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.

earth

Gravitational field

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

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

m SI unit Nkg-1

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

Will decrease when the distance of the object from the centre of earth increase.

gmoon = 1/6 gearth

Gravitational force

mass

• 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 = ma since F = W, a = gW = mg acceleration due to gravity

mass of object

Comparison between weight and mass

Weight, W Differences Mass , m

Measurement of gravitational force on the body

Definition Measurement of the quantity of matter in the body

Vector quantity Physical quantity Scalar quantity

Derived quantity Types of quantity Base quantity

Newton , N SI unit Kilogram , kg

According to the gravitational field strength

Changing of value

Constant everywhere regardless of the place and location

Example What is the weight of an object of mass 5.0 kg1. On the surface of the earth2. On the gravitational of the moon ( gravitational field strength = 10 Nkg-1)

Solution :1. We = mg

= 5.0 kg x 10 Nkg-1 = 50 N

2. Wm = mg = 5.0 kg x 1/6 ( 10 Nkg-1) = 8.33N

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

Answer : 3.87ms-1

2. 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?

1) Answer = 15 ms-1

2) Answer = 11.3 m

3) 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, find

Maximum height reached The time taken before it reaches the ground

a) Answer = 2om

b) Answer = 4s