Motion is a change of position in space. Moving things surround us. Examples include a car traveling down the highway, a ball flying through the air, and a planet orbiting the sun. When we catch a ball or safely cross a busy street, we use our understanding of motion.
Physicists study motion to better understand the world. In the 1600’s, the English scientist and mathematician Isaac Newton developed three laws to explain the motion of the planets. Newton’s laws of motion can help us understand the kinds of motion we see every day.
All motion is relative. That means that an object can only be described as moving or stationary in relation to another object. On an airplane in flight, for example, two passengers sitting in their seats are moving rapidly relative to the ground. But they are stationary relative to each other. This concept, called relativity, presents special challenges to our understanding of motion.
Subatomic particles are in constant motion. These particles are the tiny pieces that make up all matter. Physicists describe this motion using the laws of a special branch of physics called quantum mechanics.
Describing motion
We often describe an object’s motion in terms of its speed. Speed measures the distance an object travels in a certain time. We might say, for example, that a car is moving at 40 miles or 64 kilometers per hour. If the car continues at that speed for two hours, it will travel 80 miles (130 kilometers).
Knowing the direction of motion is also important. Physicists use the term velocity to mean the speed and direction of motion. For instance, the velocity of an airplane may be 535 miles (861 kilometers) per hour to the west. Quantities like velocity that include both a measure and a direction are called vectors.
A change in an object’s velocity is called acceleration. We can express acceleration as the change in speed over a certain period. For example, a car might speed up from 30 miles (48 kilometers) per hour to 60 miles (97 kilometers) per hour in 3 seconds. We could describe the car’s acceleration during that period as 10 miles (16 kilometers) per hour per second. Physicists express acceleration as a vector because knowing the direction of acceleration can be important. If the car sped up as it headed north, we could say its acceleration was 10 miles per hour per second to the north.
People often use the word acceleration to mean speeding up. In physics, acceleration includes all changes in an object’s motion. It means not only speeding up but also slowing down and changing direction. For example, Earth moves around the sun at a nearly constant speed. But Earth continuously turns to maintain its circular path. This turning is acceleration. If Earth did not accelerate in the direction of the sun, it would continue to move in a straight line, drifting out of the solar system.
Newton’s laws of motion
Newton’s laws explain how force and mass affect motion. Mass is the amount of matter an object has. The laws enable us to understand an object’s motion if we know the object’s mass and the forces acting upon it.
The first law
states that an object moving in a straight line will continue to move in a straight line unless acted on by an outside force. This law also states that an object at rest will stay at rest unless a force acts on it. Newton’s first law is known as the principle of inertia. Inertia is the tendency of an object to continue moving if it is moving and to remain motionless if it is at rest. Forces that change an object’s motion must first overcome its inertia. See Inertia .
The second law
states that a force acting on an object produces an acceleration equal to the force divided by the mass of the object. This relationship is usually written as the equation
In this equation, F is the force, m is the object’s mass, and a is the acceleration. The arrows indicate the quanties are vectors. In the case of Earth’s orbit around the sun, the force is the force of gravity, which pulls the planet toward the sun. This force accelerates Earth toward the sun, causing the planet to move in a circular path.
The second law explains that acceleration increases with force. Imagine two people pulling two identical wagons. If one person exerts more force, that person’s wagon will accelerate more. The law also explains that acceleration decreases with mass. Imagine two people using the same amount of force to pull two identical wagons. One wagon is empty. The other is loaded with rocks. The full wagon will accelerate less than the empty wagon because the full wagon has more mass.
The third law
states that for each action there is an equal and opposite reaction. For example, rockets take off by expelling gases. The downward motion of the gases creates a reaction of the rocket upward. The reaction helps it overcome gravity and fly into space. In the case of Earth, the planet tugs at the sun in reaction to the sun’s pulling on it. But because the sun has much more mass than Earth does, the sun accelerates little in response.
Motion in everyday life
To use Newton’s laws, we must understand the forces that act on an object. For a planet or a star, the forces are simple. Many kinds of motion we encounter every day are more complicated.
Imagine, for example, rolling a ball across the ground. We might expect the ball to roll forever because Newton’s first law states that an object in motion will continue moving unless acted on by an outside force. However, we know from experience that the ball will eventually slow down and stop. According to Newton’s laws, some force must have acted on the ball. Physicists call this force friction. Friction occurs when one surface moves over another. As the ball rolls, its outer surface rubs against the ground. This rubbing generates a force of friction that slows the ball’s movement.
