Physics 10 Chapter 31-D Motion

Natural Motion•Objects have a proper place •Objects seek their natural place•The heavier the object,

the faster it falls.

Violent Motion•Imposed motion•A Moving object must have a force acting on it.

The natural motion of a body is to remain in

whatever state of motion it is in unless

acted upon by net external forces.

Galileo Challenged The DogmaOf Natural Motion

Galileo Challenged The DogmaOf Natural Motion with

Experiments

Galileo’s Motion Studies

Does the speed change?

What causes the ball to speed up?

What causes the ball to slow down?

gravity

gravity

No!

Galileo’s Inclined PlanesIf I have a long horizontal plane without friction, how far will the ball go to reach the same height?

Once the ball rolls down the incline, what keeps it moving?

Inertia

The tendency of an object to resist a change in motion.

Inertia

Inertia

Inertia

Inertia Question

A quick pull will break which string?A slow and steady pull will break which string?

The Earth Revolves around the sun at 30,000 m/sec!!! (~67,000 miles/hr)

The Earth Rotates on its axis at 464 m/s (1000 miles/hr)!

The Solar System moves through the Galaxy at 220,000 m/s ( 500,000 miles/hr)

How come we can’t feel these motions?

Cuz we are moving right along with it!

Mass is a Measure of Inertia

Kilogram: KG

Mass & Weight

F ma=W mg=

210 /g m s=

Mass is how much stuff an object has – inertia.Weight is due to the force of gravity and depends on the mass

and the planet!

Mass & Weight

W mg=210 /g m s=

Calculate your weight in N.Calculate your mass in Kg.

1 lb = 4.45 N

Weight and Force

The hanging weight has a mass of 1 kg. What is its weight in Newtons?

a) 0.1 N b) 1 N c) 10 N d) 100 N

What is the reading on each of the two scales?a) 1 N b) 5 N c) 10 N d) 10 N

W mg= 210 /g m s=

Mass on the MoonGravity on the surface of the moon is only 1/6 as strong as on the Earth. What is the mass of a 12-kg object on the moon? What is the weight on the moon?

Isaac Newton(1642 -1727)

1687 Published Principia•Invented Calculus•3 Laws of Motion•Universal Law of Gravity

Newton’s First Law(Law of Inertia)

An object will remain at rest or in a constant state of motion

unless acted upon by net forces.

Newton’s First Law(Law of Inertia)

An object will remain at rest or in a constant state of motion unless acted upon

by net external forces.

0F =∑

Push or a Pull

The unit is a Newton (N)

Contact Forces

Tension Forces

Support Forces

Friction Forces

Net Force

Direction of applied forces matters!!!Opposite forces cancel!

Net Forces 0F =∑

W

T2T1

Forces up equal the forces down.Forces sideways cancel too.

Static Equilibrium 0F =∑

W

T2T1

If the Net Force is zero, then the object remains at rest – in static equilibrium.

W

Dynamic Equilibrium

0F =∑If the Net Force is zero, then the object moves at a

constant speed – in dynamic equilibrium.

Newton’s First Law 0 => No Change in MotionIf F =∑

Dynamic Equilibrium Static Equilibrium

The person weighs 250 N and the tension in each rope is 200N.What is the weight of the staging?

a) 50 Nb) 100 Nc) 150 Nd) 200 N

Net Force Question

Two 2 N weights are pulling on the spring scale as shown. The reading on the scale is:

a) 0 N b) 1 N c) 2 N d) 4 N

Galileo’s Motion Studies

0

2

f

xvt

v vv

vat

Δ=Δ+

=

Δ=Δ

gave us…

Definitions:

SpeedHow fast How far is changing.

The rate at which an object moves.

distance

VelocityVelocity is a vector. It has both magnitude (speed) and direction.

Instantaneous velocity is the velocity at any instant.

distance

total distance traveledAverage Speed = total time

dt

Δ=Δ

•Speed: Scalar (magnitude)•Velocity: Vector (magnitude AND direction)•Unit is m/s• s and v often interchanged …..

Speed & Velocity

[ ] [ ]speed , distancem ms

= =

Sense of Speed

1 / 3.6 / 2.24 /10 / 36 / 22.4 /20 / 72 / 44.8 /30 / 108 / 67.2 /

m s km hr mi hrm s km hr mi hrm s km hr mi hrm s km hr mi hr

= == == == =

1 / 2.25 /m s mi hr≈

1 / 0.62 /km hr mi hr≈

Speed QuestionWhat is its average speed? (in m/s)

total distance traveledAverage Speed = total time

Distance from Speed

d vt=

total distance traveledAverage Speed = total time

Average Speed x total time= total distance traveled

If you travel an average of 80 km/hr for 15 minutes, how far have you traveled in that time?

