FORCE – Newton’s Laws of Motion

Sir Isaac Newton’s (1643 - 1727) Contributions

• Calculus

• Light is composed of rainbow colors

• Reflecting Telescope

• Laws of Motion

• Theory of Gravitation

• Phosphorous

Kinematics

the description of how objects move without considering the reasons why they move

Dynamics

The study of the causes of motion and the reasons why a change in motion may occur

What is a force?

• A pull or a push

• May accelerate

• May change shape

• Nothing may happen

Definition of Force

• A push or pull exerted on an object that can cause a change in motion

• Is a vector quantity (needs a direction)

• Symbol: F

• Units: Newtons (N)

  • Defined as the amount of force required to give a 1 kg mass an acceleration of 1 m/s2

Types of Forces - General

Contact Force

a force that exists only when contact is being made

Ex: pushing a plate across a table, holding a glass

Types of Forces - General

Field Forces

a force that exists without contact being made

Ex: a book falling off a table,

Newton’s Laws of Motion

First Law (Law of Inertia)

An object at rest tends to remain at rest and an object in motion tends to remain in motion (in a straight line at a constant speed - no acceleration) unless acted upon by a net force.

Inertia

• The property of an object that resists changes to its motion.

  • When stationary, difficult to move

  • When moving, difficult to stop

• It depends on mass.

Inertia Examples

• Sudden acceleration in a car

• Sudden deceleration in a car

Inertia Demonstrations

• Quarter on card

• Mass on string

• Football

When calculating force..

remember, force has direction.

Newton’s Second Law

• When a net (or unbalanced) force acts on an object, it accelerates in the direction of the net force.

• The acceleration varies directly to the net force on it and inversely to its mass.

Scales and Weight

Bathroom scales measure weight - not mass

◦ Mass is the amount of matter an

object has.

◦ Weight is the force of gravity on the object.

Weight

• On or near the Earth’s surface, the acceleration due to gravity, g, is constant

  • g = 9.8 m/s2

• Force of gravity: Fg = mg

• Also called an object’s weight

• Always directed downward towards the center of the planet

Apparent Weight

the force an object experiences as a result of all the forces acting on it, giving it acceleration

Weightlessness

• What would the scale read if the cable on the elevator broke?

• Both you and the scale would be accelerating at the same rate

Newton’s Third Law

For every action force there exists a reaction force that is equal in magnitude but opposite in direction.

Recoil

• Ex. Identify the action-reaction forces in the following situations

  • A badminton racquet hitting a birdie

  • A boat being rowed

  • A gun being fired