Download net force

GPS: Measure and calculate frictional
forces and Newton’s three laws of motion
EQ: What is the difference between
contact forces and field forces?
• Force: Any push or pull
– Unit of force is a Newton (N)
– Is a vector quantity
• Symbolized using arrows
• Arrows denote direction and magnitude
(size)
• Multiple forces act on objects
• Newton’s laws deal with net force –
the leftover force after all forces
have acted on an object
Categorizing Forces
• Contact forces – object from the external
world applies a force to a system by
touching it
• Examples:
– Friction
– Normal force
– Tension
Categorizing Forces
• Field force – object from external world
applies a force to a system WITHOUT
touching it
• Example:
– Gravitational force (weight)
– Electromagnetic force
Free body diagrams
• Forces result from interactions between
masses
• Mass which causes the force is the “agent”
• The object acted on by the force is the
“system”
• Example: falling book
– Book’s mass is system
– Earth’s mass is agent
Free Body Diagrams
• Free body diagrams allow analysis of how forces
affect motion
• ID system – draw as circle – or particle (dot)
• ID agents – draw as vectors (arrows)
– ID everywhere system touches external world
– contact forces
– ID field forces
– Make vectors proportional in size
– Draw force vectors away from system (circle)
– Choose positive direction – usually in
direction of greatest force.
Forces as Vectors
• Determining net force
– Forces in same direction – add
– Forces in opposite direction – subtract
• Total is called net force
• Object moves in direction of largest
force
Forces as Vectors
10N
•Net Force up and down = 0N
10N
10m/s
10N
•Net Force left and right = 20N to
right
•Object will increase velocity and
will move to the right – positive
acceleration
10N
10N
•Net Force up and down = 0N
10N
10m/s
10N
•Net Force right and left = 0N
•Object will maintain velocity
10N
Forces as Vectors
• Balanced forces = 0 net force
• Equal in size and opposite in direction
10N
10N
10N
10N
• Result in no change in velocity – no
acceleration
Forces as Vectors
• Forces that act in opposite directions act
against each other
• Magnitudes are subtracted
• Motion is in the direction of greatest force
•
•
•
•
20N
30N
Net force is 10 N
Object will move to the right
20N
30N
Net force is 10 N
Object will move to the left
Forces as Vectors
• Forces in same direction work together to
affect motion
•Net Force
• Magnitudes are added
acting on object
20N
30N
= 50N
•Object will
move to right
5N
•Net Force acting on the object is
10N
5N
•Object will move downward
Practice gets you closer to perfect
On p. 89 in text: Complete 1-5
GPS: Measure and calculate frictional
forces and Newton’s three laws of motion
EQ: How do net forces affect
acceleration?
Newton’s 1st Law: Law of Inertia
• Objects with no net force acting on them
will not change their motion
– Objects at rest will stay at rest
– Objects in motion will maintain that
motion – no acceleration will occur
• Forces are in equilibrium
• Resistance to change in motion is due to
inertia
Newton’s 1st Law: Law of Inertia
• Equilibrium: Objects at rest
– Net force on object = 0N
• Objects sitting on a surface
– WEIGHT pushes downward FORCE
– NORMAL force pushes upward
• A contact force applied by a surface
• Total normal is equal and opposite of weight
• Hanging objects
– Weight pulls down
– Tension pulls upward
• A contact force applied by a rope/string/cable
• Total tension is equal and opposite of weight
• An object at rest on a
surface
• An object at rest
hanging from a
surface
t =5N
t =5N
w = 1N
n = 1N
• w = weight
• n = normal force
w =10N
• t = tension
Newton’s 1st Law: Law of Inertia
• Mass is NOT volume
• Volume is the amount of space an object
occupies
• Which has more mass, a pillow or a car
battery?
– Car battery
• Which has more volume?
– Pillow
• Which has more inertia?
Newton’s 1st Law: Law of Inertia
• Mass is NOT weight
• Weight is a measure of the force of gravity
on an object
• Weight can change with gravity, mass
doesn’t
• Weight is a force – units are Newtons
• The more mass, the more weight – b/c
more material for gravity to act on
Dynamic Equilibrium
• Zero net force
• Nonzero constant velocity – NOT AT REST
• Which property allows objects to keep moving
at constant velocity?
»INERTIA
• Which force acts against dynamic
equilibrium?
»FRICTION
• Friction only comes into play when
there is motion between two surfaces
GPS: Measure and calculate frictional
forces and Newton’s three laws of motion
EQ: What is the relationship between
mass, force and acceleration?
