Newton's Laws.notebook

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Newton’s Laws of Motion

I. Law of InertiaII. F=ma

III. Action­Reaction

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Newton’s Laws of Motion• 1st Law – An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

• 2nd Law – The acceleration of an object depends on the mass of the object and the amount of force applied.

• 3rd Law – When one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

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Newton’s 1st Law of Motion (Law of Inertia)

An object at rest will stay at rest, and an object in motion will stay in motion at a constant speed and in a straight line unless acted on by an unbalanced force.

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• An object at rest (not moving) isn’t going to start moving unless acted on by an unbalanced force.

• Objects that are moving will keep moving forever at the same velocity (the same speed and the same direction) unless acted on by an unbalanced force.

Newton’s 1st Law (in other words)

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Unless acted on by an unbalanced force, this golf ball would sit on the tee forever.

Newton’s 1st Law

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• Once the ball is airborne, unless acted on by an unbalanced force, it would never stop!

• Friction is an unbalanced force!

• And air­resistance is a type of friction...

Newton’s 1st Law

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Newton’s First Law is also sometimes called the Law of Inertia.

Newton’s 1st Law

• Inertia is the tendency of an object to resist changes in its velocity ­ whether it’s moving or still.

The more mass an object has means more inertia, while less mass means less inertia.

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Because of inertia, objects (including you) resist changes in their motion. When the car going 55 miles per hour is stopped by the brick wall, your body keeps moving at 55 miles hour.

Don’t let this be you. Wear seat belts!

Newton’s 1st Law and You

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• The acceleration of an object depends on the mass of the object and the amount of force applied.

• OR

• The net force of an object is equal to the product of its mass and acceleration.

Newton’s 2nd Law

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F = m x a

Newton’s 2nd Law (in other words)

a = F / maccleration = force / mass

OR

Force = mass x acceleration

Fm a

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Accleration decreases as mass increases, and increases as mass decreases.

Accleration increases as force increases, and decreases as force decreases.

Less mass means…

More mass means…

more acceleration!

less acceleration!

Newton’s 2nd Law

Bigger push means… more acceleration!

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• When mass is in kilograms and acceleration is in m/s/s, the unit for force is newtons (N).

• So, one newton (N) is equal to the force required to accelerate one kilogram of mass at one meter/second/second. (It's not just a cookie!)

Newton’s 2nd Law

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How much force is needed to accelerate a 1400 kilogram car 2 meters per second/per second?

1) Write the formulaF = m x a

2) Fill in given numbers and unitsF = 1400 kg x 2 meters per second2

3) Solve for the unknown2800 kg­meters/second2 or 2800 N

Newton’s 2nd Law in action

Fm a

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A car breaks down. It weighs 1000 kg. I can make it roll at 0.05 m/s2. Using Newton's Second Law of Motion, how much force (Newtons) am I applying to my car?

Newton’s 2nd Law Fm a

1) Force = mass x acceleration (F = m x a)

2) Force = 1000 kg x .05 m/s2

3) Force = 50 Newtons

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• What acceleration will result when a 12 N net force applied to a 3 kg object?

• How much force is needed to accelerate a 66 kg skier 1 m/sec2?

• A net force of 16 N causes a mass to accelerate at a rate of 4 m/s2. What is the mass?

Newton’s 2nd Law

2) a = 12N/ 3kg 3) a = 4 m/s21) a = F/m

3) F = 66N2) F = 66kg x 1 m/s2 1) F = m x a

3) m = 4 kg2) m = 16N / 4 m/s2 1) m = F / a

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When one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

In other words… For every action, there is an equal and opposite reaction.

Newton’s 3rd Law

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According to Newton, whenever two objects interact with each other, they exert forces upon each other. For example, when you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body.

Newton’s 3rd Law

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There are two forces resulting from this interaction ­ a force on the chair and a force on your body. These two forces are called action and reaction forces. So, forces act in pairs, but not on the same object.

Newton’s 3rd Law

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• When a fish swims through the water, it uses its fins to push the water backwards. In turn, the water reacts by pushing the fish forwards through the water.

Newton’s 3rd Law – in Nature

• The size of the force on the water equals the size of the force on the fish; but the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards). Action­reaction pairs let the fish swim.

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• A bird flies by using its wings. The wings of a bird push air downwards. In turn, the air reacts by pushing the bird upwards.

• The size of the force on the air equals the size of the force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards).

• Action­reaction force pairs make it possible for birds to fly.

Newton’s 3rd Law

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• The baseball forces the bat to the left (an action); while the bat forces the ball to the right (the reaction).

Newton’s 3rd Law

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As the hot gases push against the inside tube of the rocket and escape downward out the bottom of the tube (action); the rocket moves in the opposite direction (reaction).

Newton’s 3rd Law

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• Momentum (p) is the product of mass and velocity of an object. The more momentum, the harder it is to stop an object or change its direction.

• p = mass x velocity

Law of Conservation of Momentum

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• When a moving object hits another object, some or all of the momentum of each object can be transferred to the other object.

Law of Conservation of Momentum

• After the “collision” objects either stick together or bounce off each other.

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• If they stick together, the objects move as one object in the direction of the object with the greater momentum.

Law of Conservation of Momentum

30 N + 30 N60 N

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• If they bounce off each other, the momentum is transferred from one object to another and the objects move in different directions and at different speeds.

Law of Conservation of Momentum

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• Because the action and reaction forces are equal and opposite, momentum is neither gained or lost.

Law of Conservation of Momentum

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• The Law of Conservation of Momentum can be explained by Newton’s 3rd Law. In the pool ball example, the first ball hit the second ball with a certain amount of force (the action force). The reaction force was the equal and opposite force exerted by the other ball on the first ball.

Law of Conservation of Momentum

• The action force made the second ball start moving, and the reaction force made the first ball stop moving.

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• You can drag two, three or four balls and let them go. The same number of balls you release will be moved forward upon the collision with the moving balls. This verifies the Law of the Conservation of Momentum, which states that the momentum remains the same after a collision.

Newton’s Cradle

Newton's cradle animation

• Of course, friction will slowly reduce the speed of the balls.