Examine the forces exerted on objects by gravity

Examine the forces exerted on objects by gravity Standard IV – Objective 2

Force = Mass x Acceleration (Newton) Work = Force x Distance (Joule) Power = Work/Time (Watt)

Gravity – what is it? Simply put, it is the force that pulls objects toward Earth Universally put, it is the mutual force between all objects in the universe

What affects gravity? Gravity is affected by distance and mass The greater the distance between objects, the lower the force of gravity The greater the mass of the object, the greater its gravity

Gravity All objects have a force that attracts them towards each other - this is gravity Even you attract other objects to you because of gravity, but you have too little mass for the force to be very strong (gravity only becomes noticeable when there is a really massive object like a moon, planet or star) When you jump, the gravitational force of the Earth pulls you down, but your gravitational force also pulls the Earth towards you! You just don’t notice this effect because your mass is so much smaller than Earth’s

Mass A measure of the amount of matter in an object Measured in grams or kilograms It stays the same for an object no matter where that object is in the universe

Weight The force of gravity on an object at the surface of a planet is its weight When you step on a scale, you are measuring the force with which Earth is pulling you so your weight varies throughout the universe Measured in Newtons (1 N = 1 kg x 1 m/s 2)

Gravity Earth’s gravity pulls objects towards its center - it has a force of 9. 8 N/kg The moon has a gravity of just 1. 6 N/kg (as it is much smaller)

Acceleration due to gravity 9. 8 m/s/s (also written as 9. 8 m/s 2) at Earth’s surface

Weight = mass x acceleration due to gravity An apple weighs about 1 newton A 50 -kg person weighs 50 kg x 9. 8 m/s 2 or 490 newtons (on Earth’s surface)

Free fall When the only force acting on a falling object is gravity, the object is in free fall An object in free fall accelerates as it falls A falling object falls 9. 8 m/s faster every second that it falls

Free fall continues Therefore, after 3 seconds an object is falling at a speed of 29. 4 m/s, after 4 seconds it is falling at 39. 2 m/s All objects fall at the same rate regardless of mass

Air resistance • Even though all objects are supposed to fall at the same rate, we know they do not • All objects falling through air experience a type of fluid friction called air resistance • Air resistance (being friction) is opposite to the direction the object is falling and eventually prevents the object from accelerating faster • The maximum velocity any object can reach in free fall is called TERMINAL VELOCITY

More air resistance The greater the object’s surface area, the greater the air resistance When an object can not accelerate any more because of air resistance, it is said to be at terminal velocity In Earth’s atmosphere, leaves fall slower than rocks In a vacuum, they both fall at the same rate

Projectile motion Thrown objects are under the same gravitational acceleration as objects falling straight down so eventually they will fall straight down also For example, a cannon ball shot horizontally and one dropped vertically from the same height as the cannon will both hit the ground at the same time

Earth Gravity on Earth = 9. 8 N/kg Say you have a mass of 100 kg on Earth Can you work out your weight? Remember: weight (N) = mass (kg) x gravity (N/kg) Your weight on Earth is 100 kg x 9. 8 N Your weight = 980 N

Moon Gravity on moon = 1. 6 N/kg Say you have a mass of 100 kg on Earth What is your mass on the moon? Remember, mass is the amount of stuff there is (you still have two arms, two legs etc… on the moon) Mass = 100 kg on moon What is your weight on the moon (weight = mass x gravity)? Your weight on the moon is 100 kg x 1. 6 N Your weight = 160 N

Your mass & weight See if you can work out your mass and weight on the Earth and on the moon Remember: n Gravity on Earth = 9. 8 N/kg n Gravity on the moon = 1. 6 N/kg

Gravity Complete the gravity on other planets worksheet

Gravity Although your mass is always the same wherever you are, you would weigh the most on Jupiter, and the least on Pluto

Weight Bingo

Questions 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. On Mercury (3) the mass of 10 kg = On Venus (9) the mass of 5 kg = On Earth (10) the mass of 5 kg = On Mars (4) the mass of 4 kg = On Jupiter (23) the mass of 2 kg = On Saturn (9) the mass of 9 kg = On Uranus (8) the mass of 10 kg = On Neptune (11) the mass of 50 kg = On Pluto (0. 4) the mass of 90 kg = On Mercury (3) the mass of 30 kg = On Venus (9) the mass of 15 kg = On Earth (10) the mass of 25 kg = On Mars (4) the mass of 100 kg = On Jupiter (23) the mass of 10 kg = On Saturn (9) the mass of 20 kg = 30 45 50 16 46 81 80 550 36 90 135 250 400 230 180

Questions 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. On Uranus (8) the mass of 16 kg = On Neptune (11) the mass of 11 kg = On Pluto (0. 4) the mass of 50 kg = On Mercury (3) the mass of 70 kg = On Venus (9) the mass of 45 kg = On Earth (10) the mass of 12 kg = On Mars (4) the mass of 60 kg = On Jupiter (23) the mass of 5 kg = On Saturn (9) the mass of 52 kg = On Uranus (8) the mass of 95 kg = On Neptune (11) the mass of 54 kg = On Pluto (0. 4) the mass of 80 kg = On Mars (4) the mass of 21 kg = On Jupiter (23) the mass of 29 kg = On Venus (9) the mass of 65 kg = 128 121 20 210 405 120 240 115 468 760 594 32 84 667 585