The History of Scientific Thought about Forces & Motion

Aristotle 384 B.C. – 322 B.C. Greek scientist & philosopher Made scientific discoveries through careful observations and logical reasoning.

Aristotle believed all motion is linear on the planet Earth: meaning everything moves in a straight line. If I throw this rock, it will travel in a straight line, and also fall in a straight line….                                    The way Aristotle thought projectiles moved The way projectiles really move

When a feather and rock are dropped at the same time, the rock will fall to the ground first. Aristotle believed that the Earth pulls on objects depending on how much they weigh. This means that a large heavy object will fall to the ground faster than a large light object         Aristotle Reality      

They locked me up for this? Galileo Galilei 1564 - 1642 They locked me up for this? Astronomer & Physicist Struggled against the church to have his Sun-centered solar system ideas accepted

Falling Objects & gravitational pull Galileo experimented with ramps and dropping things to find that all objects fall at the same rate. Acceleration due to gravity 9.8 m/s2 or 9.8 m/s/s

Galileo and Constant Acceleration The inclined plane experiment

Galileo’s Projectile experiment http://www. youtube. com/watch

Galileo worked out that there was a force acting in the opposite direction to motion and that if this force was decreased (by using oil, grease etc) then the object in motion would move further before stopping. From this Galileo was to formulate his Law of Inertia Unbalanced Forces!

Sir Isaac Newton 1642 - 1727 English mathematician Built on Galileo’s work

The Nature of Force By definition, a Force is a push or a pull. Just like Velocity & Acceleration, Forces have both magnitude (size) and direction components.

One Newton A Newton (N) is the unit that describes the size of a force. One N is equal to 1 kgm/s/s.

How Forces Combine More than one force often acts on an object. When all the forces acting on an object are added together, you determine the net force on the object. An object with a net force greater than 0 N on it will change its state of motion.

When forces are applied in the same direction, they are added to determine the size of the net force.

When two forces act in opposite directions, you subtract the smaller force from the larger force to determine the net force. The net force will be in the same direction as the larger force.

Balanced & Unbalanced Forces Balanced forces: opposite and equal forces acting on the same object result in NO motion of the object Unbalanced forces – two or more forces of unequal strength or direction acting upon on an object results in the motion of the object

1st Law of Motion An object will keep the same motion unless acted on by an unbalanced force

2nd Law of Motion The force of an object depends on the mass of the object multiplied by the acceleration

3rd Law of Motion Every action force has a equal and opposite reaction force.