Mechanical Design: Mechanical Engineering for Dummies

Newton’s Laws of Motion

First Law of Motion:

An object will remain at rest or in uniform motion in a straight line unless acted upon by an external force.

Second Law of Motion:

Force = Mass * Acceleration

Third Law of Motion:

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

What is Force?

A force can be thought of as a push or a pull and it has a magnitude and a direction.

From Newton’s Second Law, F = ma, we know that a force causes an object of a certain mass to accelerate. If all the forces on an object sum to zero, then the object is not accelerating. It could be motionless (v=0) or have a non-zero constant velocity.

Forces can come in many different forms:

• Push/Pull
• Gravity (force between objects, for us one object is usually the Earth)
• Friction (opposing force caused by sliding motion)
• Drag (opposing force caused by an object moving through a fluid)

A force distributed over a certain area is called pressure.

Normal Force, F_n

The force that a surface exerts on an object resting on that surface. This force prevents the object from falling through the surface.

Assuming the surface is immovable, the Normal Force is equal to the sum of all other forces acting perpendicular to the surface. Usually, this is the object’s weight.

For multi-wheeled robots resting or moving on a level floor, the weight of the robot will be split up between all the wheels. The distribution can be calculated if you know the location of the robot’s Center of Gravity. The Normal

Force at each wheel will equal the weight on that wheel, assuming there are no other forces acting on the robot. If there are moving parts on the robot, the CG will move too.