Gold: Important concept. Very likely to appear on an assessment. Notes:Color Guide Gold: Important concept. Very likely to appear on an assessment. Blue: Supplemental information. Will not directly appear on an assessment. Red: Example. Copy if needed. White: Will be discussed by Mr. Williams.

Chapter 4: Laws of Motion Section 1: Forces & Newton’s 1st / 2nd Laws

Field Forces & Contact Forces Forces can be grouped into two major categories: Field Forces & Contact Forces Field forces act at a distance, and do not require direct contact. Examples: Gravity Magnetism Electric Force Nuclear Force Gravity is a field force.

Field Forces & Contact Forces Forces can be grouped into two major categories: Field Forces & Contact Forces Contact forces act through direct physical interaction. Examples: Friction Spring Force Normal Force Tension Force Friction isn’t always bad.

Isaac Newton was a mathematician & physicist. He was also an astronomer and natural philosopher. In 1687, he published a book named “Philosophiae Naturalis Principia Mathematica”, or “Mathematical Principles of Natural Philosophy” It is often called “The Principia”. His book is considered one of the most influential works in the history of science. It contained (among other things), his three laws of motion.

Building on what Galileo Galilei had suspected in the 1630’s (that objects tend to maintain their state of motion), Newton defined his First Law of Motion. Newton’s First Law of Motion: An object will maintain its velocity (speed & direction) unless acted on by an unbalanced force.

Newton’s first law of motion describes the inertia of an object. An object’s tendency to maintain its current state of motion is it’s inertia. Inertia causes: Objects at rest to NOT want to move. Objects in motion to NOT want to slow down or speed up. INERTIA IS NOT A FORCE.

When an object has zero net force acting on it, it is in equilibrium. Equilibrium has two conditions: Constant Velocity OR Constant Rest Both mean ZERO ACCELERATION FB FB Fg Fg Assume the boat & iceberg are stationary.

Newton’s Second Law of Motion: The acceleration of an object (with a constant net force) increases as mass decreases. The NET FORCE acting on an object is the product of its mass & acceleration. Fnet = ma

Section 2: Newton’s 3rd Law / Everyday Forces Chapter 4: Forces Section 2: Newton’s 3rd Law / Everyday Forces

Newton’s Third Law: Forces always exist in pairs Newton’s Third Law: Forces always exist in pairs. For every force, there is a force of equal magnitude in the opposite direction. Sometimes stated as: For every action force there is an equal and opposite reaction force.

When applying the 3rd Law, you must observe each object individually. The man is applying 350 N of force to the van. The van accelerates, but the man does not. Why? Although the force is equal and opposite, 350 N is not enough o move the man because of the frictional force. Do you think the man could push the van if he were on ice?

Field Forces also exist in pairs. Let’s take gravity, for example… The Earth’s gravity pulls on you, and in turn, you pull on Earth with your gravity. So…why isn’t Earth orbiting YOU? The mass of the earth is much greater than your mass. The Earth DOES accelerate towards you, but it’s acceleration is infinitely small!

All objects possess the attractive force known as gravity. Gravity is one of the four fundamental forces. Along with electromagnetic force, strong nuclear force, and weak nuclear force. Objects in a uniform gravitational field (like that of a planet) accelerate equally. Gravitational Field Strength is “g”.

An object’s weight is the force caused by a gravitational field. An object’s weight depends on it’s mass & g. When calculating Weight on Earth, g = -9.8 N/kg So, if you went to another planet, your weight would change…! For example: A person with a mass of 75 kg weighs about 165 lbs on Earth. On the moon, this person would weigh 27 lbs on the Moon. Why? The gravitational field strength on the moon is only 1.63 N/kg! Weight (Fg) = mg

So, how much would you weigh on another planet? Use the following values to find out. Remember that F=ma gives you units of Newtons. One Pound is equal to 4.45 Newtons. The Moon: g = 1.6 m/s2 Mercury: 3.59 m/s2 Venus:8.87 m/s2 Mars: 3.77 m/s2 Jupiter: 25.95 m/s2 Saturn: 11.1 m/s2 Uranus: 10.7 m/s2 Neptune: 14.1 m/s2 Pluto: 0.420 m/s2 The Sun: 274. m/s2

If one object rests on another, both provide an equal force on each other. The Normal (Perpendicular) force experienced by an object is perpendicular to the surface it is in contact with. For an object on an incline, the normal force is: Fn = mg*cosθ

Frictional force causes resistance to motion. It is present when any two surfaces are in contact. Friction is always opposite to the motion of an object.

Because each object has a different surface texture, friction can only be approximated.

There are two categories of friction, and both oppose motion. Consider this: You slide your book across the table. It soon comes to a stop. Friction causes this behavior, of course. But what KIND of friction? Kinetic friction exists between objects that are sliding against each other. (Only one has to be in motion) Kinetic friction attempts to stop the motion of objects.

Shouldn’t those two forces be equal and opposite?? Static friction exists between surface that are at rest ONLY when an outside force is attempting to set one of the objects in motion. In order to cause an object on a surface to move, enough force must be applied to overcome static friction. Once motion begins, kinetic friction takes over. Wait…WAIT! Shouldn’t those two forces be equal and opposite?? FA Fk

Consider this: You’re trying to push a heavy crate across concrete. You push with enough force to match the weight of the box (or Fg), but it doesn’t move. Again, friction causes this behavior. An opposite force responds to your push…the force of static friction. Static friction increases as the applied force increases, but it has a limit. FA Fs

How is Friction determined? Definition: coefficient of friction – a non-unit, scalar value that determines the amount of friction present. It is represented with “μ”. The higher the coefficient, the larger the force of friction can be. For example, Rubber on Pavement has a higher μ than Steel on Ice. Pronounced “mew” or “myou”.

Force of static friction: Fs = μsFn Fs equals the amount of force needed to set an object in motion. There is NO force of static friction if no force is attempting to move the object. Fk equals the applied force when an object is moving at a constant speed. Generally, Fk < Fs. Once an object begins moving, friction becomes weaker in most cases. Force of static friction: Fs = μsFn Force of kinetic friction: Fk = μkFn

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