AccelerationHow fast How fast is changing.

The rate at which the speed is changing.

Speeding up

Slowing down

Speeding up or slowing down?

change in velocitychange in time

vat

Δ= =

Δ

Acceleration is in the direction of the net Force but not necessarily

in the direction of velocity.Velocity is always in the direction of the motion!

Velocity from Accelerationchange in velocity

change in timevat

Δ= =

Δ

v a tΔ = Δ

v at=

If you start from rest and accelerate at a rate of 5m/s every second (5 m/s2) for 10 seconds, what will be your final speed?

25 / 10 50 /v at m s x s m s= = =

Quicky Question

An automobile enters a freeway on-ramp at 15.0m/s and accelerates uniformly up to 25.0 m/s in a time of 10.0s. a) What is the automobile’s average velocity?

0

2f

ave

v vv

+=

15 / 25 / 20 /2

m s m s m s+= =

Which equation?

An automobile enters a freeway on-ramp at 15.0m/s and accelerates uniformly up to 25.0 m/s in a time of 10.0s.

b) What is the distance traveled in this amount of time?

Which equation?

Quicky Question

d vt= (20 / )(10 ) 200m s s m= =

200d m=

An automobile enters a freeway on-ramp at 15.0m/s and accelerates uniformly up to 25.0 m/s in a time of 10.0s.

c) What is the automobile’s acceleration?

f iv vvat t

−Δ= =Δ Δ

225 / 15 / 1 /10

m s m s m ss−

= =

Which equation?

Quicky Question

Quizzy Questions

A car moves with an average speed of 40 km/h for 15 minutes. How far does it travel?

a) 60 km b) 10 km c) 30 km d) 30 km/h

A car can go from rest to 90 km/hr in 10 s. What is the acceleration of the car?

a) 60 km/hr/s b) 9km/hr/s c) 30 km/hr/s d) 3 km/h/s

Hill Question

On which of these hills does the ball roll down with increasing speed and decreasing acceleration? a) b) c)

a) b) c)

Tracks QuestionTwo balls are released simultaneously from rest at the left end of equal length tracks A and B as shown. Which ball reaches the end of its track first?

Free FallIn the absence of air resistance,all objects fall due to the force of gravity with an acceleration “g”:

2 29.8 / ~ 10 /g m s m s=

Free FallIn a vacuum, all objects fall with the same

acceleration due to gravity: 10 m/s2.

Falling from Rest

2 21 52

y at tΔ = =

2

:~ 10 /

Estimatea g m s=

10v at t= =

20 /20

v m sy m=

Δ =

10 /5

v m sy m=

Δ =

30 /45

v m sy m=

Δ =

40 /80

v m sy m=

Δ =

50 /125

v m sy m=

Δ =

+

0

20

12

fv v gt

y v t gt

= +

Δ = +

0 0v =

!v y≠ ΔHow FAR is notHow FAST!

Take down as +y:

What Goes Up Must Come DownSomeone standing at the edge of a cliff throws one ball straight up and one straight down at the same speed. Ignoring air resistance, which ball strikes the ground with the greatest speed?

Free Fall: Throwing UPWhat is the speed at the top of the path?(assume g = 10m/s2 )

ZERO!

What is the acceleration at the top?

g=10m/s2

What is the speed at the same height on the way down?

-30 m/s

+y

Free Fall Symmetry

SAME!The gravitational field is symmetric

(in the absence of air resistance.)

Which ball has the greatest speed when it hits the ground?

Symmetry of The G-Field

Quizzy Questions

If you throw a ball straight up in the air at 30 m/s, how fast is it going 1 second later?

10 m/s b) 5 m/s c) 20 m/s d) 0 m/s

What is the acceleration of a falling object that has reached its terminal velocity?

a) 10 m/s2 b) 5 m/s2 c) zero d) unknown

Free Fall Summary

25d t=

For an object without an initial velocity and without air resistance.

210 /a g m s= =

10v t=

If there is an initial velocity, then add it!

( ) 10iv t v t= + 2( ) 5id t v t t= +Do Aunt Minnie problems in the Practice Book.