Mechanics – gravity, weight and mass You can explain which factors acceleration depends on what the force of gravity depends on the effect of the force of gravity the difference between mass and weight

What did we learn last week? What is velocity? What is acceleration? What does a force do? Is this spinning bucket accelerating? What force is there acting? Is the earth accelerating round the sun? What force is acting?

What is Newton‘s first law? Every body stays in rest or in uniform motion unless acted upon by a force. (This means the force makes the body (or object) accelerate.)

But what does the acceleration depend on? How does the acceleration change with the force you apply? The acceleration (a) is proportional to the force (F) How does the acceleration change with the mass of the object? The acceleration (a) is inversely proportional to the mass (m). Can you work out the equation?

Space research Watch the video and answer these questions. What is Newton‘s second law? What is the difference between mass and weight? What does gravity do? Which will land first if you drop them? a) or And why? b) or And why?

Newton‘s second law We know that: The acceleration (a) is proportional to the force (F) The acceleration (a) is inversely proportional to the mass (m). You can write this as: which means that F= m. a a = F or a = F : m m

Newton‘s second law

What do you know about gravity? Gravity makes things fall. Gravity gives things their weight.

Gravity makes things fall When you drop two objects the acceleration due to gravity is always the same. If they are the same shape, the air resistance is the same so they hit the ground at the same time. Two bottles (one full and one empty) hit the ground at the same time because the acceleration due to gravity is the same and the air resistance is the same. One bottle and one piece of paper hit the ground at different times because air resistance is different. m/watch?v=5C5_dOEyA fk

Mass and weight You measure mass in kilograms (kg). Mass tells you how much of something there is (or how many particles there are of something) It does not change. You measure weight in Newtons (N). The weight tells you the force due to gravity of a body at a certain place. – Your weight would be less on moon than on the earth because gravity is less on the moon. – The weight of an object on earth is usually about 9.8 x its mass.

Summary When any object falls, it always falls because the force of __________is pulling it down. Its ____________is always equal to g (on earth g = 9.8m/s 2 ) This means that heavy and light things should fall with the same acceleration and therefore the same _________ after the same amount of time. But air resistance can make a difference – for example a feather or a parachute falls more _________ because _______________ slows them down. So a full can and an an empty can which are the same shape fall with the same acceleration.

Summary When any object falls, it always falls because gravity is pulling it down. Its acceleration is always equal to g (on earth g = 9.8m/s 2 ) This means that heavy and light things should fall with the same acceleration and therefore the same speed after the same amount of time. But air resistance can make a difference – for example a feather or a parachute falls more slowly because air resistance slows them down. So a full can and an an empty can which are the same shape fall with the same acceleration.

Vocabulary Force – Kraft Gravity – Gravitation Mass – Masse Weight – Gewichtskraft