Forces and Newton’s 3 Laws of Motion

Robert StrawnCompiled 10/16/11

A force is any push or pull that one object exerts on another that causes a change in

motion, direction, or shape.The unit for force is the Newton (N) and is equal to

1 kg x m/s2

According to newton’s 2nd LawF = ma

Weight is equal to mass times acceleration of gravity.W= Fg = mg

Mass is measured in kg and does not change as height changes. Weight decreases as you move away from the center of Earth.

Confusion of Mass and Weight

• Many students of physics confuse weight with mass.

• The force of gravity acting upon an object is sometimes referred to as the weight of the object.

• The mass of an object refers to the amount of matter that is contained by the object.

• The weight of an object is the force of gravity acting upon that object. Mass is related to how much stuff is there and weight is related to the pull of the Earth (or any other planet) upon that stuff.

About Force• Whenever there is an interaction between two

objects, there is a force upon each of the objects. When the interaction ceases, the two objects no longer experience the force.

• A force is a vector quantity. • Forces only exist as a result of an

interaction.• Force is measured using the standard

metric unit known as the Newton (N). • For simplicity sake, all forces (interactions)

between objects can be placed into two broad categories: contact forces, and forces resulting from action-at-a-distance

Contact Forces• Those types of forces which result when the two

interacting objects are perceived to be physically contacting each other. Examples of contact forces include frictional forces, tensional forces, normal forces, air resistance forces, and applied forces.

• Contact Forces– Frictional Force– Tension Force– Normal Force– Air Resistance Force– Applied Force– Spring Force

Actions at Distance

• Those types of forces which result even when the two interacting objects are not in physical contact with each other, yet are able to exert a push or pull despite their physical separation. Examples of action-at-a-distance forces include : gravitational forces, Electric forces, and magnetic forces

Drawing Free-Body Diagrams

Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation. A free-body diagram is a special example of the vector diagrams.

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Forces are Vectors so Directions are Important

Force #1

Force #2

Force #1Force #2

Total Force

Total Force = 0

Net Force

Net Force is an unbalanced force when all of the forces acting on an object are combined. It is necessary to find the net force when there are two or more forces acting on an object. If all of the forces acting on an object cancel each other out there is no net force and the forces are balanced.

Balanced Forces

• The book is said to be at equilibrium.

Balanced Forces• Balanced forces do not cause change in

motion• The Total Forces applied to an object

produce a net force of zero• They are equal in size and opposite in

direction

Unbalanced Forces

• In this case, an unbalanced force acts upon the book to change its state of motion.

Unbalanced Forces

3 N, right – 6 N, left = 3N, left

Unbalanced Forces• An unbalanced force always causes a change in

motion

• When unbalanced forces act in opposite directions you can find the net force– Net force

• Magnitude– The difference between the two forces

• Direction– Direction of the largest force

Normal Force• Normal Force is a force

exerted by one object on another in a direction perpendicular to the surface of contact.

• The normal force is always perpendicular to the surface but is not always opposite the force of gravity.

Newton's First Law of Motion

• 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.

Isaac Newton• Newton’s 3 laws– 1st : Law of Inertia An object at rest will

stay at rest, an object in motion will stay in motion, unless acted upon by an outside force.

– 2nd : F = ma An object will accelerate in the presence of an unbalanced force. The larger the mass the smaller the acceleration.

– 3rd : For every action there is an equal and opposite reaction.