Forces
Force
• A push or a pull that may cause an object’s
motion to change!
– Unit for force is the Newton(N)!
– It is a vector quantity.
• Direction is important!!
• Do forces have to be “in contact” in order to
have an affect on another object?
Balanced vs. Unbalanced Forces”
• Balanced forces—forces
that are equal in
magnitude, but
opposite in direction!!
– Will not cause a change
in the motion of the
object!!
• Unbalanced forces—
forces that are not
equal in magnitude and
opposite in direction!
– Will cause a change in
the motion of the
object!!
Are the forces Balanced or
Unbalanced?
Balanced
Balanced
Unbalanced
Unbalanced
Balanced or Unbalanced:
A.Balanced
B.Unbalanced
Balanced or Unbalanced:
A.Balanced
B.Unbalanced
Net Force(Fnet)
• Sum of all forces acting on an object.
• If there is a net force acting on an object, the
velocity of the object will change!!
Balanced or Unbalanced?
Find the Net Force
Unbalanced
FNet = 30 N left
Balanced or Unbalanced?
Find the Net Force
Balanced
FNet = 0 N
Balanced or Unbalanced?
Find the Net Force
Unbalanced
FNet = 10 N Right
Balanced or Unbalanced?
Find the Net Force
Unbalanced
FNet = 130 N down
Balanced or Unbalanced?
Find the Net Force
unbalanced
FNet = 360 N right
Find the Net force in the diagram
below:
A) 120 N right
B) 120 N left
C) 20 N right
D) 20 N left
Determine the Net Force:
A) 10 N right
B) 150 N right
C) 160 N right
D) 150 N left
Newton’s
st
1
Law of Motion
• also known as the “Law of Inertia”.
– Inertia—tendency of an object to resist change in
motion.
• Explain?????
• What does inertia depend on?
• 1st Law--An object at rest stays at rest and an
object in motion stays in motion, unless acted
on by some unbalanced force!
More thoughts on Inertia:
What gives an object inertia????
Inertia(cont):
Mass gives an object
inertia!!
It is easier to change
the motion of a small
object than it is to
change the motion of a
larger object!!
Does anything else
affect inertia?
I must think
about balancing
my bike!!!
Inertia(cont.)
Think about a
bicycle!!
Ever tried to
balance on a bike
while standing still?
Was it easy?
Was it impossible?
I must think
about balancing
my bike!!!
Inertia(cont.)
When you ride your bike down
the road, how many of you
think to yourself, “I must
balance my bike, I must balance
my bike!!”
What’s the difference?
Click me #1
Click me #2
Newton’s 1st Law of Motion(cont.)
• If you roll a ball across the floor, why does it
eventually come to a stop.
– Is there a force acting against it?
– If so, what is it?
• Friction—force that opposes motion.
– Always acts in the direction opposite of motion.
Anything slowing
this Coke as it
slides across the
bar?
Newton’s
nd
2
Law of Motion
• called the “Law of Acceleration”.
• Relates force, mass, and acceleration.
• Force and acceleration directly related.
• Mass and acceleration inversely related.
Newton’s 2nd Law of Motion(cont.)
F=ma
• Where:
F = force in Newton’s(N)
m = mass in kilograms(kg)
a = acceleration in m/s2
Ex. What force is required to accelerate a
1500 kg car by a rate of 1.5 m/s/s?
Given:
m = 1500 kg
a = 1.5 m/s/s
F=?
F=ma
F = 1500kg(1.5 m/s/s)
F = 2250 N
Ex. A force of 290 N is applied to a 20 kg
rock. At what rate will the rock
accelerate?
Given:
m = 20 kg
a=?
F = 290 N
F=ma
a = 14.5 m/s2
Ex. A force of 140 N is applied to a mover’s dolly
to accelerate a load of Cokes across the grocery
store at 3 m/s2. What is the mass of the load of
Cokes?
Given:
F = 140 N
a = 3 m/s2
m=?
A force is applied to accelerate a 50 kg
mass at a rate of 2.5 m/s/s. What
force is required?
A.
B.
C.
D.
20 N
25 N
120 N
125 N
A 250 N is applied to a 12 kg mass. At
what rate will the object accelerate?
A.3000 m/s/s
B. 240 m/s/s
C. 20.8 m/s/s
D..05 m/s/s
Newton’s
rd
3
Law
• For every action, there is an equal but
opposite reaction.
Forces always occur in pairs!!
Which of the following scenarios does
not represent Newton’s 3rd Law?
A. A rocket being launched into space.
B. A book resting on a desk.
C. A person paddling a canoe.
D. A large force pushing on a small
mass, causing it to accelerate.