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Forces in Motion
Newton’s Laws of Motion
Terms
• Inertia~the tendency of all objects to resist
any change in motion
• Momentum~ a property of a moving object
that depends on the object’s mass and
velocity
Newton’s First Law of Motion
• An object at rest remains at rest and
an object in motion remains in motion
at constant speed and in a straight line
unless acted on by an unbalanced
force.
• Objects at rest
– Not moving
– Won’t move unless a push or pull is
exerted on them
• Object in motion
– Continues forever at same speed & in
same direction unless an unbalanced force
acts on it.
Newton’s First Law of Motion
• Friction and Newton’s First Law
– Friction makes it difficult to observe
– Friction is an unbalanced force which
changes motion
• Inertia is related to mass
– 1st law sometimes called the law of inertia
– Why you slide when car makes a turn
• Mass is a measure of inertia
– Small mass → less inertia
– Easier to change motion of object with a
small mass
Newton’s Second Law of Motion
• The acceleration of an object depends
on the mass of the object and the
amount of force applied
• Acceleration depends on mass
– Acceleration decreases as mass increases
when the same amount of force is applied
• Acceleration depends on force
– Acceleration increases as force increases
when the mass is the same
• Expressing it mathematically
– F=m·a
– A=F/m
Newton’s Third Law of Motion
• Whenever one object exerts a force on
a second object, the second object
exerts an equal and opposite force on
the first.
• Force pairs do not act on the same
object
• The effect of a reaction can be difficult
to see
• More examples:
– Rabbit hopping
– Bat hitting ball
– Shuttle taking off
Momentum is a Property of
Moving Objects
• More momentum ~ harder it is to stop
or change its direction
• Momentum is Conserved
– Any time 2 or more objects interact, they
may exchange momentum, but the total
amount of momentum stays the same
– Momentum before collision = momentum
after collision
• Conservation of momentum &
Newton’s third law
– Because action & reaction forces are equal
and opposite, momentum is conserved