Forces Introduction
adapted from PPt at
http://science.pppst.com/motion.html
FORCES
Forces are pushes or pulls (a combination is a
twist).
Objects are stationary when forces are
balanced
Gravity is always acting towards the centre of
the earth. We don’t keep falling due to a
reaction force, eg the floor!
Forces can be measured using a Newton meter.
BALANCED FORCES
Unbalanced forces cause changes to objects shape or motion
The change to motion can be on the speed, the direction or both!
If a force acts on a stationary object and causes motion, the object
has gained kinetic (movement) energy.
Friction might stop the object moving.
Types of force:
Gravitational
Friction – the force that opposes motion
Magnetic
Tension – the force in rope, etc
Electrostatic
Lift – in the air (planes/birds)
Buoyancy – in the water
CONTACT FORCES
Some forces only act on contact, others can act from a distance.
Which are which?
Gravitational
Contact
Distance
Magnetic
Tension
Friction
Electrostatic
FORCE PAIRS
Forces act in pairs (e.g.
thrust and drag, weight and
lift).
Force diagrams show the
forces acting on an object
and whether they are
balanced or unbalanced.
Arrow size represents force
size if no measurements are
available.
Force pairs
• What are the
missing terms?
• Buoyancy
• Drag
• Thrust
• Weight
FRICTION
Friction is a contact force that opposes motion.
It causes heat, damage, wear and slowing.
Friction can be reduced by…
lubrication,
streamlining (aerodynamics),
smoothing surfaces in contact
Friction helps you propel yourself
forward when you walk, keeps
your bottom from sliding off chairs
and helps keep ear buds in your
ears.