chapter 13 the nature of forces forces friction inertia newton’s laws gravity air resistance
TRANSCRIPT
Chapter 13 Chapter 13 The Nature of ForcesThe Nature of ForcesForces
Friction
Inertia
Newton’s Laws
Gravity
Air Resistance
Section 1 - ForcesSection 1 - ForcesForce
a push or pull that one body exerts on another that can make an object start or stop moving, OR change its speed or direction.
What forces are being exerted on the football?
Fkick
Fgrav
Ffriction
Fgrav = Force of GravityFkick = Pushing force of the kicker’s footFfriction = molecules of air “rubbing” against the surface of the football – air resistance!
Balanced Forces forces acting on an object that are opposite in direction and equal in size
no change in velocity
no movement the upward force
is called the normal force
Section 1 - ForcesSection 1 - Forces
weight (force)
normal force
Unbalanced Forces Forces that cause a change in
motion (these forces can be in the same direction OR opposite directions.
velocity changes (object accelerates)
Section 1 - ForcesSection 1 - Forces
Ffriction Fnet
Fpull
Balanced & Unbalanced Forces
Draw the resulting force arrow for each of the 3 force combinations shown to the left!!!
Section 1 - ForcesSection 1 - Forces
=
=
=
Section 2 - FrictionSection 2 - Friction
Friction force that opposes (restricts or
”fights”) motion between 2 surfaces The amount of friction depends on
the:• types of surfaces• force between the surfaces
Friction is greater...between rough
surfaceswhen there’s a
greater force between the surfaces (e.g. more weight)
Section 2 - FrictionSection 2 - Friction
3 main types of friction:SlidingRollingFluid
What are some Pros and Cons of friction?
Section 2 - FrictionSection 2 - Friction
Decreasing Frictional Force
3 main types of friction: Sliding – when solid objects
slide past or over each other
Rolling – friction produced by solid objects such as wheels or ball bearings.
Fluid – friction produced when an object moves through a fluid such as air or water.
Section 2 - FrictionSection 2 - Friction
Section 2 - FrictionSection 2 - Friction
How do people decrease the force of friction? We use lubricants.
•Def: any substance that changes sliding friction to fluid friction.
•Ex: motor oil, grease,
Section 3 - Newton’s First LawSection 3 - Newton’s First Law
Newton’s First Law of MotionAn object at rest will remain at rest and an object in motion will remain in motion at a constant velocity unless acted upon by an outside force.
Section 3 - Newton’s First LawSection 3 - Newton’s First Law
Newton’s First Law of Motion “Law of Inertia”
Inertia tendency of an object to resist any
change in its motion (either resting or in motion)
increases as mass increases
Section 3 - Newton’s First LawSection 3 - Newton’s First Law
Restated:An object will maintain its
speed and direction (velocity) or LACK of velocity (remain motionless) unless an outside force changes it.
Test Your UnderstandingTest Your Understanding
You are a passenger in a car and not wearing your seat belt.
Without increasing or decreasing its speed, the car makes a sharp left turn, and you find yourself colliding with the right-hand door.
Which is the correct analysis of the situation?
Test Your UnderstandingTest Your Understanding
1. Before and after the turn, there is a rightward force pushing you into the door.
2. Starting at the time of collision, the door exerts a leftward force on you.
3. Both of the above
4. Neither of the above
2. Starting at the time of the turn, the door exerts a leftward force on you.
Section 3 - Newton’s Second Section 3 - Newton’s Second LawLaw
Newton’s Second Law of MotionNewton’s Second Law
shows how force, mass, and acceleration are related.
F = ma
Section 3 - Newton’s Second Section 3 - Newton’s Second LawLaw
F = maF: force (N)m: mass (kg)a: accel (m/s2)
1 N = 1 kg ·m/s2
m
F
a
Force CalculationsForce CalculationsWhat force would be required to
accelerate a 40 kg mass by 4 m/s2?
GIVEN:
F = ?
m = 40 kg
a = 4 m/s2
WORK:
F = ma
F = (40 kg)(4 m/s2)
F = 160 N
m
F
a
Force CalculationsForce CalculationsMr. Miller weighs 557 N. What is his
mass?
