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Force and Laws of MotionS.8.C.3.1.1,2

Unit 3

Lesson 7

The physics classroom website

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Force• A push or pull on an object.• Can change the motion of an object• Measured in Newtons (N) (kg*m/s2)

• Friction and gravity are examples.

Friction • A force that acts against motion. • Between objects that are touching

• Between the legs of a desk and the floor when you push a desk

1 N= 0.225lbs

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Gravity• A force that pulls matter together• Between objects that are not touching• Isaac Newton

– “the force of gravity acts between all objects in the universe”

• Gravity exists between all matter – Gravity exists between you and your pencil

however it is very weak

• The more mass an object has, the stronger its gravity

• Keeps objects in orbit around a star, planet or moon.

• Makes objects “fall”

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Combining ForcesForce Diagrams: • We use arrows to show the different forces acting on

objects• Direction of arrow shows the direction the force is acting• Length of the arrow shows the strength (size or magnitude)

of the force

• Net force: the combination of all forces acting on an object

5N5N

= 10N

5N 10N

=

=

5N 5N 0 N=

5N

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• When two forces are the same strength (magnitude or size) but acting in opposite directions on a the same object.

• Cannot change an objects motion (speed and direction)

• An object at rest to start with balanced forces will continue at rest

• An object moving at 30 mph to the left with balanced forces acting on it will continue to move at 30 mph to the left

Balanced Forces

• Forces are equal• Motion remains the same

• If not moving, it continues to not move

• If moving , it continues to move exactly the same

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Balanced ForcesForces on the person are balanced: Gravity pulls down on person, chair pushes up on person

10 N 10 N

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Unbalanced Forces

• Net force of 40 N to the Right• 40 N• New motion depends on the

starting motion

• Will change an objects motion (speed and direction)

• A net force results.

• Net force is calculated by adding forces in the same direction or subtracting forces in opposite directions

• If an object is starting at rest when a net force of 40 N is applied, the object will move to the right

• If an object is already moving to the right at a constant speed when a net force of 40 N is applied, the object will speed up.

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Newton’s 1st Law of Motion

Law of Inertia

– Object at rest will remain at rest– Object in motion will continue in

motion in a straight line

Unless an unbalanced force acts upon it

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

• The higher the mass of an object , the higher its inertia is.

Circular motion animation

car making a sharp turn

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Inertia

Video

Clip

Bill Nye- In

ertia

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Newton’s 2nd Law of Motion• F = MA• The larger the net force on an object , the more the

objects motion will change (greater acceleration).• Objects with larger masses, need more force to

change their motion.

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Newton’s 2nd Law of Motion

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Newton’s 3rd Law of Motion• When one object exerts a force on another object,

the second object exerts an equal but opposite force on the first object

• They are not balanced forces because the forces act on different objects

100 N100 N

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Newton’s 3rd Law• Action-Reaction• For every action (force), there is an equal but

opposite reaction (force)

3 laws Video clip

Newton’s 3 Laws