Gravity and motion


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  • This slide shows the force on the ping pong ball when dropped.
  • To stop it falling an upward force on the ball is needed. This is supplied when they suck.
    If the ball does not fall the upward force must be equal to the force of gravity on the ball.
    Ask them to explain why the diagram shows this. (Arrows the same length.)
  • Gravity pulls objects towards the Earth.
    In Australia people are attracted to the Earth as we are – they do not fall off!
    The children might think people should be standing on their heads.
  • This is a story about air resistance.
    Ask what has happened? Hippo does not have a parachute and is falling.
  • What is happening?
    The force of gravity is pulling Hippo down. He will be moving faster and faster! (The word acceleration is not on the curriculum.)
    Ask what could save Hippo? They will suggest a parachute.
  • Will the parachute in the picture help?
    Get them to explain that the parachute would not catch the air and slow Hippo down.
  • Ask if Hippo is safe. – Still falling very quickly.
  • Ask what the force arrows show
    They should spot that the force of the air resistance is equal to the force of gravity on Hippo.
    Ask them what change can be made so that Hippo slows down and is not hurt when he reaches the ground.
    This could lead into an investigation if wanted by the teacher.
  • Get them to suggest that Hippo will slow if the parachute is larger.
    (This could lead into an investigation with the class teacher at another time.)
  • Astronauts on the Moon are not pulled towards the Moon with as large a force as on Earth.
    Note they will think there is no gravity on the Moon.
    Point out there is no air but there is gravity.
    The space suits enable the astronauts to breathe and keep on living.
    Also they have very heavy boots.
  • Which answers are correct?
    - Because they weigh very much less than on Earth
    - To stop huge bounces
    - The Moon’s gravity is very much less than the Earth’s gravity
  • Einstein thought more about gravity
    He answered “what happens when - -?”
    But no-one knows “how?” and “why?”
  • Explain that scientists ask questions.
    Then try to find the answers.
    There are lots of questions that we do not know the answer to.
    So if they become a scientist they might be able to explain some of the puzzles.
  • Gravity and motion

    1. 1. Gravity and Motion
    2. 2. 2 DO TRY THIS AT HOME Lifting a ping pong ball . straw Suck It! You can use a drinking straw to lift a ping pong ball by sucking on the opposite end of the straw.
    3. 3. 3 The force of Gravity Should Cause the Ping pong ball to fall The arrow represents the force of gravity
    4. 4. 4 The ball does not fall when your sucking force = force of gravity straw Suck gravity
    5. 5. Isaac Newton • Sir Isaac Newton was the first person to hypothesize that the force that pulls an apple to the ground also pulls the moon toward Earth, keeping it in orbit.
    6. 6. Gravity • Gravity is a force that attracts all objects toward each other. • The force of gravity is measured in units called Newtons (N). • The strength of gravity between two objects depends on two factors: 1. masses of the objects (If mass increases, force also increases) 2. distance between the objects (If distance increases, force decreases)
    7. 7. Gravity
    8. 8. Gravity
    9. 9. Gravity The greater the mass, the greater the forceforce The greater the distancedistance, the lessless the force
    10. 10. Newton’s Law of Universal Gravitation • Any two objects attract each other with a gravitational force, proportional to the product of their masses and inversely proportional to the square of the distance between them. • The force acts in the direction of the line connecting the centers of the masses.
    11. 11. Newton’s Law of Universal Gravitation
    12. 12. Newton’s First Law of Motion • An object at rest will remain at rest unless acted on by an unbalanced force. • An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force. • This law is often called "the law of inertia".
    13. 13. Why do Earth and the moon remain in their orbits? • Inertia and gravity combine to keep Earth in orbit around the sun and the moon in orbit around the Earth. • A combination of gravity and inertia keeps the moon in orbit around the Earth. If there were no gravity, inertia would cause the moon to travel in a straight line. If only gravity existed, the earth would be pulled into the sun.
    14. 14. 15 Gravity down under • People do not fall off the Earth
    15. 15. Mass vs. Weight • Mass- is the amount of matter in an object • Weight- is the force of gravity on an object • The greater the mass the greater the force (weight) • Force = Mass X Acceleration (F=ma) • Acceleration due to gravity (g)= 9.8 m/s/s or 9.8 m/s2 • Force of Gravity (Fg) or Weight (Fw) = Mass X Acceleration due to Gravity • Fg or Fw = m g
    16. 16. 17 Hippo jump
    17. 17. 18 Force of gravity Mass of Hippo is 3000 kg Fg = m X a Fg = 3000 kg X 9.8 m/s2 Fg = 29400 kgm/s2 Fg = 29400 N If we could put a scale under the Hippo right now, could you measure this weight? Hippo jump
    18. 18. 19 Saved?
    19. 19. 20 Hippo is still falling quickly
    20. 20. 21 Hippo is still falling quickly but not getting faster • How do we know? Force of gravity air resistance
    21. 21. 22 How can we slow Hippo down? • What can you change?
    22. 22. 23 On the Moon NASA pictures
    23. 23. 24 Very heavy boots- Why? Answers? • To stop huge bounces • To stop them floating away • The Moon’s gravity is very much less than the Earth’s gravity • They weigh very much less than on Earth
    24. 24. 25 GRAVITY • Things fall • Things have weight • Things stay in place
    25. 25. 26 No Gravity • Would an apple fall? • Would things stay on earth? • What would happen? • What? • Why? • How? keep asking questions -like Newton !