ch. 11 motion & forces i. newton’s laws of motion “if i have seen far, it is because i have...

Ch. 11Ch. 11Motion & ForcesMotion & Forces

I. Newton’s Laws of Motion “If I have seen far, it is because I have

stood on the shoulders of giants.”- Sir Isaac Newton (referring to Galileo)

A. Newton’s First LawA. Newton’s First Law

Newton’s First Law of MotionAn object at rest will remain at

rest and an object in motion will continue moving at a constant velocity unless acted upon by a net force.

B. Newton’s Second LawB. Newton’s Second Law

Newton’s Second Law of MotionThe acceleration of an object is

directly proportional to the net force acting on it and inversely proportional to its mass.

F = ma

C. Newton’s Third LawC. Newton’s Third Law

Newton’s Third Law of MotionWhen one object exerts a force

on a second object, the second object exerts an equal but opposite force on the first.

Ch. 11Ch. 11Motion & ForcesMotion & Forces

II. Describing Motion Motion Speed & Velocity Acceleration

Newton’s First LawNewton’s First Law



motion

constant velocitynet force

A. MotionA. Motion

Problem: Is your desk moving?

We need a reference point...nonmoving point from which

motion is measured

A. MotionA. Motion

MotionChange in position in relation to

a reference point.

Reference point

Motion

A. Motion: DisplacementA. Motion: Displacement

The distance an object has been moved from one position to another.

A. MotionA. Motion

Problem:You are a passenger in a car

stopped at a stop sign. Out of the corner of your eye, you notice a tree on the side of the road begin to move forward.

You have mistakenly set yourself as the reference point.

B. Speed & VelocityB. Speed & Velocity

Speed rate of motion distance traveled per unit time

time

distancespeed

vd

t


Instantaneous Speedspeed at a given instant

Average Speed

time total

distance totalspeed avg.


Problem:A storm is 10 km away and is

moving at a speed of 60 km/h. Should you be worried?

It depends on the storm’s direction!


Velocityspeed in a given directioncan change even when the

speed is constant!

C. AccelerationC. Acceleration

Acceleration the rate of change of velocitychange in speed or direction

t

vva if

a: acceleration

vf: final velocity

vi: initial velocity

t: time

a

vf - vi

t

C. AccelerationC. Acceleration

Positive acceleration “speeding up”

Negative acceleration “slowing down”

D. CalculationsD. CalculationsYour neighbor skates at a speed of 4 m/s.

You can skate 100 m in 20 s. Who skates faster?

GIVEN:

d = 100 m

t = 20 s

v = ?

WORK:

v = d ÷ t

v = (100 m) ÷ (20 s)

v = 5 m/s

You skate faster!vd

t

D. CalculationsD. CalculationsA roller coaster starts down a hill at 10 m/s.

Three seconds later, its speed is 32 m/s. What is the roller coaster’s acceleration?

GIVEN:

vi = 10 m/s

t = 3 s

vf = 32 m/s

a = ?

WORK:

a = (vf - vi) ÷ t

a = (32m/s - 10m/s) ÷ (3s)

a = 22 m/s ÷ 3 s

a = 7.3 m/s2a

vf - vi

t

D. CalculationsD. CalculationsSound travels 330 m/s. If a lightning bolt

strikes the ground 1 km away from you, how long will it take for you to hear it?

GIVEN:

v = 330 m/s

d = 1km = 1000m

t = ?

WORK:

t = d ÷ v

t = (1000 m) ÷ (330 m/s)

t = 3.03 s

vd

t

D. CalculationsD. CalculationsHow long will it take a car traveling 30 m/s

to come to a stop if its acceleration is -3 m/s2?

GIVEN:

t = ?

vi = 30 m/s

vf = 0 m/s

a = -3 m/s2

WORK:

t = (vf - vi) ÷ a

t = (0m/s-30m/s)÷(-3m/s2)

t = -30 m/s ÷ -3m/s2

t = 10 sa

vf - vi

t

E. Graphing MotionE. Graphing Motion

slope =

steeper slope =

straight line =

flat line =

Distance-Time Graph

A

B

faster speed

constant speed

no motion

speed


Who started out faster? A (steeper slope)

Who had a constant speed? A

Describe B from 10-20 min. B stopped moving

Find their average speeds. A = (2400m) ÷ (30min)

A = 80 m/min B = (1200m) ÷ (30min)

B = 40 m/min

Distance-Time Graph

A

B

0

100

200

300

400

0 5 10 15 20

Time (s)

Dis

tan

ce (

m)

Distance-Time Graph


Acceleration is indicated by a curve on a Distance-Time graph.

