Chapter 4: Linear Kinetics, Force and Newton’s Laws of Motion
4.2 Newton’s First Law
Authors: William Moebs, Samuel Ling, Jeff Sanny
Adapted by: Rob Pryce, Alix Blacklin
Learning Objectives
By the end of this section, you will be able to:
- Describe Newton’s first law of motion
- Recognize friction as an external force
- Define inertia
- Identify inertial reference frames
- Calculate equilibrium for a system
Experience suggests that an object at rest remains at rest if left alone and that an object in motion tends to slow down and stop unless some effort is made to keep it moving. However, Newton’s first law gives a deeper explanation of this observation.
Newton’s First Law of Motion
A body at rest remains at rest or, if in motion, remains in motion at constant velocity unless acted on by a net external force.
Note the repeated use of the verb “remains.” We can think of this law as preserving the status quo of motion. Also note the phrase “constant velocity;” this means that the object maintains a path along a straight line, since neither the magnitude nor the direction of the velocity vector changes. In other words, if we see either the speed (magnitude) or the direction of an object changes, then unbalanced forces must be acting on an object. Netwon’s First Law can also be stated as “Every body remains in its state of uniform motion in a straight line unless it is compelled to change that state by forces acting on it.” We can use Figure 5.7 to consider the two parts of Newton’s first law.
Rather than contradicting our experience, Newton’s first law says that there must be a cause for any change in velocity (a change in either magnitude or direction) to occur. This cause is a net external force, which we defined earlier in the chapter. An object sliding across a table or floor slows down due to the net force of friction acting on the object. If friction disappears, will the object still slow down?
The idea of cause and effect is crucial in accurately describing what happens in various situations. For example, consider what happens to an object sliding along a rough horizontal surface. The object quickly grinds to a halt. If we spray the surface with talcum powder to make the surface smoother, the object slides farther. If we make the surface even smoother by rubbing lubricating oil on it, the object slides farther yet. Extrapolating to a frictionless surface and ignoring air resistance, we can imagine the object sliding in a straight line indefinitely. Friction is thus the cause of slowing (consistent with Newton’s first law). The object would not slow down if friction were eliminated.
Consider an air hockey table (Figure 4.8). When the air is turned off, the puck slides only a short distance before friction slows it to a stop. However, when the air is turned on, it creates a nearly frictionless surface, and the puck glides long distances without slowing down. Additionally, if we know enough about the friction, we can accurately predict how quickly the object slows down.
Newton’s first law is a general or universal law, which means it can be applied to anything from a textbook sliding on a table, to a person jumping in the air, to blood pumped from the heart. In other words, it is a basic feature of all laws of physics (and therefore biomechanics). Identifying these laws is like recognizing patterns in nature from which further patterns can be discovered. The genius of Galileo, who first developed the idea for the first law of motion, and Newton, who clarified it, was to ask the fundamental question: “What is the cause?” The ability to think in terms of cause and effect is the ability to make a connection between an observed behavior and the surrounding world.
Mass, Gravity and Inertia
Gravity and inertia are two closely related aspects in biomechanics. Both are connected to mass. Roughly speaking, mass is a measure of the amount of matter in something. While gravitation is the attraction of one mass to another, such as that between yourself and the Earth. The magnitude of this attraction is your weight, and it is a force.
However, mass is also related to inertia, which is the ability of an object to resist changes in its motion—in other words, to resist acceleration. Newton’s first law is often called the law of inertia. As we know from experience, some objects have more inertia than others. It is more difficult to change the motion of a large boulder than that of a basketball, for example, because the boulder has more mass than the basketball. In other words, the inertia of an object is measured by its mass. The relationship between mass and weight is explored later in this chapter.
Newton’s First Law and Equilibrium
Newton’s first law also tells us about equilibrium, which is defined as the state in which the all forces acting on a system are balanced. Returning to the ice skaters in Figure 4.3, we know that the forces [latex]{\overset{\to }{F}}_{1}[/latex] and [latex]{\overset{\to }{F}}_{2}[/latex] combine to form a resultant force, or the net external force: [latex]{\overset{\to }{F}}_{\text{R}}={\overset{\to }{F}}_{\text{net}}={\overset{\to }{F}}_{1}+{\overset{\to }{F}}_{2}.[/latex] To create equilibrium, we require a balancing force that will produce a net force of zero. This force must be equal in magnitude but opposite in direction to [latex]{\overset{\to }{F}}_{\text{R}},[/latex] which means the vector must be [latex]\text{−}{\overset{\to }{F}}_{\text{R}}.[/latex]
We can give Newton’s first law and static equilibrium in equation form:
Newton’s first law is deceptively simple. If a car is at rest, the only forces acting on the car are weight and the contact force of the pavement pushing up on the car (Figure 4.9). It is easy to understand that a non-zero net force is required to change the state of motion of the car. However, if the car is in motion with constant velocity, a common misconception is that the force propelling the car forward is larger in magnitude than the forces that opposes its forward motion (which are friction and drag). In fact, the force propelling the forces is identical in magnitude to the drag and friction forces. Therefore, the car is said to be in equilibrium.
Check your Understanding
A skydiver opens his parachute, and shortly thereafter, he is moving at constant velocity.
(a) What forces are acting on him?
(b) Which force is bigger?
Solution:
a. His weight acts downward, and the force of air resistance with the parachute acts upward.
b. neither; the forces are equal in magnitude
Interactive
Engage this simulation to predict, qualitatively, how an external force will affect the speed and direction of an object’s motion. Explain the effects with the help of a free-body diagram. Use free-body diagrams to draw position, velocity, acceleration, and force graphs, and vice versa. Explain how the graphs relate to one another. Given a scenario or a graph, sketch all four graphs.