Physical Science 9

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Physical

Science 9 Newton’s Laws of Motion

My name is

Newton, and

these are my

laws!

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Objects in motion stay in

motion unless acted upon by

a net force…

Objects at rest stay that way

unless acted upon by a net

force.

Some people call this the law of inertia.

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An example of this law is when a car suddenly

stops. Everything in the car wants to keep

going forward.

Seat belts are used to keep you attached to the

car which is now stopped. Without seat belts,

your body flies forward at the speed the car

was going.

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What happens when you don’t wear your seat belt?

The arrow

shows the

body of the

driver being

ejected from

his car.

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What happens when you don’t wear your seat belt?

The arrow

shows the

body of the

driver being

ejected from

his car.

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Newton’s Second Law of Motion

When an unbalanced force acts on an

object, the amount of force is equal to the

product of the object’s mass and

acceleration.

Force = (mass)(acceleration)

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This is another SI formula. Mass MUST be

in kg, and acceleration MUST be in m/s2.

F=ma

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Basically, the amount of force required to

move a heavier object faster is greater

than moving a light object slowly.

F=ma

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F=ma

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Sample Problem:

How much force is required to accelerate a

10kg ball at 10m/s2.

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1kg x m/s2 = 1 Newton or 1N

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Here’s another problem:

Petey Piranha threw the 27kg Mario at an

acceleration of 4.3m/s2. What force did Petey

Piranha apply to Mario?

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Here’s another problem:

Petey Piranha threw the 27kg Mario at an

acceleration of 4.3m/s2. What force did Petey

Piranha apply to Mario?

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Please do the practice problems on your

handout (1-3).

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Please turn to page 270 and do the practice

problems 1-3.

Problem #1 Solution:

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problems 1-3.


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problems 1-3.


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Science 9 Free Fall and Weight

In the absence of air, objects near the Earth’s

surface accelerate at a constant rate.

9.8m/s2

Assign some Brain Cells to this Number!!!

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In our calculations, you will always neglect air

resistance.

9.8m/s2

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This means a falling object will continue to

accelerate towards the ground until it hits

something (like the ground).

9.8m/s2

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After 1

second, the

cat would be

traveling at

9.8m/s.

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After 2

seconds, the

cat would be

traveling at

19.6m/s.

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After 3

seconds, the

cat would be

traveling at

29.4m/s.

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The speed of an object with

a constant acceleration can

be calculated with the

formula…

s=at

In the case of a falling object,

you can replace ‘a’ with ‘g’, or on

Earth 9.8m/s2

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s=at

Here’s a sample problem…

A cat jumps from an airplane.

What is the cat’s speed after

12.3s (ignore air resistance).

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s=at

That’s about 270 mph!!!

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Science 9 Weight

In Physics, weight and mass ARE NOT the

same thing. Mass is the amount of material

that makes up a substance.

Weight is a force depending on gravity and

the mass.

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Science 9 Weight

Your mass does not change unless you add

to or get rid of mass, no matter where you

are.

If your mass is 42kg on Earth, your mass

would be 42kg on the moon, on Mars, and in

space.

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Science 9 Weight

Your WEIGHT does change depending on

where you are. The value of ‘g’ (gravity)

changes depending on your location.

If your weight is 100N on Earth, your weight

would be 17N on the moon, since the value of

‘g’ is 1/6 of that on Earth. Remember, ‘g’ is

determined by the mass and distance of the

objects involved.

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Science 9 Weight

You can calculate the weight of an object

by using Newton’s 2nd Law…

F=ma

We can substitute w (weight) for F since it is a

force, and ‘g’ for ‘a’ since gravity is an

acceleration.

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Science 9 Weight

That gives us the formula…

w=mg

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Science 9 Weight

Sample Problem

w=mg What is the weight of an object on the

Earth’s surface that has a mass of 10kg?

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Science 9 Weight

Sample Problem

w=mg

As an example, an apple has a weight of

about 1N. This is equivalent to about ½

pound.

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Science 9 Weight

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Science 9 Weight

What causes astronauts to be weightless if

they are in orbit around the Earth?

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Science 9 Weight

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Science 9 Weight

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Science 9 Weight

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Science 9 Weight

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Science 9 Weight

A cat who jumped out of

an airplane with a

bathroom scale would

weigh zero, because they

are both in free fall.

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Science 9 Weight

At some point, the falling

cat would stop

accelerating towards the

Earth because the

amount of air resistance

would balance out the

acceleration due to

gravity. When a constant

falling speed is reached,

this is called…

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Science 9 Weight

Terminal

Velocity

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Science 9 Weight

Remember, FORCE is measured in

NEWTONS.

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Science 9 Newton’s Third Law of

Motion

For every action, there is an equal and

opposite reaction.

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Motion

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Motion

If you push on a wall with a force of 20N, the

wall pushes back with an equal force of 20N –

so there is no motion.

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Motion

If the wall is not able to push back with 20N, it

will move in the direction you are pushing it.

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Motion

Keep

pushing

harder!

The wall

keeps

pushing

back!!!

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Motion

This “action-

reaction” force is

what makes

rockets & jets

propel…

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Science 9 That’s it for Chapter 8

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Physical Science 9