Explain the motion of objects on the basis on
Newton’s three laws of motion: inertia; the
relationship among force, mass, and
acceleration; and action and reaction forces.
Explain the relationship between mass and
weight by using the formula Fw = mg.
What is a Force?
• Force – push or pull one body
exerts on another
– SI Unit of force: Newton (N)
• Net force – the combining of forces
when two or more forces act on an
• Balanced forces – forces that are
equal in size and opposite in
– Forces are acting on the same
– Do not cause the motion of the
object to change
• Unbalanced forces – forces that
are not equal and/ or opposite in
– Net force causes object to move in
direction of force
Check it out…
• Gravity – the attraction two objects
have for one another
• Air resistance – opposing force an
object feels when traveling
• Friction – resistance to motion
caused by two objects in contact
with one another.
Inertia and Mass
• Inertia – the tendency of an object
to resist changes in its motion
– The greater the mass of an object
the greater its inertia
• Newton’s First Law of Motion –
‘Stuff keeps doin’ what its doin’
until somethin’ else messes with
• Force, mass,
– For any object,
the greater the
force applied to
it, the greater its
• If two equal forces act on an
object in opposite directions,
what is the acceleration?
• According to balanced forces,
the acceleration of the object
would be zero.
• Calculate the force on an
object that has a mass of
12 kg and an acceleration
of 4 m/s/s.
• F = mx a
• F = 12 kg x 4 m/s/s
• F = 48 kg x m/s/s = 48 N
What is the acceleration of a
200 kg object with 3,000 N of net
force acting on it?
a = 3,000 kg x m/s/s / 200 kg
a = 15 m/s/s
Newton’s Third Law
• When one object exerts a force
on a second object, the second
one exerts a force on the first
that is equal in size and
opposite in direction.
– “To every action, there is an equal
and opposite reaction.”
– Example – when you jump on a
trampoline the trampoline exerts the
same force on you in the opposite
How are these not balanced forces?
• When a force is exerted on
a box, an equal and
opposite force is exerted by
the box. These forces are
known as _____ forces.
• These forces are action-
• What is the difference
between weight and mass?
• Mass is a measure of the
amount of matter in an object,
while weight is a measure of
the force of gravity on the