GIVEN:
F(W) = 557 N
m = ?
a(g) = 9.8 m/s2
SHOW YOUR WORK:
m = F ÷ a
m = (557 N) ÷ (9.8 m/s2)
m = 56.8 kg
m
F
a
Force CalculationsForce CalculationsA 4.0 kg shotput is thrown with 30 N of
force. What is its acceleration?
GIVEN:
m = 4.0 kg
F = 30 N
a = ?
SHOW YOUR WORK:
a = F ÷ m
a = (30 N) ÷ (4.0 kg)
a = 7.5 m/s2
m
F
a
Section 3 - Newton’s Third LawSection 3 - Newton’s Third Law
Newton’s Third Law of Motion When one object exerts a force
on a second object, the second object exerts an equal but opposite force on the first.
Section 3 - Newton’s Third LawSection 3 - Newton’s Third Law
Simply put….•Every action has an equal and opposite reaction.
Section 3 - Newton’s Third LawSection 3 - Newton’s Third Law
How do the arrows indicate how the forces are acting in the picture?
Section 3 - Newton’s Third LawSection 3 - Newton’s Third Law
Explanation: The force of the water (blue arrows) is counterclockwise. The resulting force (red arrows) against the sprinkler head is in the opposite direction – clockwise.
This is why the sprinkler spins!!
Section 3 - Newton’s Third LawSection 3 - Newton’s Third Law
Action-Reaction Pairs The hammer exerts a
force on the nail to the right.
The nail exerts an equal but opposite force on the hammer to the left.
However, even though the nail exerts as much force as it can, it is not as large as the hammer’s force and the net result is the nail moving toward the right.
Section 4 - GravitySection 4 - GravityGravity
force of attraction between any two objects in the universe
increases as...
• mass increases
• distance decreases
G = m1 m2
d2
Section 4 - GravitySection 4 - GravityWho
experiences more gravity - the astronaut or the politician?
less distance
more mass
Which exerts more gravity - the Earth or the moon?
Section 4 - GravitySection 4 - Gravity
Weight the force of gravity on an object
W = mgW: weight (N)m: mass (kg)g: acceleration due
to gravity (m/s2)
WEIGHTdepends on gravity
(N or gxm/s2)
MASSalways the same
(kg or g)
Section 4 - GravitySection 4 - Gravity
Would you weigh more on Earth or Jupiter?
Therefore it has greater gravity
And you would have greater weight
Jupiter has a greater mass
Jupiter because...
Test Your UnderstandingTest Your Understanding
Is the following statement true or false? An astronaut has less mass on the
moon since the moon exerts a weaker gravitational force.
False! Mass does not depend on gravity, weight does. The astronaut has less weight on the moon.
Section 4 - GravitySection 4 - Gravity
Acceleration due to gravity (g)
In the absence of air resistance, all falling objects have the same acceleration!
On Earth: g = 9.8 m/s2
mW
g =
elephant
m
Wg =
featherAnimation from “Multimedia Physics Studios.”
Section 4 - Air ResistanceSection 4 - Air Resistance
Air Resistance “fluid friction” or “drag” force that air exerts on a moving
object to oppose its motion depends on:• speed of object• surface area of object• shape of object• density of fluid
Section 4 - Air ResistanceSection 4 - Air Resistance
Terminal Velocity
maximum velocity reached by a falling object
reached when…
Fgrav = Fair
Fair
Fgrav
no net force no acceleration constant velocity
Section 4 - Air ResistanceSection 4 - Air Resistance
Terminal Velocity
increasing speed increasing air resistance until…
Fair = FgravAnimation from “Multimedia Physics Studios.”
Section 4 - Air ResistanceSection 4 - Air Resistance
Falling with air resistance
Fgrav = Fair
Animation from “Multimedia Physics Studios.”
heavier objects fall faster because they accelerate to higher speeds before reaching terminal velocity
larger Fgrav
need larger Fair
need higher speed