Gravity and Free Fall

 Gravity - attraction between objects with mass  Gives weight  Causes free fall Gravity

Weight  Weight – force caused by gravity F w = mg  measured in N  depends on mass, gravity, and location  weight is less on the moon due to less gravity Weight Gravity Mass

 Mass (amount of matter)  measured in kg  does NOT depend on gravity or location  mass is the same on Earth and on the moon Mass

 Gravity on earth = 9.8 m/s 2  Gravity on the moon = 1.6 m/s 2  F w = mg  The weight of 1 kg object ON EARTH (1 kg)(9.8 m/s 2 ) = 9.8 N  The weight of 1 kg object ON THE MOON (1 kg)(1.6 m/s 2 ) = 1.6 N

Practice Unless the problem says otherwise, assume you are on Earth, where g = 9.8 m/s 2  What is the weight of a 100 kg person? ( 100 kg) x (9.8 m/s 2 ) = 980 N  What is the mass of a rock that weighs 196 N? ( 196 N) ÷ (9.8 m/s 2 ) = 20 kg

What’s your weight on Mars?  Calculate and compare the weight of a 100 kg person on Earth and on Mars. (g = 3.7 m/s 2 on Mars) Earth = (100 kg) × (9.8 m/s 2 ) = 980 N Mars = (100 kg) × (3.7 m/s 2 ) = 370 N  The MASS did NOT change!!!

Free Fall  Free Fall – when gravity is the only force acting on the object  Mass of object does not affect free fall  Falling objects speed up 9.8 m/s 2 for every second they fall

Practice The force of gravity on the surface of Mercury is much less than on Earth. A rock has a weight of 100 N on Earth. What do you know about the weight of the rock on Mercury? A. The rock will weigh exactly 100 N B. The rock will weigh more than 100 N C. The rock will weigh less than 100 N D. There is not enough information

Practice The force of gravity on the surface of Mercury is much less than on Earth. A rock has a weight of 100 N on Earth. What do you know about the weight of the rock on Mercury? A. The rock will weigh exactly 100 N B. The rock will weigh more than 100 N C. The rock will weigh less than 100 N D. There is not enough information

Practice A chair has a mass of 10 kilograms on Earth. What is the mass of the chair on Mercury? A. Exactly 10 kg B. More than 10 kg C. Less than 10 kg D. Not enough information

Practice A chair has a mass of 10 kilograms on Earth. What is the mass of the chair on Mercury? A. Exactly 10 kg B. More than 10 kg C. Less than 10 kg D. Not enough information

Practice If an astronaut, with a mass of 90 kg, has a weight of N on the Moon, what is the value of the Moon’s gravitational acceleration? (145.8 N) ÷ (90 kg) = 1.62 m/s 2