Section 1: The First Two Laws of Motion

• Newton’s Laws of Motion

  • The British scientist Isaac Newton, who lived from 1642 to 1727, published a set of three rules in 1687 that explained how forces and motion are related.

  • First Law of Motion: states that if the net force acting on an object is zero, the object remains at rest, or if the object is moving, it continues moving in a straight line with constant speed.

    • The first law of motion connects forces with changes in motion

  • Inertia: the tendency of an object to resist a change in its motion.

    • The inertia of an object depends on the object’s mass. The greater the mass of an object, the greater its inertia.

    • The first law of motion sometimes is called the law of inertia.

  • Second Law of Motion: states that the acceleration of an object is in the same direction as the net force on the object, and that the acceleration can be calculated from the following equation:

    • acceleration (in meters/second (squared) = net force (in newtons) / mass (in kilograms)

Section 2: Gravity

• Gravity: an attractive force between any two objects that depends on the masses of the objects and the distance between them.

  • This force increases as the mass of either object increases, or as the objects move closer,

  • Gravity is one of the four basic forces.

    • The Law of Universal Gravitation: gravitational force = (constant) x (mass 1) x (mass 2) / (distance) (squared)

  • According to the law of universal gravitation, the gravitational force between two masses decreases rapidly as the distance between the masses increases.

• Earth’s Gravitational Acceleration

  • When all forces except gravity acting on an a falling object can be ignored, the object is said to be in free fall.

  • Close to Earth’s surface, the acceleration of an object in free fall is 9.8 m/s2.

  • Gravitational Force Equation: gravitational force(N) =mass (kg) X gravitational acceleration (m/s2)

  • Weight: The gravitational force exerted on an object

    • The weight of an object on Earth is equal to the gravitational force exerted by Earth on the object.

    • Weight Equation: weight (N) = mass (kg) X gravitational acceleration (m/s2)

• Weightlessness and Free Fall

  • According the law of universal gravitation, at 400-km altitude the force of Earth’s gravity is about 90 percent as strong as it is at Earth’s surface.

  • On the Moon, the gravitational force on the astronaut is less than it is on Earth. As a result, the astronaut can take longer steps and jump higher than on Earth.

• Projectile: Anything that’s thrown or shot through the air

• Centripetal Force

  • Centripetal Acceleration: Acceleration toward the center of a curved or circular path

  • Centripetal Force: The net force exerted toward the center of a curved path

  • The Moon would move in a straight line except that Earth’s gravity keeps pulling it toward Earth. This gives the Moon a nearly circular orbit.

Section 3: The Third Law of Motion

• Newton’s Third Law

  • Third Law of Motion: describes action-reaction pairs this way: When one object exerts a force on a second object, the second one exerts a force on the first that is equal in strength and opposite in direction

  • When a force is applied in nature, a reaction force occurs at the same time.

  • According to the third law of motion, action and reaction forces act on different objects. Thus, even though the forces are equal, they are not balanced because they act on different objects

• Momentum: the product of its mass and velocity.

  • Formula: momentum (kg m/s) = mass (kg) X velocity (m/s)

  • The momentum of an object doesn’t change unless its mass, velocity, or both change.

  • Momentum can be transferred from one object to another.

  • Law of Conservation of Momentum: if a group of objects exerts forces only on each other, their total momentum doesn’t change.