Chapter 13 The Nature of Forces Forces Friction Inertia Newton’s Laws Gravity Air Resistance

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)


Ffriction Fnet

Fpull

Balanced & Unbalanced Forces

Draw the resulting force arrow for each of the 3 force combinations shown to the left!!!


=

=

=

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)


3 main types of friction:SlidingRollingFluid

What are some Pros and Cons of 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.



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.


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


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?


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.


Simply put….•Every action has an equal and opposite reaction.


How do the arrows indicate how the forces are acting in the picture?


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!!


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)


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...


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.


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


Terminal Velocity

maximum velocity reached by a falling object

reached when…

Fgrav = Fair

Fair

Fgrav

no net force no acceleration constant velocity

Section 4 - Terminal VelocitySection 4 - Terminal Velocity


Terminal Velocity

increasing speed increasing air resistance until…

Fair = FgravAnimation from “Multimedia Physics Studios.”


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

JET CAR CHALLENGEJET CAR CHALLENGE

CHALLENGE:

Construct a car that will travel as far as possible (at least 3 meters) using only the following materials.scissorstape4 plastic lids2 skewers

2 straws1 balloon1 tray

How do each of Newton’s Laws apply?

Documents

Chapter 13 The Nature of Forces Forces Friction Inertia Newton’s Laws Gravity Air Resistance