Newton

1
Unit 6-2

• Newtons Law of Universal Gravitation

2
Objectives

• MCAS Physics Frameworks 1.1.11 (bold) Students
  should conceptually understand Newtons Law of
  Universal Gravitation
• Objectives I G 1. Students should know Newtons
  Law of Universal Gravitation so they can
• a) Determine the force that one spherically
  symmetrical mass exerts on one another
• b) Determine the strength of the gravitational
  field at a specific point outside a uniform
  sphere, and calculate how the field at the
  surface depends on the radius and density of the
  sphere.

3
Recap

• From last lesson we learnt Keplers Three Laws
• 1. Planets travel in elliptical orbits around the
  sun
• 2. They sweep out equal areas in equal times
• 3. R13/R23 T12/T22

4
Newton

• In 1666, Sir Isaac Newton, a brilliant
  mathematician and scientist, was living in rural
  England because the plague had closed the
  Universities. He used mathematical arguments to
  show that the Force F varies with the inversely
  with the square of the distance between the
  planet and the sun
• F a 1/d2

5

• The sight of a falling apple made him think that
  the force that pulled it to the Earth (and vice
  versa) might be the same as the force that kept
  the planets in orbit around the sun.
• The force (F) between two masses m1 and m2 must
  be proportional to the product of their masses
  and inversely proportional square of their
  distance (d) apart

6
Newtons Law of Universal Gravitation

• By including a Universal Gravitational Constant G
  6.67 x 1011 N m2/kg2 . This gives us the
  equation
• F G m1m2/r2

7
Calculations

• What is the attraction between two people each of
  mass 50 kg a distance of 2 meters apart?
• Calculate the force of attraction between the sun
  and Earth, if the Earth has a mass of 5.979 x
  1024 kg and the mass of the sun is 1.991 x 1030
  kg and the distance of the earth from the sun is
  1.4957 x 10 11

8

• Calculate the force of attraction between a mass
  of 10 kg and the Earth if the mass of the Earth
  is 5.979 x 10 24 kg and the radius of the earth
  is 6.3713 x 10 6 m
• Since acceleration Force (F) divided by mass
  then gravitational acceleration (g) for a mass m
  on a planet of mass mp
• g F/m (Gmpm/r2)/m
• g Gmp/r2
• Use the information on p159 of your textbook to
  calculate the acceleration due to gravity on each
  of the planets of the solar system
• Homework H6-2 Answer questions 1-5 and 20-25 p172
• http//www.bbc.co.uk/science/space/playspace/games
  /jigsaw/jigsaw.shtml