The ball also collides with air molecules as it moves. It thus generates a kind of friction called air resistance. The faster the ball moves, the more molecules it collides with in a given period. For this reason, air resistance becomes greater as the ball’s velocity increases.
Many types of friction affect the movement of a more complex object, such as an automobile. There is friction between the tires and the road. There is also friction between the parts of the engine and transmission. There is air resistance as air flows over the outside of the car. The friction between moving parts tends to decrease at higher speeds. Air resistance, however, increases with speed. These factors can make it difficult to predict the car’s motion based only on the force generated by the engine.
The effects of friction complicated early efforts to understand motion. Newton developed his laws by studying planets. The planets experience almost no friction as they move through nearly empty space.
Motion is relative
An object may move in relation to another object and yet stand still with regard to a third object. Suppose, for example, you are riding on an airplane that passes a person standing on the ground. The person on the ground will see you and everyone else on the airplane as being in motion. A passenger sitting next to you on the plane, however, will appear stationary with respect to you.
Galileo’s principle of relativity.
In the 1600’s, the Italian astronomer and physicist Galileo developed a principle to describe the relative nature of motion. This principle became known as Galilean relativity. According to Galilean relativity, observers describe motion in a particular frame of reference. To the person watching the plane fly overhead, the ground and other objects, such as trees and houses, make up a frame of reference in which the plane is moving. To the airplane passenger, however, the plane and everything it holds make up a different frame of reference. In the passenger’s frame of reference, the aircraft and its contents appear to stand still as the ground below slips by.
Galilean relativity states that the laws of motion appear the same for any frame of reference traveling at a constant velocity. For example, if an airplane passenger releases a ball, it will fall to his or her feet just as it would if he or she were standing on the ground. In fact, as long as the plane maintains a constant velocity, everything inside behaves as it would if the plane were not moving at all.
Galilean relativity can help us describe the differences in motion seen by two different observers. Imagine, for example, a passenger on an airplane traveling at 500 miles (800 kilometers) per hour. The passenger throws a ball toward the front of the plane at a speed of 10 miles (16 kilometers) per hour. The passenger will see the ball moving at 10 miles per hour. To an observer on the ground, however, the ball’s speed includes the speed of the throw and the speed of the plane. The ball appears to move 510 miles (821 kilometers) per hour.
Einstein’s theories of relativity.
In the early 1900’s, the German-born physicist Albert Einstein noted that the laws of physics seemed to show that nothing could travel faster than the speed of light. This appeared to contradict Galileo’s relativity principle. Imagine, for example, that an airplane passenger shines a flashlight toward the front of the plane. To the passenger, the light appears to travel at the speed of light. Using Galilean relativity, we might expect an observer on the ground to see the light moving 500 miles per hour faster than the speed of light.
Einstein proposed that the speed of light appeared the same to all observers, regardless of their relative motion. He replaced Galileo’s principle with a principle called special relativity. In special relativity, both time and space are different in frames of reference that are moving in relation to each other. Experiments soon confirmed many predictions of special relativity. However, Galilean relativity remains useful because it provides an accurate description of motion at speeds well below the speed of light.
In 1916, Einstein realized that Newton’s laws of motion could not be exactly correct for objects moving under the influence of gravity. He developed a theory called general relativity to explain the effects of gravity on motion. General relativity holds that mass and energy distort the structure of space and time, affecting the motion of objects. Scientists still use Newton’s laws for many things. In most cases, the predictions of Newton’s laws agree closely with general relativity.
The motion of subatomic particles
Subatomic particles, such as electrons and quarks, are in constant motion. Quarks are the particles that make up protons and neutrons. They move in ways that cannot be fully understood using Newton’s laws.
For example, electric current is the flow of electrons through a wire. The electrons move in response to an electric field. An electric field is a region of electric influence surrounding an electric charge. A typical electron is sped up by the electric field. It starts to move along the wire, only to collide with another electron or an atomic nucleus. The collision may slow down the electron or reverse its direction. The individual electrons bounce from collision to collision, drifting down the wire.
The motion of electrons in atoms is governed by the laws of quantum mechanics. These laws determine the probability that an electron will be in a particular place or will have a particular velocity. We cannot determine exactly where an electron is and how fast it is moving at the same time. The more precisely we know the electron’s location, the less we know about its speed, and vice versa. This rule is called the uncertainty principle. Despite this uncertainty, experiments have shown that the laws of quantum mechanics can be used to describe the behavior of subatomic particles with precision. The laws of quantum mechanics also apply to atomic nuclei and quarks.
