When building a CNC router or upgrading a 3D printer, the first question is usually: "Is NEMA 17 enough, or do I need NEMA 23?" Most beginners look at the Holding Torque and stop there. This is a mistake. A NEMA 23 motor isn't just "stronger"—it is physically different in ways that affect your speed, your driver choice, and your machine's ability to avoid missed steps. If you choose a NEMA 17 for a heavy gantry, it is far more likely to overheat or lose steps under cutting load. If you choose NEMA 23 for a fast 3D printer, it might actually run slower than the smaller motor. This guide explains the engineering limits of each frame size. Table of Contents 1. Physical Difference (The Frame Size) 2. Torque & Speed (The Inductance Trap) 3. Driver Compatibility 4. Selection Summary Advertisement 1. Physical Difference (The Frame Size) "NEMA" is just a standard for ...
Figure 1: Mass is intrinsic (amount of matter). Weight is extrinsic (force of gravity acting on that matter).
Let us return to the legend of Newton and the falling apple. From the study of statics, we know that the apple remains attached to the tree as long as the apple stalk is strong enough to support the weight of the apple.
As the apple grows, its mass increases. Eventually, the gravitational force (weight) exceeds the stalk's strength, and it snaps. But why does it fall? And what is the difference between the "stuff" inside the apple and the force pulling it down?
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1. Mass vs. Weight: The Critical Distinction
In everyday language, we use "mass" and "weight" interchangeably. In engineering, confusing them causes catastrophic calculation errors.
Mass (m):
The amount of matter contained in a body. It is a measure of inertia (resistance to acceleration).
Unit: Kilograms (kg)
Weight (W):
The force exerted on a mass by a gravitational field.
Unit: Newtons (N) or Pounds-force (lbf)
The amount of matter contained in a body. It is a measure of inertia (resistance to acceleration).
Unit: Kilograms (kg)
Weight (W):
The force exerted on a mass by a gravitational field.
Unit: Newtons (N) or Pounds-force (lbf)
The Space Station Example:
Consider fruits grown inside an orbiting spacecraft. In that environment, objects are effectively weightless and float freely. However, they still possess mass. If an astronaut tried to shake a heavy floating pumpkin, it would still resist moving due to its inertia (mass), even though it has zero weight.
2. The Second Law: F = ma
When the apple separates from the tree, the downward force of gravity is no longer balanced by the tension in the stalk. There is a Net Force acting on the mass.
Newton formalized this relationship in his Second Law of Motion:
Figure 2: Force causes acceleration. The magnitude of that acceleration depends inversely on the mass.
F = m × a
This equation reveals two fundamental truths of mechanics:
- Direct Proportion: For a given mass, doubling the force will double the acceleration.
- Inverse Proportion: For a given force, doubling the mass will cut the acceleration in half.
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3. Engineering Applications
Newton’s second law isn't just for falling apples. It is the backbone of dynamic analysis in mechanical design.
Figure 3: Calculating the torque required for a robot to accelerate a load requires F=ma (plus gravity).
Example: Sizing a Motor
When designing a conveyor belt or a robotic arm, you cannot simply calculate the static weight. You must calculate the force required to accelerate the load from a stop.
Engineering Calculation: Orientation Matters
Gravity always acts downwards, but how it opposes your motor depends on the direction of motion:
Gravity always acts downwards, but how it opposes your motor depends on the direction of motion:
- Vertical Lift (e.g., Elevator):
Force = (Mass × Gravity) + (Mass × Acceleration) - Horizontal Move (e.g., Conveyor):
Force = (Friction Coefficient × Mass × Gravity) + (Mass × Acceleration)
If you only design for gravity (static weight), your motor will stall or move too sluggishly to meet cycle time requirements.
Related: Newton's Three Laws Overview
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