What is Force?
Common Units
of Force

SI:
newton (N)
   1 N = 0.225 lb

English:
Pound (lb)
   1 lb = 4.448 N

One type of force that everyone is familiar with is weight. This is the amount of force that the Earth exerts on you. There are two interesting things about this force:

  • It pulls you down, or, more exactly, toward the center of the Earth.
  • It is proportional to your mass. If you have more mass, the Earth exerts a greater force on you.

When you step on a bathroom scale, you exert a force on the scale. The force you apply to the scale compresses a spring, which moves the needle. When you throw a baseball, you apply a force to the ball, which makes it speed up. An airplane engine creates a force, which pushes the plane through the air. A car's tires exert a force on the ground, which pushes the car along.

Force causes acceleration. If you apply a force to a toy car (for example, by pushing on it with your hand), it will start to move. This may sound simple, but it is a very important fact. The movement of the car is governed by Isaac Newton's Second Law, which forms the foundation for classical mechanics. Newton's Second Law states that the acceleration (a) of an object is directly proportional to the force (F) applied, and inversely proportional to the object's mass (m). That is, the more force you apply to an object, the greater the rate of acceleration; and the more mass the object has, the lower the rate of acceleration. Newton's Second Law is usually summarized in equation form:

a = F/m, or F = ma

To honor Newton's achievement, the standard unit of force in the SI system was named the newton. One newton (N) of force is enough to accelerate 1 kilogram (kg) of mass at a rate of 1 meter per second per second (m/s2). In fact, this is really how force and mass are defined. A kilogram is the amount of weight at which 1 N of force will accelerate at a rate of 1 m/s2. In English units, a slug is the amount of mass that 1 pound of force will accelerate at 1 ft/s2, and a pound mass is the amount of mass that 1 lb of force will accelerate at 32 feet/s2.

The Earth exerts enough force to accelerate objects that are dropped at a rate of 9.8 m/s2, or 32 feet/s2. This gravitational force is often referred to as g in equations. If you drop something off a cliff, for each second it falls it will speed up by 9.8 m/s. So, if it falls for five seconds, it will reach a speed of 49 m/s. This is a pretty fast rate of acceleration. If a car accelerated this quickly, it would reach 60 miles per hour (97 kph) in less than three seconds!

Usually, when we talk about force, there is more than one force involved, and these forces are applied in different directions. Let's look at a diagram of a car. When the car is sitting still, gravity exerts a downward force on the car (this force acts everywhere on the car, but for simplicity, we can draw the force at the car's center of mass). But the ground exerts an equal and opposite upward force on the tires, so the car does not move.


Figure 1. Animation of forces on a car

When the car begins to accelerate, some new forces come into play. The rear wheels exert a force against the ground in a horizontal direction; this makes the car start to accelerate. When the car is moving slowly, almost all of the force goes into accelerating the car. The car resists this acceleration with a force that is equal to its mass multiplied by its acceleration. You can see in Figure 1 how the force arrow starts out large because the car accelerates rapidly at first. As it starts to move, the air exerts a force against the car, which grows larger as the car gains speed. This aerodynamic drag force acts in the opposite direction of the force of the tires, which is propelling the car, so it subtracts from that force, leaving less force available for acceleration.

Eventually, the car will reach its top speed, the point at which it cannot accelerate any more. At this point, the driving force is equal to the aerodynamic drag, and no force is left over to accelerate the car.