Mass, Weight and Gravity Physics Mr. Maloney

© 2002 Mike Maloney Objectives Students will be able to  explain how mass and weight are related to each other.  explain where gravitational forces arise from.  develop and use a universal and local model of gravity  apply the local model of gravity to describe a model of freefall

© 2002 Mike Maloney Mass and weight  Mass and weight are different from each other.  Mass is how much stuff (matter) you are made out of.  Weight is the gravitational force exerted by a large body (Earth)

© 2002 Mike Maloney Mass and Weight  When you get on a scale what are you measuring … mass or weight?  Right WEIGHT  What about when you put an object on a triple beam balance?  MASS it is.

© 2002 Mike Maloney What is Weight?  The full model for the force of gravitational attraction between two objects as discovered by Newton looks like:

© 2002 Mike Maloney What is Weight?  The full use is saved for AP, but we can use an approximation of it if we are near the surface of the Earth. At the surface of the Earth, what is constant in this equation?  The full model for the force of gravitational attraction between to objects as discovered by Newton looks like:

© 2002 Mike Maloney What is Weight?  What is the value of a constant created by combining the three constant values here?  9.8 m/s 2  We will call this little g (g = 9.8 N/kg)  G = x m 3 / (kg∙s 2 )]  M E = × kg  r = m NOTE G is a UNIVERSAL constant, it is the same everywhere you go in the universe. g is a LOCAL constant. It is only good at the surface of the Earth. Other planets have their own g, based on their size and mass. As you move father away from the surface of the Earth, g changes.

© 2002 Mike Maloney Mass and Weight  So what is the weight of a 2 kg mass?  Fg = m ∙ g = 2 kg ∙ 9.8 N/kg  Right 19.6 N  What is the mass of a 1000 N person?  F g = m ∙ g  m = F g /g = 1000 N / 9.8 N/kg  Right again, about 102 kg

© 2002 Mike Maloney  Let’s look at Newton’s Law of Gravitation one last time.  We used it to find the Force of the Earth on you, or anything near the Earth.  But we learned from Newton’s 3 rd Law that forces are the way things interact.  What does that really mean?  WE ALSO PULL ON THE EARTH!  So, why do we fall to the Earth if we are supposedly pulling it towards us? Mass and Weight

© 2002 Mike Maloney Other local g’s Local gravitational constants  g Earth : 10 N/kg.  g Moon : 1.6 N/kg  g Jupiter : 26 N/kg  g you : ~ N/kg F G,green F G,red F G,green F G,red F G,orange F G,blue

© 2002 Mike Maloney Consequence (freefall)  Let’s now apply Newton’s 2 nd law and our understanding of gravity to an object in freefall.  What is the acceleration of an object in freefall? FgFg

© 2002 Mike Maloney Consequence (freefall)  So even though a g and g are the same, they are really different.  g is the local gravitational constant.  a g is the acceleration of an object when the only force acting on it is gravity.  It is better to say g = 10 N / kg and a g = 10 m/s 2.

© 2002 Mike Maloney Objectives Are you able to  explain how mass and weight are related to each other.  explain where gravitational forces arise from.  develop and use a universal and local model of gravity  apply the local model of gravity to describe a model of freefall