Newton’s 1st Law: Law of Inertia
• Inertia is measured as mass
• Mass is the amount of material in an
object
• More mass, more inertia, more
resistance to change
• More massive objects are harder to
get moving and harder to stop
• Friction: A force between two surfaces
that ALWAYS OPPOSES motion
• Has to be acted against to get things
moving
• Has to be reduced to keep things moving
• Is always between two surfaces and
always makes a moving object slow down
• Friction converts moving energy to heat
– Causes surfaces to heat up
– Causes surfaces to wear down
Newton’s
nd
2
Law
• Changing Velocity – aka ACCELERATION
–Net force acting on a body causes
acceleration
• If velocity changes, there’s a net force
acting on it
• Any change in speed or direction
Newton’s 2nd Law
• The net force (Fnet) causing acceleration
can be calculated:
• Net Force = (mass) acceleration
• Fnet = ma
or
a = Fnet or
m
m = Fnet
a
Newton’s 2nd Law
• Fnet = ma
• 1kg ball to accelerate 10m/s2 requires 10N
of net force
• m = 1kg a = 10m/s2 Fnet = ? Fnet =ma
• Fnet = 1kg(10m/s2) Fnet = 10kg٠m/s2
• 1kg ٠m/s2 = 1N = unit of force
• Increasing mass requires a greater
net force to get the same
acceleration
• m = 1kg a = 10m/s2 Fnet = 10N
• m = 2kg
a = 10m/s2 Fnet = 20
N
• Mass doubled, net force needed doubled
• INERTIA doubled – more resistance
• Semi-Truck has much bigger engine than minicooper – bigger engine, more Force
• Increasing acceleration requires a
greater net force
• m = 1kg a = 10m/s2 Fnet = 10N
• m = 1kg
a = 20m/s2 Fnet = 20 N
• Acceleration doubled, net force
needed doubled
Mustang vs Mustang GT : Same mass
• GT accelerates faster – bigger engine
(V-8) – more force
• Maintaining net force on a greater mass
will result in LESS acceleration
• a = Fnet/m
• m = 1kg Fnet = 10N a = 10N/1kg = 10m/s2
• m = 2kg Fnet = 10 N a = 10N/2kg = 5m/s2
• Mass was doubled, acceleration cut in ½
• Mass cut in ½, acceleration doubled
• There is an inverse relationship between
“m” and “a”
• IF Fnet is not changed, ↑mass causes
↓acceleration
• IF Fnet is not changed, ↓ mass causes
↑ acceleration
• Camaro (↓ m) accelerates faster than Cadillac
SUV (↑m) with same 3.6L engine size (same
Fnet)
2010 Chevrolet Camaro Performance Specs
Mass is about 1542kg
Engine: 3.6L V6
Power:
300 HP SAE @ 6,400 rpm
Torque:
273 ft. per lb. @ 5,200 rpm
2009 Cadillac SRX Performance Specs
Mass is about 2028kg
Engine: 3.6L V6
Power:
255 HP SAE @ 6,500 rpm
Torque:
254 ft. per lb. @ 2,800 rpm
• WEIGHT = Fw = m(“a” due to gravity)
• On Earth “a” due to gravity = g = 10m/s2
Fw = mg
• If a 1kg ball hangs from a spring scale.
What is its weight?
• m = 1kgg = 10m/s2 Fw = ? Fw = mg
• Fw = 1kg(10m/s2) Fw = 10N
GPS: Measure and calculate frictional
forces and Newton’s three laws of motion
EQ: How can Force,mass, and
acceleration be derived using
Newton’s 2nd Law ?
Practice
• Calculate the acceleration of a 2000kg
airplane just before takeoff when it’s thrust is
500N.
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Practice
• Calculate the acceleration of a 300,000kg jumbo
jet just before takeoff when the thrust of each of
its 4 engines is 30,000N
G
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Practice
• Calculate the horizontal force that must be
applied to produce an acceleration of 1g for a
1.3kg puck on a horizontal friction free table.
G
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Practice
A 10kg screen hangs from the ceiling on two wires.
What is its weight?
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What is the tension force on each wire?
S
• Falling with AIR RESISTANCE (friction)
• Air resistance increases with speed
• Air resistance increases with greater
surface area
• Increased air resistance means increased
friction --- opposes motion
Terminal Velocity
• Air resistance ↑ = weight ↓
• 0 net force – falling body reaches constant
velocity
• 0 acceleration
• Why does body keep falling down?
INERTIA! -- resists change in motion
Newton’s 3rd Law
• Newton’s 3rd Law
• When one object exerts a force on
another, the second object exerts a force
equal and opposite direction on the first
object
• “ For every action there is an equal and
opposite reaction”
Newton’s 3rd Law
• Examples:
Newton’s 3rd Law
Newton’s 3rd Law
• If a horse pulls on a cart and the cart pulls
on the horse with equal and opposite
force, How does the cart ever move
forward?
• Remember: Motion depends on NET
FORCE acting on a body.
Newton’s 3rd Law
If a brick is dropped from a bridge into a valley. Earth
pulls on the rock.
Newton’s 3rd Law states the rock pulls on Earth with
equal and opposite force. Why don’t we see Earth’s
acceleration?
Find the acceleration
of the ball and the
Earth: F = ma
massR = .2kg
g = 10m/s2
massE = 6 x 1024 kg