1. and Free Fall

2. Gravity  The attraction between objects with mass  Gives smaller objects weight  Causes free fall

3. Weight  Weight is a force caused by gravity (Remember Newton’s 2 nd Law: F=ma) F w = mg F w = weight m = mass g = acceleration due to gravity

4. a cceleration d ue t o g ravity ( g ) ‘g’ is different on different planets and at different altitudes, so your weight depends on:  Mass  Acceleration due to gravity (g)  Location (Earth, Mars, moon…)

5. O n E arth :  Gravity is g = 9.8 [ m/s 2 ] or 9.8 [N/ kg]  At Earth's surface, gravity exerts a force of 9.8N on every kilogram of mass  What is the weight of a 1kg mass on Earth? F w = mg = (1kg)(9.8N/kg) = 9.8N  What is the weight of a 2 kg mass on Earth? F w = mg = (2kg)(9.8N/kg) = 19.6N

6. Mass vs. Weight Mass – The amount of matter in an object  measured in [kg]  does NOT depend on gravity or location  your mass is the same on Earth and on the Moon Weight - a force due to gravity’s pull on an object  measured in [N]  depends on mass, gravity and location  you weigh less on the moon due to less gravity (g = 1.62 N/kg on moon and g = 9.8N/kg on Earth)

7. What’s your weight on Mars? Looking to lose weight? Try the MARS DIET!!  Calculate and compare the weight of a 100kg person on Earth and on Mars. (g = 3.7 N/kg on Mars) Earth: F w = mg = (100)(9.8) = 980N Mars: F w = mg = (100)(3.7) = 370N (much less!)  The MASS did not change!!!

8. Free Fall  When the only force acting on an object is gravity, it is in free fall!  Object has not been projected (thrown, launched…)  Ignore air resistance (no drag)  In free fall problems, g = 9.8[m/s 2 ]  This means falling objects speed up at 9.8 [m/s] each second they fall.

9.  If you drop ANY two objects, they will hit the ground at the same time in a vacuum  vacuum = The absence of all air and wind resistance.  Why? Because ALL objects accelerate at g = 9.8 [m/s 2 ]!  VIDEO: http://mit.tv/yeuWADhttp://mit.tv/yeuWAD Free Fall in a vacuum

10.  Acceleration (g) is constant; speed increases at the same rate over time.  The Mass of and object doesn’t affect free fall! Free Fall

11. Practice (Unless the problem says otherwise, assume you are on Earth, where g = 9.8N/kg)  What is the weight of a 100kg person? F w = mg = 100 x 9.8 = 980N  What is the mass of a rock that weighs 196N? F w = mg 196= 9.8m(divide both sides by 9.8) m = 20kg

12. Activities HOOK: http://www.exploratorium.edu/ronh/weight/ GIZMO: Weight and Mass (on Earth, Mars, Jupiter, and the moon) http://www.explorelearning.com/index.cfm?method=cReso urce.dspView&ResourceID=653&ClassID=2710305