Motion

Newton's laws

The three Newton's laws of motion:

First law

A body remains at rest, or in motion at a constant speed in a straight line, unless acted upon by a force.

• Define force and inertia
• A stationary object will only begin to move if there is an unbalanced force acting on it
• Moving object will only change speed or direction if there is an unbalanced force acting on it

Examples

• An object resting on the ground is influenced by the gravity and a reaction force (the ground). As the forces are balanced, the object is not moving.
• In outer space, where there is no "natural" forces (friction, air resistance, gravity), an object would keep on moving in a straight line.

Second law

When a body is acted upon by a force, the time rate of change of its momentum equals the force.

$$F=ma=m\frac{dv}{dt}=\frac{dp}{dt}$$

• Net force: $F$
• Mass: $m$
• Acceleration: $a=\frac{dv}{dt}$ (see: derivative)
• Velocity: $v$
• Momentum: $p=mv$
• The momentum $p$ of an object is the product of its mass $m$ and velocity $v$

Examples

• The weight of an object is it's mass multiplied by gravity (~9.8m/sec. on Earth)

Third law

If two bodies exert forces on each other, these forces have the same magnitude but opposite directions.