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. SCIENCE IS EXCITING!
Gravity and motion
Gravity and Motion
DO TRY THIS AT HOME
Lifting a ping pong ball
You can use a
drinking straw to
lift a ping pong
ball by sucking on
the opposite end
of the straw.
The force of Gravity Should
Cause the Ping pong ball to fall
The arrow represents the
force of gravity
The ball does not fall when your
sucking force = force of gravity
• 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.
• 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)
The greater the mass, the greater the
The greater the distancedistance, the lessless the
Newton’s Law of Universal
• 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
• The force acts in the direction of the line
connecting the centers of the masses.
Newton’s Law of Universal
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".
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.
Gravity down under
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
• 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
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?