Forces and motion
Force and mass
Gravity and weight
Action and reaction
Vectors and scalars
Circular motion
Forces and their effects
• You cannot see a force but you can see what it
does.
• You can also feel effects of a force on your
body
•What can a force
do?
• Make an object move
• Make a moving object stop
• Change the speed of a moving object
• Change the direction of a moving object
• Change the shape of an object deformed
Forces in balance
• A force is a push or a pull, exerted by one object on
another. It has direction as well as magnitude (size),
so it is a vector.
• The SI unit of force is the newton (N).
• Example:
Motion without force
• Newton’s first law of motion-law of inertia.
• If no external force is acting on it, an object will
• - if stationary, remain stationary
• - if moving, keep moving at a steady speed in a
straight line.
• On Earth, unpowered vehicles soon come to rest because of
friction.
• With no friction, gravity, or other external force on it, a
moving object will keep moving for ever- at a steady speed in
a straight line.
Balanced forces
• An object may have several forces on it.
• If the forces are in balance, they cancel each
other out.
• The object behaves as if there is no force on it
at all.
• With balanced forces on it, an object is either
at rest, or moving at a steady velocity.
Terminal velocity
• When a skydiver falls from a hovering
helicopter, as her speed increases, the air
resistance on her also increases.
• Eventually, it is enough to balance her weight,
and she gains no more speed. She is at her
terminal velocity
• At a steady velocity, the forces must be in
balance.
• That follows from Newton’s first law.
Linking force, mass, and acceleration
• Resultant force=mass x acceleration
• F=ma
• This relationship between force, mass, and acceleration
is called
• Newton’s second law of motion
• 1 newton is the force required to give a mass of 1kg an
acceleration of 1m/s2
• (defining the newton)
Example:
• What is the acceleration of the model car on
the picture?
• First, work out the resultant force on the car.
• A force of 18N to the right combined with a
force of 10N to the left
• 18N-10N=8N
• Next, work out the acceleration when F=8N
and m=2kg
• F=ma
• 8N=2kg x a
• a=4m/s2
Gravity and weight
• Gravitational force
• If you hang an object from a spring balance,
you measure a downward pull from the Earth.
• This pull is called a gravitational force.
No one is sure what causes
gravitational force
• All masses attract each other
• The greater the masses, the stronger the force
• The closer the masses, the stronger the force
Weight
• Weight=mass x g
(g, gravitational field)
• W=mg
( W, gravitational force)
• Unit for weight is N (newtons)
Summary
• Newton’s first law of motion-law of inertia.
• If no external force is acting on it, an object will
• - if stationary, remain stationary
• - if moving, keep moving at a steady speed in a straight
line
• Resultant force=mass x acceleration
• F=ma
• This relationship between force, mass, and acceleration is
called
• Newton’s second law of motion
Example:
• What is the acceleration of the rocket on the
right?
•
•
•
•
•
W=mg=200kg x 10N/kg=2000N
Resultant force=3000N-2000N=1000N
F=ma
1000N=200kg x a
A=5m/c2
Action and reaction
• A single force cannot exist by itself. Forces are
always pushes or pulls between two objects.
So they always occur in pairs.
Newton’s third law of motion
• If object A exerts a force on object B, then
object B will exert an equal but opposite force
on object A.
• Another way of stating the same law:
• To every action there is an equal but opposite
reaction.
More about vectors
• Quantities such as force, which have a
direction as well as a magnitude (size), are
called vectors.
• Quantities such as mass and volume, which
have magnitude but no direction, are called
scalars.
The parallelogram rule
• The parallelogram rule is a method of finding
the resultant (two vectors acting at a point can
be replaced by a single vector with the same
effect), where the vectors are not in line.
• The parallelogram rule also works in reverse:
• A single vector can be replaced by two vectors
having the same effect.
Q
• 1) How is a scalar different from a vector?
• Give an example of each.
• 2) Force of 12N and 5N both act at the same
point, but their directions can be varied.
• a) What is their greatest possible resultant?
• b) What is their least possible resultant?
• c) If the two forces are at right angles, find by
scale drawing the size and direction of their
resultant.
Moving in circles
• Centripetal force
• If someone whirling a ball around in a
horizontal circle at a steady speed.
• Fc=mv2/r
• This inward force
• Centripetal force isn’t produced by circular motion. It is the
force that must be supplied to make something move in a
circle rather than in a straight line
Centrifugal force
When you whirl a ball araund on the end of some
string, you feel an outward pull on your hand
Orbits
• Satellites around the Earth.
• A satellites travels round the Earth in a curved
path called an orbit.
• Gravitational pull provides the centripetal force
needed.
• Electron around the nucleus
• In atoms, negatively charged electrons are in
orbit around a positively charged nucleus. The
attraction between opposite charges provides the
centripetal force needed