2.6 UNDERSTANDING GRAVITY

Is a region around the earth where an object experiences a force acting on it towards the centre of the earth.Gravitational field

It is the force of gravity that pulls all objects towards the centre of the earth.

Gravitational force acting on a unit mass in the field.Formula: g = F m

Also can be define as the ratio of the weight to the mass of the object Formula : g = W weight mSI unit Nkg-1

When an object is at rest, g (= 9.8 Nkg-1) is the earths gravitational strength acting on it.

Will decrease when the distance of the object from the centre of earth increase.gmoon = 1/6 gearthGravitational forcemass

Acceleration of an object due to the pull of the gravitational force.

The acceleration due to gravity for a freely falling body has a value of about 9.8 ms-2. (+g = object moving downward, -g = object moving upward )

Free fall:Refer to an object which is falls due to gravitational force only.Not effected by mass and shape of the object.Only occur in vacuum.Assumed that air resistance is zero.g = 9.8 ms-2

Weight is the force of gravity acting on the object.Symbol : WVector quantitySI unit Newton, NFormula : F = masince F = W, a = gW = mg acceleration due to gravity mass of object

Comparison between weight and mass

Weight, WDifferences Mass , mMeasurement of gravitational force on the bodyDefinitionMeasurement of the quantity of matter in the bodyVector quantityPhysical quantityScalar quantityDerived quantityTypes of quantityBase quantityNewton , NSI unitKilogram , kgAccording to the gravitational field strength Changing of valueConstant everywhere regardless of the place and location

Example What is the weight of an object of mass 5.0 kgOn the surface of the earthOn the gravitational of the moon ( gravitational field strength = 10 Nkg-1)

Solution :We = mg = 5.0 kg x 10 Nkg-1 = 50 NWm = mg = 5.0 kg x 1/6 ( 10 Nkg-1) = 8.33N

Wei Bo is a basketball player. His vertical leap is 0.75m. What is his take-off speed?Answer : 3.87ms-1

A coconut 1.5 seconds to fall to the ground. What is Its speed when it strike the ground?The height of the coconut tree?Answer = 15 ms-1Answer = 11.3 m

A ball is thrown vertically upwards with an initial velocity of 20 ms-1. If g = 10 ms-2 and air resistance can be neglected, findMaximum height reachedThe time taken before it reaches the ground Answer = 2omAnswer = 4s