Date: October 2020
Friction Is a force that opposes movement. This means that friction slows down or sop moving objects. Without friction we would not be able to walk, drive a car, ride a bicycle or lift things up. Friction slows down all moving objects. Air rubs against a car or a plane and slows it down. Water rubs against a boat or ship and slows it down. A spaceship can go on forever as there is no air in space to slow it down. Friction can be reduced by lubricating the surface with oil or grease, using rollers or ball bearings or smoothing the surface. Gravity is the invisible force that pulls objects to the center of the earth. The point at which an objects weight is concentrated is called the centre of gravity. When you are standing, for example, the centre of gravity is found in the middle of the body, which is the navel. Understanding the concept of mass is further necessary to understand gravity. Mass does not change and remains the same in every situation. Weight is related to mass but it is incorrect to use the terms interchangeably. Weight is the measure of a force of gravity on an object or the gravitational force exerted on an object. It measures how strongly gravity pulls on the object. In everyday situations the word "weight" is continued to be used when the word "mass" is meant. For example, we say an object "weighs one kilogram" even though kilogram is a unit of mass. Mass is a measure of the amount of material in an object. The mass of an object on earth is exactly the same as the mass of the object on the moon. In contrast the weight of an object on the moon is one-sixth of its weight on earth. For example an object that weighs 100kg (how hard it presses down on the scale) on Earth will only weigh 60kg on the moon. The reason is that the force of gravity is one-sixth of what it is on Earth. The greater the force of gravity on an object the greater the weight. Similarly, an astronaut can jump for a longer distance and higher height on the moon than on Earth because there is less gravity on the Earth pulling down. Falling Things The shape of things determine how fast they fall. Two stones of different sizes dropped from the same height will fall at the same speed and hit the ground at exactly the same time. Although one stone is heavier it doesn't fall faster than the other. The weight of an object does not determine hoe fast it falls. Activity Answer the following questions 1. Friction is a force that opposes _______________. That means that friction slows down or ___________ moving objects 2. Without friction we would not be able to ___________________________ ____________________________________________________________________ _____________________________________________________________________ ___________________________ 3. Friction ___________________ all moving objects. 4. Air rubs against a car or a plane and slows it down. Water rubs against a boat or ship and and slows it down. A spaceship can go on forever in outer space because there is no ______________. 5. Friction can be reduced by _______________________________________ ________________________________________________________________ __________________________________________________. 6. Gravity is the invisible force that pulls things to the centre of the _______________. 7. The point at which an objects weigh is concentrated is called _______________ ___________. 8. When you are standing, for example, the centre of your gravity is your ______________ _______. 9. Give two definition of weight 1.______________________________________________________________ ___________________________________________________________________ _____________________________. 2. __________________________________________________________________ ____________________________________________________________________ _________________________. 10. The measure of the amount of material in an object is called it's ____________________.