11BCnewton


 * Newton's Laws of Motion **

**1st Law**
 * If the Resultant Force (R) acting on a body is zero then the velocity will remain constant (or zero)


 * {SInce R is the vector sum of all forces acting on the body, the vector sum must be zero}

**2nd Law**
 * __R__ = m__a__


 * {Resultant Force (vector) = mass (constant) × acceleration (vector)}


 * Examples

**3rd Law**


 * For every action by object A on object B there is an equal magnitude and opposite direction reaction force by object B on object A.


 * If object A exerts a force of F on object B,
 * then object B exerts a force of (– F) on object A

**Newton's 3rd Law**


 * // For every action, there is an equal and opposite reaction. //

Newton's 3rd law refers to an action force and a reaction force that acts on __different__ bodies.

For example, if a man stands on a platform, there are two forces acting __on the man__ -- gravity pulling down and the normal reaction force of the platform pushing up. These two forces are NOT an action/reaction pair. They are both acting on the SAME body.

The action/reaction pair in this example is the force acting ON THE PLATFORM of the man pushing down and the force acting ON THE MAN of the platform pushing up. These are the two forces that are opposite and equal as described by Newton's 3rd Law.

A second action/reaction pair in this same example is the force acting ON THE MAN of the earth pulling down (using the gravitational field of the earth) and the force acting ON THE EARTH of the man pulling the earth up (using the gravitational field of the man). Those two forces are also equal and opposite by Newton's 3rd law. Since the relative masses are so different, the acceleration of the earth upwards caused by the man's gravitational field is so small that it can't be measured.

**Example** (not directly in the course)

An 80kg man stands at sea level. The gravitational force acting on the man due to the earth's gravitational field is therefore 80g or 784 Newtons.

Therefore an equivalent force of 784 Newtons is acting on the Earth and pulling it upwards.

Since the mass of the Earth is approximately 5.97 x 10 24 kg, we can use R = ma to get: math \\ . \qquad 784 = a \times 5.97 \times 10^{24} \\. \\ . \qquad a = 784 / (5.97 \times 10^{24}) \\. \\ . \qquad a = 131 \times 10^{-24} \\. \\ . \qquad a = 1.31 \times 10^{-22} m/s^2 math

Since one nanometer is 10 –9 m and the radius of the nucleus of an atom is in the vicinity of 10 –15 m, we can see that 10 –22 m is very small indeed.

The population of the world is currently close to 7 x 10 9.

If, instead of a single man, we took the entire population of the world and gathered them in one spot on the surface of the Earth (in one big, uncomfortable heap), they would still cause a combined gravitational acceleration on the Earth of less than the diameter of one atom per second per second.

Since the population is spread out approximately uniformly over the surface of the Earth, their gravitational effects are more or less cancelling each other out so the Resultant force exerted by people on the Earth is effectively zero. .