Download Free Body Diagrams and Net Force

WARM
UP:
1. Calculate the acceleration of a bag of softball gear
weighing 50 N if pulled with a force of 25 N.
2. Calculate the force of a cars tires on the road if the car
has a mass of 2000 kg and is traveling at a constant
velocity of 60 mph north
•
Quiz TOMORROW!
Free Body
Diagrams and
Net Force
Terms used with Forces
Net Force: the sum of all forces acting
on an object.
F
Expressed in N.
https://www.youtube.com/watch?v=YyJSlcIbd-s
Terms used with Forces
Balanced Forces net force on an object
is zero, the object is in equilibrium—the
forces are balanced.
The object is either:
1. Not moving
2. Moving at a constant velocity
Terms used with Forces
UNBalanced Forces Whenever the
forces do not cancel each other out.
The object is ACCELERATING:
1. changing direction
2. changing speed
Free Body
F
1. Show all forces acting
F
Diagrams
tension
Hanging objects
F air
normal
supporting
Falling – terminal
velocity - gliding
2.
Think only about the
objects acceleration
F applied
F friction
Always
opposes
motion –
direction
of
movement
on one object
3. Length of vector
matters
F grav
On every
diagram
4.
Be careful with
constant velocity
problems
A physics book rests upon
a level table.
F norm
F grav
A physics book is pushed
across the desk with a
F norm
rightward acceleration.
But
does not move
F applied
F friction
F grav
A physics book is pushed
across the desk with a
F norm
rightward acceleration.
F applied
F friction
F grav
A physics book is pushed
across the desk and moves
with a constantF norm
velocity.
F applied
F friction
F grav
A ball is moving upward
after being kicked
F grav
An egg is free falling
from a nest.
F grav
A car hits the brakes and
is coasting
F norm
F friction
F grav
An elevator is rising at a
constant velocity.
F tens
F grav
• Determining Net Force: Free-body
diagrams for four situations are shown
below. The net force is known for each
situation. However, the magnitudes of a
few of the individual forces are not
known. Analyze each situation individually
and determine the magnitude of the
unknown forces.
If a = 1.5 m/s2 determine
the net force and the mass of
the car.
• Unfortunately for Vanessa, the wheels on
her suitcase are not working. She pulls on
the strap in an effort to budge it from rest
and drag it to the curbside check-in desk.
Use force values to determine the net
force, the mass and the acceleration of
the suitcase. The values of the individual
forces are:
F = F = 207 N
grav
norm
Ftens = 182 N
Ffrict = 166 N.
Which one(s) of the following
force diagrams depict an
object moving to the right
with a constant speed? List
all that apply.
Newton’s 1st law
He is providing a pushing
force.
Constant velocity
Newton’s 1st law
There is an equal and
opposite friction force.
Applied force
friction
Constant velocity
Remember when the forces are
balanced (net force is zero) he
travels at constant velocity.
Applied force
friction
Constant velocity
Newton’s 2nd law
Now lets imagine what
happens if he pedals faster.
Applied force
friction
Newton’s 2nd law
His velocity changes (goes
faster). He accelerates!
Remember from that
acceleration is rate of change of
velocity. In other words
Applied force
acceleration = (change in
velocity)/time
friction
acceleration
Newton’s 2nd law
Now imagine what happens if he
stops pedalling.
friction
Newton’s 2nd law
He slows down (decellerates). This
is a negative acceleration. But he
is still moving forward.
friction
Newton’s 2nd law
So when the net force is
greater than zero, an object
accelerates (changes
velocity)
Mrs. Rice’s Porsche
applied force
friction
An example
We can use the diagram to
determine his acceleration?
Applied force
(100 N)
Mass of cyclist
and bike = 100
kg
Friction (60 N)
An example
Net force = 100 – 60 = 40 N
FNET = ma
40 = 100a
a = 0.4 m/s2
Mass of cyclist and bike = 100 kg
Pushing force (100
N)
Friction (60
N)