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# Physical Science Study Guide for McGraw-Hill's ASVAB

By McGraw-Hill Professional
Updated on Jun 26, 2011

### The Laws of Motion

The English scientist Sir Isaac Newton (1642–1727) formulated three laws of motion.

Newton's First Law The first law reads as follows: An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. Basically, this means that an object will keep doing what it is doing unless something makes it change. The something that makes an object change its state is called a force. A force is a push or pull upon an object resulting from one object's interaction with another object. An example of this is a person pushing a swing or a person pulling a suitcase.

An object has inertia when it is moving. Inertia is merely the resistance to change. An example is the tendency of your body to keep moving forward when your car comes to a sudden stop. In a collision, a seatbelt can keep you in place when otherwise you might hurtle through the windshield because of inertia.

Friction is a force that results when the surface of one object touches the surface of another. Friction causes moving objects to slow down or stop. For example, if you shove a book across a table, it will soon slow to a stop because it is rubbing on the surface of the table. Friction between two objects causes heat.

Newton's Second Law   According to Newton, an object will accelerate only if there is an unbalanced force acting upon it. The presence of an unbalanced force will accelerate an object, changing its speed, its direction, or both its speed and its direction. The second law states that the acceleration of an object is dependent upon two variables—the force acting upon the object and the mass of the object. The formula for acceleration is .

Newton's Third Law   Newton's third law reads: For every action, there is an equal and opposite reaction. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. There are two forces resulting from this interaction—a force on the chair and a force on your body. When a bird flaps its wings downward, the air pushes it upward, allowing it to fly. A gun recoils when it is fired.

Describing Motion   In physics, there are various ways to describe motion. Speed is how fast an object is moving. A fast-moving object has a high speed, while a slow-moving object has a low speed. An object with no movement at all has a zero speed. The average speed during the course of a motion can be calculated using the following equation:

Velocity is defined as the rate and direction at which an object's position changes. Velocity includes both speed and direction, such as moving 55 miles/hour in a westerly direction or 6 meters/second upward.

Average velocity can be calculated using the equation:

Acceleration is defined as "the rate at which an object changes its velocity." An object is accelerating if it is changing its velocity. If an object is not changing its velocity, then the object is not accelerating. A falling object accelerates as it falls. If you could measure the motion of a falling object, you would notice that the object has an average velocity of 5 m/s in the first second, 15 m/s in the second second, 25 m/s in the third second, 35 m/s in the fourth second, and so on. By pushing down the gas pedal, you can accelerate your car.