# Section 1: Newton’s Second Law

• Force, Mass, and Acceleration

• Newton’s second law of motion describes how the forces exerted on an object its mass, and its acceleration are related.

• The acceleration of an object depends on its mass as well as the force exerted on it. Force, mass, and acceleration are related.

• Newton’s Second Law of Motion: states that the acceleration of an object is in the same direction as the net force on the object.

• Newton’s second law also can be used to calculate the net force if mass and acceleration are known.

• Friction: the force that opposes the sliding motion of two surfaces that are touching each other.

• According to Newton’s first law of motion, if the net force acting on a moving object is zero, it will continue to move in a straight line with constant speed.

• The amount of friction between two surfaces depends on two factors—the kinds of surfaces and the force pressing the surfaces together.

• Friction is due to microwelds formed between two surfaces. The larger the force pushing the two surfaces together is, the stronger the microwelds will be.

• According to Newton’s second law, if the acceleration is zero, then the net force on the box is zero.

• Static Friction: the frictional force that prevents two surfaces from sliding past each other.

• Sliding friction acts in the direction opposite the motion of the sliding box.

• Sliding Friction: the force that opposes the motion of two surfaces sliding past each other.

• Sliding friction is caused by microwelds constantly breaking and then forming again as the box slides along the floor.

• When a wheel is spinning there is sliding friction between the wheels and surface.

• As a wheel rolls over a surface, the wheel digs into the surface, causing both the wheel and the surface to be deformed.

• Rolling friction is the frictional force between a rolling object and the surface it rolls on.

• Air Resistance: opposes the motion of objects that move through the air.

• Air resistance causes objects to fall with different accelerations and different speeds.

• Air resistance acts in the opposite direction to the motion of an object through air.

• The size of the air resistance force also depends on the size and shape of an object.

• The amount of air resistance on an object depends on the speed, size, and shape of the object.

• As an object falls, the downward force of gravity causes the object to accelerate.

• The terminal velocity depends on the size, shape, and mass of a falling object.

# Section 2: Gravity

• What is gravity?

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

• Gravity is one of the four basic forces.

• The other basic forces are the electromagnetic force, the strong nuclear force, and the weak nuclear force.

• The two nuclear forces only act on particles in the nuclei of atoms.

• Electricity and magnetism are caused by the electromagnetic force.

• Chemical interactions between atoms and molecules also are due to the electromagnetic force

• The Law of Universal Gravitation

• For thousands of years people everywhere have observed the stars and the planets in the night sky.

• The law of universal gravitation enables the force of gravity to be calculated between any two objects if their masses and the distance between them are known.'

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

• Earth’s motion around the Sun is affected by the gravitational pulls of the other planets in the solar system.

• The motion of every planet in the solar system is affected by the gravitational pulls of all the other planets.

• Earth’s Gravitational Acceleration

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

• Weight is a force and mass is a measure of the amount of matter an object contains.

• The weight of an object usually is the gravitational force between the object and Earth.

• But the weight of an object can change, depending on the gravitational force on the object.

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

• 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.

• A space shuttle in orbit is in free fall, but it is falling around Earth, rather than straight downward.

• Projectile Motion

• Anything that’s thrown or shot through the air is called a projectile.

• Earth’s gravity causes projectiles to follow a curved path.

• Centripetal Force

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

• According to the second law of motion, when the ball has centripetal acceleration, the direction of the net force on the ball also must be toward the center of the curved 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 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.

• Momentum : the product of its mass and velocity.

• A moving object has a property called momentum that is related to how much force is needed to change its motion.

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

• Momentum is transferred in collisions.

• The results of a collision depend on the momentum of each object