Gravity

Describe the gravitational forceDefine weight and relate to the

gravitational forceDistinguish between weight and

massDetermine the weight of objects.

Objectives

Gravitational Force

Attractive force between two objects.

1. If the masses of the objects increase, the magnitude of the force increases.

2. If the distance between the objects increase, the magnitude of the force decreases.

GRAVITATIONAL FORCE

Thus you cannot feel the gravitational force between you and the book you are reading because your mass and the mass of the book are not significantly big.

You cannot feel the gravitational force between you and the sun because the sun is too far ( distance is too big)

ButYou can feel the gravitational force between you and earth ( mass of earth is big and distance is relatively not big)

This force is the reason you feel yourself pulled towards the Earth.

The gravitational force that you experience depends on the mass of the Earth, and on the separation between you and the mass of the Earth.

Gravitational Force

Do all objects experience the

same gravitational force?

IN FACT,

No, the gravitational force

depend on the mass of the object

of the earth’s surface

Gravitational Force

Example of Gravitational Forces

- Gravitation is responsible for keeping the Earth and the other planets in their orbits around the Sun.

- Earth's gravity pulls other objects twords it's center without touching them. An objects weight is the amount of gravitatinal force.

GRAVITATIONAL FORCE

According to the second law of motion, whenever a force is acting on an object, it produces acceleration in it.

So:If we consider the

example of the stone (Rf: the figure at the right), the gravitational force produces an acceleration on both the earth and the stone.

GRAVITATIONAL ACCELERATION

•This acceleration usually called g is equal to 9.8m/s² and is equal on all objects on the earth’s surface.

g= 9.8m/s²• This acceleration changes from one planet to another,. On the moon, the gravitational acceleration is much less.

Weight and gravitational forceEven if you are

not moving , the earth is pulling you downward . Whether you are moving, falling or standing, Earth exerts a gravitational force on you. This force is called: weight.

Weight and gravitational force

According to Newton’s second Law ,Force (newtons)= mass (kg) x

acceleration (m/s²)Where:

Force is the weight (gravitational force)

Acceleration = gravitational acceleration

= 9.8m/s²

Thus, we can write:

Weight (N)= mass (kg) x g (m/s² or N/kg)

Applications:

1. Calculate the weight of a 5 kg object (g= 9.8m/s²)

2. Find the mass of an object if its weight is 4N.

3. Calculate the weight of a 400 g object on the moon if the gravitational acceleration on the moon is : 1.67 m/s².

+ Solve questions: p: 82 no.6, 7 & 8

Difference between mass and weight

In everyday life people often confuse mass with weight. In science we should distinguish between them. The weight of an object is the gravitational force acting on it by the Earth. Like other force, weight is measured in newtons. If the object falls freely, its weight gives it an acceleration of g

Difference between mass and weight

Mass is the measure of amount of matter in a certain substance.

Mass is a physical quantity with no direction.

Mass doesn't from a place to another

Mass is measured using a balance (scale): its basic unit is: Kg

Weight is the gravitational force between the earth and the object.

Weight is a force which is always downward.

Weight changes from a place to another.

Weight is measured using a spring balance: its basic unit is Newton.

MASS AND WEIGHT

The picture at the right shows a spring scale used to measure the weight of a substance and a balance beam used to measure the mass of an object

Look at the following diagram and conclude:

DO NOT FORGET

Gravitational force is the force of attraction between two masses.

Weight is the gravitational force that pulls objects downward towards the center of the earth.

Mass is different than weight: it is a constant quantity which only depends on the amount of the substance

For Next Time

Solve section review p: 82 no. 1-5Download PowerPoint

presentationRevise section : 3.1 & 3.2System wide quiz 2:- Chapter 1: section 1.3- Chapter 2: sections 2.1 , 2.2 &

2.3

Good Luck!!!