Download net force

Acceleration depends on both
mass and the net force
Newton’s 2nd law:
The acceleration produced by a net force
on a body is directly proportional to the
magnitude of the net force.
It is in the same direction as the net force,
and is inversely proportional to the mass of
the body.
Acceleration depends on both
mass and the net force
F
a
m
F
m
a
F  ma
If the net force is zero, there is
no acceleration
If the net force is not zero, there is
acceleration
Acceleration depends on net force
• A force of 10N accelerates the box
Acceleration depends on net force
• A force of 20N accelerates the box twice
as fast
Acceleration depends on mass
• A force of 10N accelerates the box
Acceleration depends on mass
• A force of 10N accelerates the smaller box
faster
• An airplane weighs 2,000,000N and needs
to accelerate at 4m/s2. What thrust (force)
must the engines produce?
• A train engine has a mass of 120,000kg
and pulls a 75,000kg car. The train can
pull with a force of 585,000N. What is its
maximum acceleration?
• Now suppose a second car is added to the
train in the last problem. What is the new
maximum acceleration?
• A train engine mass = 120,000kg
• Train car1 = 75,000kg.
• Train car2 = 75,000kg.
In order for sink a ship in the harbor, a
cannon ball of 25kg must be accelerated at
44m/s2.
How much force is required to hit the ship?
• A stunt penguin want to fly around the room and
needs to accelerate enough to compensate for
gravity.
• If his thruster can produce 88N of force, what is
the maximum mass of the penguin and his
thruster?
Friction
• Friction is a force like any other force
• Friction acts on materials that are in
contact with each other
• Friction slows down motion
• Forces due to friction are always in the
opposite direction of the motion.
Find the net force
5N
5N
5N
5N
5N
10N






5N
5N
10N
10N
15N
5N






10N
0N
15N
5N
10N
5N
Free-body diagrams
Draw the free-body diagram for a book is at
rest on a table top.
A girl is suspended motionless from a bar
which hangs from the ceiling by two ropes. A
free-body diagram for this situation looks like
this:
An egg is free-falling from a nest in a tree.
Neglect air resistance. A free-body diagram
for this situation looks like this:
A rightward force is applied to a book in
order to move it across a desk at constant
velocity. Consider frictional forces. Neglect
air resistance. A free-body diagram for this
situation looks like this:
A college student rests a backpack upon his
shoulder. The pack is suspended motionless
by one strap from one shoulder. A free-body
diagram for this situation looks like this:
A skydiver is descending with a constant
velocity. Consider air resistance. A free-body
diagram for this situation looks like this:
A force is applied to the right to drag a sled
across loosely-packed snow with a rightward
acceleration. A free-boy diagram for this
situation looks like this
A football is moving upwards towards its
peak after having been booted by the
punter. A free-body diagram for this situation
looks like this:
A car is coasting to the right and slowing
down. A free-body diagram for this situation
looks like this:
Net Force
• If there is no movement
–The net force is zero
• If there is no acceleration
–The net force is zero
• If there is acceleration
–The net force is not zero.
Free Body diagrams
• If the net forces are zero, the FBD is
balanced
Examples of zero net force?
• A book on a desk
• A sky diver with a parachute open
• A bike moving at a constant speed.
Free Body diagrams
• If the net forces are NOT zero, the FBD is
NOT balanced
Free Body diagrams
• If the net forces are NOT zero, the FBD is
NOT balanced
Fnorm
Fapp
Ffric
Fw
Examples of non zero net force
• A book sliding across a desk
• A sky diver falling without a parachute
Pressure
• P pressure (units are pascals Pa)
• P = F/A
• A Area
• F Force
Pressure
Hydraulic Pressure
Pressure
• Does a book’s weight change depending
on the angle it is held at?
• No
W=mg
• Does the pressure exerted by a book
change depending on the angle it is held
at?
• Yes
Pressure
• What is the difference between
weight and pressure?
• Weight is measured in
N
• Pressure is
N / Area
• P=F/A