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Forces: Falling
Objectives
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Understand the effect of a force
Know that objects fall towards the Earth because of the gravitational force
Plenary
Pupils should revise what they know about forces by reading the fairy tale and classifying the
forces (in bold) as contact forces or non-contact forces.
Contact forces: crushing watermelons, opening the door, pulling the lever, arm wrestling,
pushing at the magnet. Non-contact forces: picking up pins, armour attracted to the magnet,
falling to the ground (this final force is not familiar yet).
The force of gravity
Remind pupils that a force can change the motion of an object; it could cause it to move, to
get faster or slower or change direction.
Pupils must note:
A force can change the motion of an object.
When an object is dropped, it falls faster towards the ground. Allegedly, when an apple fell
from a tree and knocked Isaac Newton on the head, he realised that there was an attractive
force acting between the Earth and the apple. This force causes objects to be pulled towards
the (centre of the) Earth.
Gravity does not just cause objects to fall towards Earth, it also responsible for holding
objects on the ground. Astronauts float due to the lack of gravity in space.
Pupils must note:
The force of gravity causes objects to fall towards the Earth.
Newton’s work on gravitation, forces and motion was published more than 300 years ago in
‘Principia Mathematica’ and is the foundation of classical physics. Classical physics still
largely holds up today: we used classical physics to put a man on the moon.
Observing the effect of gravity
It is difficult to observe that the force of gravity causes objects to accelerate, as objects in
freefall drop so quickly. This activity demonstrates that gravity does cause objects to
accelerate.
Find a high window to drop a tennis ball (or other brightly coloured ball) from. Set a video
camera on a tripod, then drop the tennis ball and record the fall. On a large screen, play the
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video back frame by frame. Pupils should see that although the images are separated by the
same time interval, the ball appeared to move further between frames as it falls. This is
because it is accelerating; the force of gravity affects its motion.
The strength of gravity (non-statutory)
Everything on Earth falls at the same rate. The reason why a feather appears to fall more
slowly than a hammer is due to air resistance, which will be studied in this unit. However,
objects do not fall at the same rate all over the universe.
Show the pupils footage of Gene Cernan (Apollo 17) bunny hopping on the moon. Pupils will
notice that Cernan falls back to the ground more slowly than he would on Earth. Explain that
this is because the moon’s gravity weak; 1/6 that of Earth’s gravity. This is due to the size of
the moon. Smaller bodies have weaker gravitational fields. The sun is so massive that its
gravity causes all the planets to orbit it.
Possible extra-curricular questions
What causes gravity?
A gravitational field arises due to an object having mass. Every object with mass has gravity,
but only when an object is as heavy as the Earth or the Moon is its gravitational field strong
enough to notice. The gravitational force occurs when two objects with mass (and hence
gravitational fields) interact.
Why may you not use classical physics in some situations?
Classical physics works on a macroscopic scale where objects act predictably. But on a very
small scale, objects work in different ways (e.g.: a particle can be in two places at once) so
need new physics - quantum mechanics. When physicists deal with the very fast movement of
galaxies, objects also work in different ways (e.g.: time slows down for moving objects) so
need new physics - relativity.
What other objects have gravity?
Every object with mass has an attractive gravitational field. However, a person’s
gravitational field is too weak to have any significant effect on their surroundings. If two
people were floating in space, they would die before their gravity made them float together.
Gravity is the weakest fundamental force. It is weaker than the electromagnetic force (which
holds atoms together) by about 10000000000000000000000000000000000000000x
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