Forces

Force: A Push or Pull in a Given Direction Forces are vector quantities since they have both magnitude (size) and direction.

Gravity Force (Weight): The Earth’s Pull on an Object Mass is the amount of material or stuff in an object. Mass is measured by a balance. Mass is measured in grams (g) or kilograms (kg).

Inertia : The Tendency of an Object to Resist a Change in Motion An object’s inertia is directly proportional to its mass.

Measuring Mass with an Inertial Balance

Gravity Force (Weight): The Earth’s Pull on an Object Gravity is the earth’s pull on an object. This pull is an object’s weight. Weight is a force measured by a spring balance. Weight is measured in newtons (N).

The Relationship Between Mass and Weight

Defining The Force Unit, Newton (N) A newton is the amount of force that will accelerate a 1 kg mass at a rate of 1m/s/s (1m/s2).

What is Earth’s Gravity Force on 1 Kg? Since the earth accelerates all objects at 9.8 m/s2, there must be a force of 9.8 N on 1 kg.

Calculating an Object’s Weight From its Mass An object’s weight in N is equal to its mass in kg x 9.8 N/kg. W = m x 9.8 N/kg

Mass vs Weight The mass of an object remains the same anywhere in space, but the weight of an object varies depending on the gravity pull on the object.

Gravitational Field Strength The Gravitational Field Strength is the force of gravity on a mass of 1 kg at some point near a large mass (like a planet). The gravitational field strength varies on planet Earth and from planet to planet. It also varies (gets less) as a 1 kg mass is moved away from planet Earth. The units of gravitational field strength are N/kg

Gravity: A force of attraction between two masses

Gravity: Depends on Quantity of Mass and Distance

Newton’s Law of Universal Gravitation The force of gravity is directly proportional to the product of the masses and inversely proportional to the square of the distance between their centres.

Gravity Varying With Mass and Distance

Cavendish’s Experiment to Measure G, the Gravitation Constant

A Escaping Gravity – orbits and final escape are dependent on velocity of object.

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