Laws of motion

• Newton’s first law of motion states that an object at rest remains at rest and an object in motion maintains its velocity unless it experiences an unbalanced force.

• Objects tend to maintain their state of motion.

• Inertia is the tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction until an outside force acts on the object.
• Inertia is related to an object’s mass. Mass is a measure of inertia.

• Seat belts and car seats provide protection.
• Because of inertia, you slide toward the side of a car when the driver makes a sharp turn.
• When the car you are riding in comes to a stop, your seat belt and the friction between you and the seat stop your forward motion.
• Newton’s second law of motion states that the unbalanced force acting on an object equals the object’s mass times its acceleration.

• Force equals mass times acceleration.

Force = mass ´ acceleration

F = ma

• Force is measured in newtons (N).

1 N = 1 kg ´ 1 m/s2

• Sir Isaac Newton (1642–1727) generalized his observations on gravity in a law now known as the law of universal gravitation.

• Universal Gravitation Equation

F = G ((m1m2)/d)

• m1 and m2 are the masses of the two objects
• d is the distance between the two objects
• G is a constant

• All matter is affected by gravity.
• Two objects, whether large or small, always have a gravitational force between them.
• When something is very large, like Earth, the force is easy to detect.

• Gravitational force increases as mass increases.

• Gravitational force decreases as distance increases

• Free fall is the motion of a body when only the force of gravity is acting on the body.

• Free-fall acceleration near Earth’s surface is constant.
• If we disregard air resistance, all objects near Earth accelerate at 9.8 m/s2.
• Freefall acceleration is often abbreviated as the letter g, so g = 9.8 m/s2.
• Weight is equal to mass times free-fall acceleration.

weight = mass ´free-fall acceleration

w = mg

• Weight is different from mass.
• Mass is a measure of the amount of matter in an object.
• Weight is the gravitational force an object experiences because of its mass.

• Orbiting objects are in free fall.

• The moon stays in orbit around Earth because Earth’s gravitational force provides a pull on the moon.

• Two motions combine to cause orbiting.

• Projectile motion is the curved path an object follows when thrown, launched, or otherwise projected near the surface of Earth.

• Projectile motion applies to objects that are moving in two dimensions under the influence of gravity.

• Projectile motion has two components—horizontal and vertical. The two components are independent.
• Projectile motion has some horizontal motion.
• Horizontal motion is motion that is perpendicular (90º) to Earth’s gravitational field.
• The horizontal velocity is constant.

• Projectile motion also has some vertical motion.
• The vertical motion is the same as downward free-fall motion.

Newton’s 3rd law

• Newton’s third law of motion states that for every action force, there is an equal and opposite reaction force.

• Forces always occur in action-reaction pairs.

• Action-reaction force pairs are equal in size and opposite in direction.
• Force pairs do not act on the same object.

• When one object exerts an action force on a second object, the second object exerts a reaction force on the first object.

• Equal forces don’t always have equal effects.
• For example, the action force of Earth pulling on an object and causing it to fall is much more obvious than the equal and opposite reaction force of the falling object pulling on Earth.

Momentum

• Momentum is a quantity defined as the product of the mass and velocity of an object.

momentum = mass ´ velocity

p = mv

• Moving objects have momentum.
• For a given velocity, the more mass an object has, the greater its momentum is.
• Likewise, the faster an object is moving, the greater its momentum is.

• Force is related to change in momentum.
• When you force an object to change its motion, you force it to change its momentum.

• Momentum is conserved in collisions.
• The law of conservation of momentum states that the total amount of momentum in an isolated system is conserved.

• Conservation of momentum explains rocket propulsion.

• Momentum is transferred.
• When a moving object hits a second object, some or all of the momentum of the first object is transferred to the second object.
• Momentum can be transferred in collisions, but the total momentum before and after a collision is the same.

• Action and reaction force pairs are everywhere.