Newton’s Second Law of
Motion-Force and Acceleration
Chapter 5
Change in Motion-Acceleration
Newton’s First Law of Motion pertains to
objects either at rest or undergoing constant
velocity
Objects in equilibrium and moving at a
constant velocity have no net force acting
on them
Most of the motion we experience in real
life undergoes change- accelerated motion
Acceleration revisited
Acceleration describes
how quickly motion
changes
Acceleration is the
change in velocity per
certain time interval
a = v / t
Acceleration is caused
by force
Force Causes Acceleration
Apply a force to a hockey puck at rest on the ice
and it moves
The puck has accelerated, or changed it state of
motion
When the hockey stick no longer pushes it, it
moves at a constant velocity (minimal friction)
Apply another force by striking the puck again and
its acceleration changes again
Force causes acceleration
Questions
Is an object at rest
accelerating?
How much acceleration
does an object moving at a
constant speed have?
Both examples are of
objects in equilibrium
No net force is acting on
the objects
Acceleration is zero
Acceleration depends on net
force
The combination of forces acting on an object is
the net force
Net force must be increased to increase
acceleration
If net force is doubled, acceleration will double
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If a hockey puck is hit, it will accelerate, or change
velocity
If the hockey puck is hit twice as hard, it will move
twice the speed
If it’s hit three times as hard, the puck will travel three
times the speed
Acceleration ~ Net Force
Mass resists acceleration
Push on an empty shopping cart, then push equally
hard on the same shopping cart loaded with
groceries
Which one will accelerate more?
Acceleration depends on the mass being pushed
The same force applied to twice the mass results
in half the acceleration
If the loaded cart is twice as heavy as the empty
cart, what is the loaded cart’s relative
acceleration?
Acceleration ~ 1 / Mass
Newton’s Second Law of Motion
Newton came up with one of the most important
rules of nature ever proposed:
The acceleration produced by a net force on an
object is directly proportional to the magnitude of
the net force, is in the same direction as the net
force, and is inversely proportional to the mass of
the object
Acceleration ~ Net Force / Mass
What units should be used for each variable in this
equation?
Questions
If a car can accelerate at 2m/s2, what
acceleration can it attain if it is towing
another car of equal mass?
What kind of motion does a constant force
produce on an object of fixed mass?
What acceleration is produced by a force of
2000N applied to a 1000-kg car?
What if the force is 4000N?
What if the car had a mass of 500-kg?
Friction
Friction is a force that also affects motion
Friction acts on materials that are in contact with
each other and always acts in a direction to oppose
motion
Friction is due to irregularities in the surfaces of
the two objects in contact
Friction depends on the materials an object is
made of
Fluid friction occurs when an object moves
through a liquid or gas (materials that flow)
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Water resistance
Air resistance
Friction
When the frictional
force equals the
applied force, the
object moves at a
constant velocity
The friction force
balances the applied
force and the
acceleration is zero