Newton’s Laws of Motion - A heavy parachutist falls faster than a lighter one and, there fore, has a rougher landing but why? Have you tried the party trick where you pull a tablecloth out from under place settings and the dishes stay put? How does this “trick” work, and what law of motion does it demonstrate? Have you heard the expression “You can’t touch without being touched”? Does this statement about the objective world of physics have a corollary in the world of human emotions? How did Newton’s laws get us to the Moon? How do birds ﬂy, rockets take off, and people walk? How do Newton’s laws of motion interface with modern discoveries about motion gained from relativity and quantum mechanics?
Newton’s First Law of Motion
Galileo’s work set the stage for Isaac Newton, who was born shortly after Galileo’s death in 1642. By the time Newton was 23, he had developed his famous three laws of motion, which completed the overthrow of Aristotelian ideas about motion. These three laws ﬁrst appeared in one of the most famous books of all time, Newton’s PhilosophiaeNaturalisPrincipiaMathematica,*often simply known as the Principia.
The ﬁrst law is a restatement of Galileo’s concept of inertia;
the second law relates acceleration to its cause—force; and the third is the
law of action and reaction. Newton’s ﬁrst law is:
Every object continues in its state of rest, or a uniform speed in a straight line, unless acted on by a nonzero force. The key word in this law is continues; an object continues to do whatever it happens to be doing unless a force is exerted upon it. If the object is at rest, it continues in a state of rest. This is nicely demonstrated when a tablecloth is skill fully whipped from beneath dishes sitting on a tabletop, leaving the dishes in their initial state of rest (Figure 1).
On the other hand, if an object is moving, it continues to move without changing its speed or direction, as evidenced by space probes that continually move in outer space. This property of objects to resist changes in motion is called inertia (Figures1 and 2).
|Figure 1. Inertia in action.|
|Figure 2. Rapid deceleration is sensed by the driver,who lurches forward inertia in action!|
When a space shuttle travels in a nearly circular orbit around the Earth, is a force required to maintain its high speed? If the force of gravity were suddenly cut off, what type of path would the shuttle follow?
CHECK YOUR ANSWER
There is no force in the direction of the shuttle’s motion, which is why it coasts at a constant speed by its own inertia. The only force acting on it is the force of gravity, which acts at right angles to its motion (toward the Earth’s center). We’ll see later that this right-angled force holds the shuttle in a circular path. If it were cut off, the shuttle
would ﬂy off in a straight line at a constant velocity.
Newton’s Second Law of Motion
Isaac Newton was the ﬁrst to recognize the connection between force and mass in producing acceleration, which is one of the most central rules of nature, as expressed in his second law of motion. Newton’s second law states:
The acceleration produced by a net force on an object is directly proportional to the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object.
|Figure 3. When you accelerate in the direction of your velocity, you speed up; when you accelerate against your velocity, you slow down; when you accelerate at an angle to your velocity, your direction changes.|
Acceleration equals the net force divided by the mass. If the net force acting on an object is doubled, the object’s acceleration will be doubled. Suppose instead that the mass is doubled. Then the acceleration will be halved. If both the net force and the mass are doubled, then the acceleration will be unchanged.
An object accelerates in the direction of the net force acting upon it. Speed changes when the net force acts in the direction of the object’s motion. When the net force acts at right angles to the object’s motion, then the direction of the object changes. A net force acting in any other direction results in a combination of
speed change and deﬂection (Figure 3).