Lesson 10.4.2: Newton's 2nd and 3rd Law of Motion

• Describe Newton's 2nd Law of Motion

  • Calculate Acceleration when given a Force and a mass

• Describe Newton's 3rd Law of Motion

• Acceleration is the change in velocity over time

  • Involves changes in speed as well as changes in direction​

• Acceleration occurs in both horizontal movement as well as vertical movement

• More massive Objects are more difficult to accelerate

  • Experiments show that doubling the mass of an object halves the acceleration, tripling thirds it, etc.

  • Acceleration and mass are inversely related​

• ​When an Unbalanced Force is applied to a mass, it causes it to accelerate

  • The acceleration of an object is directly proportional to the net force on the object and is inversely proportional to the mass of the object

    • ​a = f/m

    • f=m*a​

A Force is Part of an Interaction

• Force is not a trait of an object, it is something an object does

• Force is more than just a single push or a pull and is an interaction between two objects

  • Both the object exerting the force and the object receiving the force is part of the interaction

• Example: If a truck collides with a car, the truck exerts a force on the car and the car exerts a force on the truck at the same time

  • These forces are equal in strength and occur in opposite directions

Newton's 3rd Law: The Law of Action & Reaction

• Whenever one object exerts a force on an second object, the second object exerts an equal and opposite force on the first

  • To every action, there is an equal and opposite reaction

  • One force is the action force, the other force is the reaction force

Identifying Action and Reaction

• First identify the interaction

  • Example: Object A interacts with Object B

    • ​Action: Object A exerts a force on Object B

    • Reaction: Object B exerts a force on Object A

  • If the action is A on B the reaction is B on A

Action & Reaction on Objects of Different Masses

• Recall Newton's Second Law: a= F/M

• Newton's Third law​ states that every action has an equal and opposite reaction, so why do objects respond differently?

  • Because they have Different Masses

  • A given Force Exerted on a small mass produces a large acceleration, while the same force exerted on a large mass produces a small acceleration

Action & Reaction on Objects of Different Masses

• ​If Forces are equal and Opposite, why don't they cancel out to zero?

  • Because they act on different bodies

    • When you kick a ball, an action force acts on the ball, the reaction force is on the foot​

Defining a System

• A System is any object or collection of objects that you are studying​

  • Can be adjusted to focus on different parts​

Summary of Newton's Laws

• Newton's First law: an object in motion stays in motion, an object at rest stays at rest

• Newton's Second Law: a=F/m

• Newton's Third Law: every action has an equal and opposite reaction​