Intro to Forces

(1)

Ch. 12 Forces

Defining Force



Force



Newton’s First Law



Friction

(2)

Force

 Force

 a push or pull that one body exerts on another  What forces are being

exerted on the football?

Fkick

(3)

Force

 Balanced Forces

 _{forces acting on}

an object that are opposite in direction and equal in size

 _{no change in}

(4)

Force

 _{N = Normal}

Force – the

force that one surface exerts on another

surface in the absence of

(5)

Force

 W (weight) = mass x gravity

 _F_g_{= mg}

 g= 9.80 m/s2

 g is acceleration due to force of gravity

(6)

Force

 Net Force

 Overall force acting on an object after all the forces are combined.

 Net force is equal to “0” when the forces are equal and opposite. (Ex. Tug of War – No team is moving)

Ffriction

W

F_pull

F_net

(7)

Force

 Net Force

 Unbalanced vs. Balanced

 When the net force acting on an object causes it to move (accelerate) it becomes unbalanced.

Ffriction

W

F_pull

F_net

(8)

Friction

 Friction

 _{force that opposes motion between}

2 surfaces

 _{depends on the:}

•

types of surfaces

(9)

Friction

 Friction is greater...

 between rough surfaces

 when there’s a greater force between the surfaces

(e.g. more weight)

 4 Types of Friction

static, sliding, rolling, and fluid.

(10)

Air Resistance

 Air Resistance

 _{a.k.a. “fluid friction” or “drag”}

 _{force that air exerts on a moving}

object to oppose its motion

 _{depends on:}

•

speed

•

surface area

•

shape

(11)

Air Resistance

 Terminal Velocity

 _{maximum velocity reached}

by a falling object

 _{reached when…}

F

_grav

= F

_air

F

_air

F

_grav

 _{no net force}

 no acceleration

(12)

Newton’s First Law



Newton’s First Law of Motion



An object at rest will remain at

rest and an object in

motion

will continue moving at a

constant velocity

unless acted

(13)

 Newton’s First Law of Motion

 _{“Still don’t understand”}

 _{Objects resist (don’t like) change}

in motion (whether moving or not) unless they are forced to do so.

(14)

Newton’s First Law

 Newton’s First Law of Motion

 “Law of Inertia”

 Inertia

 a property of matter

 tendency of an object to resist any change in its motion

(15)

ConcepTest 1

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.

(16)

ConcepTest 1

1. Before and after the collision, 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

(17)

Newton’s Second Law

 Newton’s Second Law of Motion

 _{The acceleration of an object is}

directly proportional to the net force acting on it and inversely

proportional to its mass.

(18)

 Newton’s Second Law of Motion

 _{In other words, If a net force acts on}

an object, it will cause acceleration of that object. The forces are

unbalanced.

(19)

Newton’s Second Law

F

=

ma

F: force (N) m: mass (kg) a: accel (m/s2)

1 N = 1 kg ·m/s2

m

F

(20)

2

ndnd

Law application: Gravity

 Gravity

 _{force of attraction between any two}

objects in the universe

 _{increases as...}

•

mass increases

(21)

Gravity

 Who experiences more gravity - the

astronaut or the politician?

less distance

more mass

 Which exerts more gravity - the

(22)

Gravity

 Weight

 _{the force of gravity on an object}

MASS

always the same (kg)

WEIGHT

depends on gravity (N)

F

g

=

mg

Fmg: mass (kg): weight (N)

(23)

Gravity

 Would you weigh more on Earth

or Jupiter?

greater gravity

greater weight greater mass

(24)

Gravity

 Accel. due to gravity (g)

 _{In the absence of air}

resistance, all falling objects have the same acceleration!

 On Earth: g = 9.8 m/s2

(25)

Calculations

 What force would be required to

accelerate a 40 kg mass by 4 m/s2?

GIVEN:

F = ?

m = 40 kg

a = 4 m/s

2

WORK

:

F = ma

F = (40 kg)(4 m/s

2

)

F = 160 N

m

F

(26)

Calculations

 A 4.0 kg shotput is thrown with 30 N of

force. What is its acceleration?

GIVEN:

m = 4.0 kg

F = 30 N

a = ?

WORK

:

a = F ÷ m

a = (30 N) ÷ (4.0 kg)

a = 7.5 m/s

2

m

F

(27)

Calculations

 Mr. Shea weighs 868 N. What is his

mass?

GIVEN:

F(W) = 868 N

m = ?

a(

g

) = 9.8 m/s

2

WORK

:

m = F ÷ a

m = (868 N) ÷ (9.8 m/s

2

)

m = 88.6 kg

m

F

(28)

ConcepTest 2

 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}

(29)

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

(30)

 Problem:

 _{How can a horse}

pull a cart if the cart is pulling back on

the horse with an equal but opposite force?

NO!!!

 _{Aren’t these “balanced forces”}

(31)

Newton’s Third Law

 _{forces are equal and opposite but}

act on different objects

 _{they are not “balanced forces”}  _{the movement of the horse}

depends on the forces acting on the horse

(32)

 Action-Reaction Pairs

 The hammer exerts

a force on the nail to the right.

 The nail exerts an

equal but opposite force on the

(33)

Newton’s Third Law

 The rocket exerts a

downward force on the exhaust gases.

 The gases exert an

equal but opposite upward force on the rocket.

F_G

(34)

 Both objects accelerate.

 The amount of acceleration

depends on the mass of the object.

 Small mass  more acceleration

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