Mechanics is the area of physics that encompasses the laws of motion, energy, and forces. Velocity, momentum, and Newton’s second law relating force, mass, and acceleration are all mechanical concepts. All mechanical devices, including the simple machines discussed in this chapter, are based on the application of force in order to achieve a movement or change in position of a mass.
Each of the physical quantities reviewed in this section were first introduced in the Physical Science section of chapter 9: General Science. This brief review will focus only on the concepts from that chapter which most directly underlie mechanical operation.
All matter has mass. Mass is a measure of the total quantity of matter in an object. Generally speaking, larger objects tend to have greater mass, but some materials contain more mass than others per unit volume (are denser), so bigger doesn’t always mean more massive. Unlike the force of weight, which will be discussed in more detail a little later in the chapter, mass is not a vector quantity, since it has magnitude but no direction.
For the purposes of mechanics, the most important thing to know about mass is that it corresponds to how much force is required to achieve a particular acceleration. In other words, the more mass something has, the more difficult it is to change its motion. The term inertia is sometimes used to refer to an object’s resistance to changes in its motion, but inertia is just a property of mass.
A force is a push or pull. Forces are everywhere in the world. Some are fairly obvious, such as a tractor pulling a plow, a baseball being hit into the stands, or a person shoving his way past others in a crowded store. Other forces are often taken for granted. Earth’s gravity applies a force (weight) that always pulls down, and it is what keeps objects and people on the ground instead of floating around in midair. All forces are vector quantities and therefore act in a single direction. The term net force refers to the total force acting on an object.
Without a force being applied to them, objects would not move (or, if they were already in motion, they would remain in motion without stopping). Objects with a great deal of mass (such as a freight train) require a large force to alter their motion. The goal of all mechanical technology is to most effectively apply forces to the movement of varying mass quantities.
All mechanical devices take Newton’s laws for granted in their operation. Newton’s first law describes the unchanging state of motion of an object when it experiences no net force. A heavy crate suspended by a pulley is only able to avoid crashing to the ground because the attached ropes fully counteract the force of gravity pulling the mass downwards.
Newton’s second law can be summarized by the formula
F = ma
where m is measured in kilograms (kg), a is in meters per second squared (m/s2) and F is in newtons (N). This law expresses the linear relationships between either the mass of the moved object or the desired acceleration, with the force required to achieve that movement.
One of the most important historical roles of machines has been in multiplying the force a human or work animal provides in order to move very massive objects that would be otherwise immobile. Ancient structures including Stonehenge and the Egyptian pyramids are evidence of a long human tradition of mechanical aids to labor.
Newton’s third law of motion states that for every action there is an equal and opposite reaction. This law applies to all applications of force, including those mediated by simple machines. It’s possible to pull on a rope and pulley to lift a weight only because the force of tension is carried through a rope by a daisy chain of action-reaction force pairs from one section of rope to the next.
| Question | Analysis |
| Simple machines and other mechanical devices | Step 1: The question asks which statement applies to mechanical devices. |
| Step 2: Think about the traits that simple machines and other mechanical devices have in common: they are used to transfer force, and they obey Newton’s laws. | |
| Step 3: That’s as close as you can get to a prediction with a general question like this, so start evaluating answer choices. | |
| (A) are technologies used to get around certain laws of physics (B) are exclusively based on momentum (C) operate according to the rules of force and mass (D) transmit and multiply force in order to increase the amount of energy available |
Step 4: Mechanical devices use the laws of physics; they don’t circumvent them, so (A) is not correct. Momentum is important, but it is not the only basis for any and all machines, so eliminate (B). Choice (C) is true. Select it. Choice (D) sounds almost correct, but how does a device increase the energy available? By burning fuel? By creating energy from nothing? Simple machines like levers don’t themselves burn up fuel. And although force can be multiplied with mechanical advantage (more on that later), energy cannot be created out of nothing. |
Now try one on your own.
Choice (C) is correct. There must be a non-zero net force for any object to accelerate. Objects in motion where the net force is zero will continue moving at a constant velocity, but will not accelerate. Both (C) and (D) talk about a difference between two values, but answer choice (D) compares the applied force to the object’s mass rather than weight. Since these quantities have different physical meanings, it doesn’t make sense to say one is greater than another.