object. So why do they move downward if nothing is acting on it? But Newton explained this motion with gravity. He said that gravity is a force that the earth has upon all objects, something invisible that pulls us down at all times at a constant acceleration. There is a myth that the way Newton thought of the idea of gravity was when he was thinking about it under an apple tree when an apple fell on Newton’s head and at that moment, he figured out that there must be a force pulling the object down. This is also why apples are used to demonstrate Newton’s force, but no one knows definitively if the myth is true or not. Read the rest of this entry »


Concentrating Sunlight: It’s Easy!


Martin Sagendorfby: Martin Sagendorf

On a Bright Day:

A great deal of energy falls on the Earth’s surface – roughly 1 kW per square meter.  This is about 0.6 Watt per square inch.  This doesn’t sound like much energy, but suppose we collect and concentrate 63 square inches of this sunlight?  These 63 square inches would collect about 38 Watts of energy.  This doesn’t sound like much, but…

Suppose We Could Then:

Concentrate these 38 Watts into an area of only 1/8 of a square inch?  This is exactly what we can do with an inexpensive plastic Fresnel lens.  We’ll focus the sunlight into an area 3/8” in diameter – this is the equivalent of 300 Watts per square inch!  With this energy level, we can easily ignite a piece of wood, boil some water, and even melt a penny.

A Suitable Device:

Is described in the book, Physics Demonstration Apparatus and in the blog The Sun’s Energy.

Physics Demonstration Apparatus Book

Now we’re going to describe how to build a much simpler version that works just as well – one that uses a very inexpensive Fresnel lens and is very easy to construct. Read the rest of this entry »