Newton's Third Law of Motion: Momentum
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### Lecture Description

The only way I can walk forward is to have the Earth to push on! We discover this when we try to walk on sheer clear smooth ice! So - too - I can jump upward only by pushing downward on the Earth. Invoking Newton's Second Law and the Third Law we are led to say that if I go up the Earth must go down! and indeed this is so. But the Earth is so massive that ITS motion is not de¬tected. You see ME go up but you do not see the Earth go down - yet it most certainly does.
We show an array of DEMONSTRATIONS bearing on this Third Law.

A - A balloon attached to a little car is blown up. When the air comes out this way the car goes the other way.

B - A rocket goes best where "there is nothing" . The STUFF coming out be¬hind DOES NOT need the atmosphere to push against.

C - In the Case of The Two Cars (See Program *3) the force which the spring exerts on both cars is the same. We see that the little car of mass m acquires a higher velocity than the big car of mass M. Thus we can write mV - Mv which is to say that the momenta of both cars are the same. (Momenta is the plural of momentum).^

D - With the "Sputnik" demonstration the gas comes out one way and the vehicle goes the other way. We find the CO2 cartridge VERY COLD after the gas comes out! The gas, experiencing free expansion, drops in temperature. We shall learn more of this in later programs. You can discover this same thing by letting the HOT air out of a HOT tire on a HOT day. As the HOT air IN the tire emerges it feels COLD!

E - The elastic steel spheres on the track demonstrate CONSERVATION of LINEAR MOMENTUM. As many "go away" after the collision as collide before.
Toys can demonstrate in a pretty way the principles of Physics.
1 - A spinning propeller goes UP only because it PUSHES the air DOWN.
2 - The air in a blown-up balloon comes out one way; the balloon goes the
other way.
3 - Hero's Engine - known as early as the 5th century B.C. - shows the idea
of Newton's Third Law.
4 - To demonstrate that I PUSH BACKWARD when I WALK FORWARD I stand on a free-wheeling cart like a skate-board. As I move forward the cart goes backward. So does the Earth!

### Course Description

Demonstrations in Physics was an educational science series produced in Australia by ABC Television in 1969. The series was hosted by American scientist Julius Sumner Miller, who demonstrated experiments involving various disciplines in the world of physics. The series was also released in the United States under the title Science Demonstrations.

This program was a series of 45 shows (approximately 15 minutes each) on various topics in physics, organized into 3 units: Mechanics; Heat and Temperature / Toys; and Waves and Sound / Electricity and Magnetism.