Download Ch. 6 – Newton`s Second Law of Motion – Force and Acceleration1

Ch. 6 – Newton’s Second Law of Motion – Force and Acceleration1
What happens when forces are in
equilibrium (ΣF=0)?
What happens when the sum of the forces
is not zero? Example a kicked ball.
Force Causes Acceleration

An object at rest (ex. A hockey puck) …

Forces are balanced (equilibrium)


Hit the puck


Apply an unbalanced force
Object experiences a change in motion
(acceleration)
Acceleration
o Change in velocity in a
given time period
o m/s/s or m/s2
Motion undergoes a change
Curves downward due to gravity
Look at the cause of acceleration – force
When the hockey stick is no longer applying a
force to the puck …


No longer any unbalanced forces acting
Puck moves at a constant velocity
Unbalanced forces acting on an object cause
the object to accelerate.
Usually the applied force from us is not the
only force acting on the object.


Gravity
Air resistance
Mass Resists Acceleration
Combination of forces is the net force

Acceleration depends on the net force.
o To increase the acceleration of an
object, you must increase the force.
o Double the force, you double the
acceleration. Same for triple …
Acceleration is directly proportional to the net
force acting on it.
Acceleration ~ net force
Acceleration
Shopping at Home Depot …
Push an empty cart.
Now, load that cart with wood, concrete,
nails, etc.
What is pushing it like now?
The loaded shopping cart will accelerate
much less than the empty cart.

net force
For a given force, the acceleration produced is
inversely proportional to the mass.
Acceleration ~
Acceleration depends on mass
For a constant force, an increase in the
mass will result in a decrease in the
acceleration.

Ex. The same force applied to twice
the mass will have half the
acceleration.
Ch. 6 – Newton’s Second Law of Motion – Force and Acceleration2
Newton’s Second Law
Newton was the first person to realize that acceleration depends on more than just force.


Acceleration depends on both force and mass
Newton’s second law of motion is an important rule of nature.
Newton’s second law states that the acceleration produced by a net force on an object is
directly proportional to the magnitude of the net force, and is in the same direction as
the force. Acceleration is inversely proportional to the mass of the object.
a = net force
mass
a = fnet
m
F = ma
Units: m = kg
a = m/s2
F = newton (N) = kg•m/s2
A car has a mass of 1000 kg. What is the acceleration produced by a net force of 2000 N?
If the force is 4000 N, what is the acceleration?
How much force (thrust) must a 30,000-kg jet airplane develop to achieve an acceleration of
1.5 m/s2?
Friction
Friction is a force and affects motion.


Acts on objects in contact with each other
o Due to irregularities of surfaces touching
Opposes motion
Force of friction between the surfaces depends on the kinds of
material in contact and how much the surfaces are pressed together.

Ex. Concrete has a greater frictional force on rubber than steel
(road dividers are now being made out of concrete rather than steel)
Ch. 6 – Newton’s Second Law of Motion – Force and Acceleration3
Friction (continued)


Friction occurs when solids slide (or roll) past each other
Friction occurs in liquids and gases
o Liquids and gases are called fluids because they flow
o Fluid friction occurs when an object pushes the fluid aside as it moves
through
o Air resistance is friction of objects moving through air
 Notice air friction more at higher speeds (bicycling, skiing, driving a
car, sky diving)
Even with friction present, an object can move with a constant velocity even when
an outside force is applied.

Friction balances applied force
o Net force = 0
o Acceleration = 0

Pushing the crate just balances the force of friction, so it moves at a constant
velocity
Sack falls with constant velocity once air resistance (air friction) balances the
effect of gravity

A diagram showing all the forces acting on an object is called a free-body diagram.
Applying Force – Pressure
A book lying on a table exerts the same force as a book
standing upright on the table (assuming the same mass).
Force is not the same as pressure.
Pressure is the amount of force per unit area, as in the
greater the amount of surface area, the less pressure.
Read page 92.
BLN activity:
nd
p. 92, 2
paragraph
For a constant force, an increase in the area of contact
will result in a decrease in pressure. When force is
perpendicular to the surface area:
Pressure = Force
area
P=F
A
units: N = 1 pascal (Pa)
m2
Ch. 6 – Newton’s Second Law of Motion – Force and Acceleration4
Falling and Air Resistance
Free Fall Explained
Free fall occurs when a falling object
encounters no air resistance.

Falling objects accelerate equally if air
resistance is negligible
o Also approximately true if air
resistance is very small compared
with the mass of the falling object.
Why is mass unimportant in free fall?
Mass (a quantity of matter) and weight (force
due to gravity) are proportional.
nd
Newton’s 2 law of motion means that mass
must be considered, along with the force.
Since a =
The force due to air resistance
diminishes the net force acting on the
falling objects.
Speed and Area
Air resistance force an object
experiences depends on the object’s
speed and area.
Air resistance ~ speed x frontal area
force
Air resistance is directly proportional to
the speed and frontal area of an object.

a 10x larger object would have
10x the force acting on it, so it has the same
acceleration as the smaller object.

All freely falling objects have the same
acceleration at the same place on Earth.
Terminal speed: speed at which
the acceleration of a falling
object is zero because friction
balances the weight
Terminal velocity: same as
terminal speed, but with
direction (downward).
All freely falling objects fall
with the same acceleration
because the net force on an
object is only its weight, and
the ratio of weight to mass is
the same for all objects!
Air resistance is affected by weight and
body position.



Greater weight is better at “plowing
through the air” faster.
Spreading out (like the flying
squirrel) increases air resistance and
therefore falls slower.
Air resistance builds up at higher
speeds.
o Low speeds: air resistance is
often negligible
o High speeds: effect of air
resistance is more pronounced
on lighter objects.