can change ! L-3 Review – inertia • Î No is required to keep an object moving with constant velocity. „ Tendency of objects to resist changes in . • What can change the velocity of an „ The inertia of an object is measured by object ? Î FORCES its – the quantity of matter in it. • is a change in velocity „ If an object is at rest is stays at rest. • forces produce „ If an object is moving with constant • for example- or air resistance velocity, it continues moving until something stops it.

The force of and gravity

• Today we will explore one force that can • All objects exert an attractive force on change the velocity of an object each other – Universal Law of Gravity • Î GRAVITY • Your weight is the attractive force that the • Everything that has mass is affected by earth exerts on you- it’s what makes things gravity fall! • All objects are pulled toward the center of • It is the most common force we have to the earth by gravity. deal with – it’s what keeps us on earth and • The sun’s gravity is what holds the solar the earth revolving around the SUN. system together.

The sun is the most massive object in the solar system, about 3 million the earth’s mass and 1000 times more massive than the most A little massive planet-Jupiter • The planets revolve around the sun in SUN approximately circular paths (Kepler) Uranus Pluto Mars • The further the planet is from the sun the longer it takes to go around (Kepler) • The to go around the sun is a year * the earth spins on its axis once every day * the revolves around the earth Mercury, Venus, Earth, Jupiter, Saturn, Neptune once every month

Astronomers have recently decided that PLUTO is too small to be Considered a planet. IN THIS COURSE PLUTO IS A PLANET!

1 What does your weight depend on? What is this thing called g? • g is something you often hear about, for example • The weight w of an object depends on its • You might hear that a fighter pilot experienced so mass and the local many g’s when turning his jet plane. strength of gravity- we • Î g is the acceleration due to gravity. call this g – the • When an object falls its increases as it acceleration due to gravity descends • Weight points toward • acceleration is the rate of change of velocity the earth’s center • g is the amount by which the speed of a falling • Sometimes down is up! object increases each second – about 10 m/s each second (9.8 m/s/s to be exact)

Example – a falling object How to calculate weight

time velocity • Weight = mass x acceleration due to gravity 0 s 0 m/s + 10 m/s 1 s 10 m/s •Or w = m x g (mass times g) + 10 m/s 2 s 20 m/s 3 s 30 m/s + 10 m/s • In this formula m is given in kilograms (kg) and g ≈ 10 meters per second per second + 10 m/s 4 s 40 m/s (m/s2), then w comes out in force units – 5 s 50 m/s + 10 m/s Newtons (N) approximately equal

You weigh more on Jupiter and example less on the moon • What is the weight of a 100 kg object? • The value of g depends on where you are, • w = m x g = 100 kg x 10 m/s2 = 1000 N since it depends on the mass of the planet ______• On the moon g ≈ 1.6 m/s2 ≈ (1/6) g on • One is equal to 0.225 lb, so in earth, so your weight on the moon is only these common units 1000 N = 225 lb (1/6) your weight on earth 2 • Often are given by the equivalent • On Jupiter g ≈ 23 m/s ≈ 2.3 g on earth, mass in kilograms, we would say that a so on Jupiter you weigh 2.3 times what 225 lb man “weighs” 100 kg. you weigh on earth.

2 Get on the scale: Free Fall How to weigh yourself spring • Galileo showed that all objects (regardless of mass) fall to earth with the same force acceleration Æ g = 10 m/s2 • This is only true if we remove the effects of m air resistance. demos • We can show this by dropping two very weight different objects inside a chamber that has the air removed. mass

Galileo’s experiments On the other hand . . .

• To test this we must • If you drop an object from a small height it drop two objects from the same height and falls so quickly that it is difficult to make an measure the time accurate measurement of the time they take to fall. • We can show experimentally that it takes H • If H isn’t too big, then less than half a second for a mass to fall 1 the effects of air meter. (demo) resistance are minimized • How did Galileo deal with this?

What did Galileo learn from the Galileo made g smaller! inclined plane experiments? inclined plane • He measured the time it took for different to fall down the inclined plane. • He found that different masses take the same h D h D time to fall down the inclined plane. • Since they all fall the same , he concluded that their accelerations must also be 2 the same. gstraight =10ms / down Can be made • By using different he was able to h small by using a discover the relation between time and distance. small h or big D ggdown=× straight • How did Galileo deal with friction? ramp down D

3 How did Galileo measure the time? Problem - Around the world • How long would it take for the B2 • Galileo either used bomber to fly around the world? his own pulse as a • The B2 can fly at Mach 0.85 clock (he was trained (subsonic) which means 0.85 to be a physician) times the speed of sound 730 mph (330 m/s). • speed = 0.85 x 730 mph=618 mph • Or, a . • Earth’s circumference = 25,000 mi • time = distance ÷ speed B2 Stealth = 25,000 mi ÷ 618mph = 40 hr Bomber

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