Changing slope = changing velocity


0

1

2

3

0 2 4 6 8 10

Time (s)

Sp

ee

d (

m/s

)

Speed-Time Graph

slope =

straight line =

flat line =

acceleration +ve = speeds up -ve = slows down

constant accel.

no accel. (constant velocity)


0

1

2

3

0 2 4 6 8 10

Time (s)

Sp

ee

d (

m/s

)

Speed-Time GraphSpecify the time period

when the object was... slowing down

5 to 10 seconds speeding up

0 to 3 seconds

moving at a constant speed 3 to 5 seconds

not moving 0 & 10 seconds

Ch. 3 & 4Ch. 3 & 4Motion & ForcesMotion & Forces

III. Defining Force Force Newton’s First Law Friction

A. ForceA. Force

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

exerted on the football?

Fkick

Fgrav

A. ForceA. Force

Balanced Forces

forces acting on an object that are opposite in direction and equal in size

no change in velocity

A. ForceA. Force

Net Force unbalanced forces that are not

opposite and equal velocity changes (object accelerates)

Ffriction

W

Fpull

Fnet

NN

B. Newton’s First LawB. Newton’s First Law



B. Newton’s First LawB. 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 increases as mass increases

Concept Test 1Concept Test 1

TRUE or FALSE?

The object shown in the diagram must be at rest since there is no net force acting on it.

FALSE! A net force does not cause motion. A net force causes a change in motion, or acceleration.

Taken from “The Physics Classroom” © Tom Henderson, 1996-2001.


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 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

2. Starting at the time of collision, the door exerts a leftward force on you.

C. FrictionC. Friction

Friction force that opposes motion between

2 surfaces depends on the:

• types of surfaces• force between the surfaces

C. FrictionC. Friction

Friction is greater... between rough surfaces when there’s a greater

force between the surfaces (e.g. more weight)

Pros and Cons?

Ch. 3 & 4Ch. 3 & 4Motion & ForcesMotion & Forces

IV. Force & Acceleration Newton’s Second Law Gravity Air Resistance Calculations

A. Newton’s Second LawA. 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.

F = ma

A. Newton’s Second LawA. Newton’s Second Law

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

1 N = 1 kg ·m/s2

am

F

a

Fm

B. GravityB. Gravity

Gravity force of attraction between any two

objects in the universe

increases as...

•mass increases

•distance decreases


Who experiences more gravity - the astronaut or the politician?

less distance

more mass

Which exerts more gravity - the Earth or the moon?


Weight the force of gravity on an object

MASSalways the same

(kg)

WEIGHTdepends on gravity

(N)

W = mgW: weight (N)m: mass (kg)g: acceleration due

to gravity (m/s2)


Would you weigh more on Earth or Jupiter?

greater gravity

greater weight

greater mass

Jupiter because...


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

mW

g

elephant

m

Wg

featherAnimation from “Multimedia Physics Studios.”

C. Air ResistanceC. 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• 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


Terminal Velocity

increasing speed increasing air resistance until…

Fair = FgravAnimation from “Multimedia Physics Studios.”

C. Air ResistanceC. Air ResistanceFalling 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

D. CalculationsD. 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

D. CalculationsD. 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 = ?

WORK:

a = F ÷ m

a = (30 N) ÷ (4.0 kg)

a = 7.5 m/s2

m

F

a

D. CalculationsD. CalculationsMrs. J. weighs 557 N. What is her

mass?

GIVEN:

F(W) = 557 N

m = ?

a(g) = 9.8 m/s2

WORK:

m = F ÷ a

m = (557 N) ÷ (9.8 m/s2)

m = 56.8 kg

m

F

a

Concept TestConcept Test

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.

Motion & ForcesMotion & Forces

V. Free-Fall Motion Free-fall Circular Motion Free-fall

A. Free-FallA. Free-Fall

Free-Fall when an object is influenced only

by the force of gravity

Weightlessness sensation produced when an object

and its surroundings are in free-fall object is not weightless!

CUP DEMO

A. Free-FallA. Free-Fall

Weightlessness surroundings are falling at the same

rate so they don’t exert a force on the object

Motion & ForcesMotion & Forces

VI. Action and Reaction Newton’s Third Law Momentum Conservation of Momentum

A. Newton’s Third LawA. 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.


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” resulting in no acceleration?


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

Explanation:


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.


Action-Reaction Pairs

The rocket exerts a downward force on the exhaust gases.

The gases exert an equal but opposite upward force on the rocket.

FG

FR


Action-Reaction PairsBoth objects accelerate.The amount of acceleration

depends on the mass of the object.

a Fm

Small mass more accelerationLarge mass less acceleration

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?

ch. 11 motion & forces i. newton’s laws of motion “if i have seen far, it is because i have...

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