Transcript Force and Acceleration

Force Causes Acceleration
change_in_ velocity
accelerati on 
time_inter val
•
•
•
•
•
Acceleration deals with changes in motion
There is zero acceleration at constant velocity
The cause of acceleration is force
A constant force produces motion at a constant acceleration
Acceleration ~ net force
– The ~ symbol means, “directly proportional to”
The great acceleration of the racing
car is due to its ability to produce
large forces.
Mass Resists Acceleration
• For a given force the acceleration is inversely
proportional to the mass
1
acceleration ~
mass
Mass Resists Acceleration
1
acceleration ~
mass
6 times the mass
1/6 the acceleration
Newton’s Second Law
net _ force
acceleration ~
mass
• By using consistent units: N for force, kg for
mass and m/s2 for acceleration the equation
becomes exact:
F
a
m
1N
1m / s 
1kg
2
What acceleration is produced by a force
of 2000 N applied to a 1000 kg car?
F
a
m
2000 N/1000 kg = 2 m/s2
If the force is 4000 N, what is the acceleration?
Doubling the force on the same mass simply doubles the acceleration.
Newton’s Second Law
• If mass and acceleration are known then the
equation would look like this:
F  ma
Newton’s Second Law
Head = 4 kg
F  ma
Newton’s Second Law
Head = 4 kg
F  ma
Acceleration of head = 25 m/s2
F  4kg  25m / s
F  100N
2
Friction
Friction acts on materials
that are in contact with each
other, and it always acts in a
direction to oppose motion.
The force of friction between the
surfaces depends on the kinds of
material in contact and how much
the surfaces are pressed together.
• Rubber against concrete produces more friction
than steel against steel.
Why are concrete road dividers more
effective than steel rails?
Friction also occurs in liquids and gases (fluids).
Air resistance – friction
acting on something
moving through the air
Applying Force--Pressure
• Pressure—The amount of force per unit
of area
• One Newton per square meter is equal
to one pascal (Pa)
Force
Pressure
F
P
A
Area of application
Applying Force--Pressure
• Pressure—The amount of force per
unit of area
Pressure
F
P
A
Force
Area of application
The smaller the area supporting a given
force, the greater the pressure on that
surface.
• You exert more
pressure on the
ground when you
stand on one foot
than when you stand
on both feet.
(Decreased area of
contact)
• Stand on one toe
like a ballerina and
the pressure is huge.
Why don’t the nails puncture the skin?
The driving
force
per nail is
not enough
to puncture
the skin.
Free Fall Explained
A 10 kg cannonball
will hit the ground at
the same time as a 1
kg stone!
F
m
=
F
m
A 10 kg cannonball
will hit the ground at
the same time as a 1
kg stone!
g = acceleration of free fall
The weight at the Earth’s
surface is 9.8 N per 1 kg.
1 kg stone
F weight 9.8 N
2
a 

 9.8m / s  g
m
m
1kg
10 kg cannonball
F weight 98 N
2
a 

 9.8m / s  g
m
m
10kg
Free Fall
• The net force on each object is only its weight.
• The ratio of weight to mass is the same for
both.
• All freely falling objects undergo the same
acceleration at the same place on Earth.
If you were on the moon and dropped a hammer
and a feather from the same elevation at the
same time, would they strike the surface of
the moon at the same instant?
Answer
• Yes
• On the moon, the hammer and
feather weigh only 1/6 of their
Earth weights, and there is no
air to provide friction.
• The ratio of moon-weight to
mass for each object is the
same, and they both accelerate
at (1/6)g.
Air Drag
Falling and Air Resistance (drag)
Terminal speed
(acceleration terminates)
150-200 km/h
A heavier person will
attain a greater terminal
speed than a lighter
person.
15-25 km/h
A parachute increases
air resistance.
• Air drag builds up as
speed increases. The
result is reduced
acceleration.
• More reduction can
occur by increasing
the surface area
encountered by the
air. (Diver spreads
out)
• If there were no air
drag, like on the
moon, there would be
no terminal speed.
(free fall and each
object hits the ground
at the same time).
Falling and Air Resistance
• When air resistance equals
the weight, the net force is
zero and no further
acceleration occurs.
Acceleration terminates; the
object has reached its
terminal speed.
• If we are concerned with
direction (such as down for
falling objects), the object
has reached its terminal
velocity.
a.
His weight is 100 kg, so his weight at the Earth’s
surface is a constant 1000 N (9.8 N rounded).
Air resistance (R) varies. Find his acceleration at
each position.
R=0
A = 10 m/s2
W=1000N
net
b.
R=400N
W=1000N
A = 6 m/s2
c.
R=1000N
W=1000N
A = 0 m/s2
d.
R=1200N
W=1000N
A = -2 m/s2
e.
R=2000N
W=1000N
A = -10 m/s2
f.
R=1000N
W=1000N
A = 0 m/s2
F
W R
a

m
m