Newtons Law

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Newton’s Laws of Motion

Chapter 6

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•Aristotle (384-322 BC) believed that all object had a “natural place” and that the tendency of an object was to reside in its “natural place.”

•All objects were classified into categories of earth, water, air, or fire.

•“Natural motion” occurred when an object sought to return to its “natural place” after being moved from it by some type of “violent motion.”

•To keep an object moving would require a force.

Causes of Motion

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These views remained widelysupported until the 1500swhen Galileo Galilei (1564-1642)popularized experimentation.

Isaac Newton (1642–1727)proposed that the tendency ofan object was to maintain itscurrent state of motion.

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Sir Isaac Newton (1642 – 1727)

• Built on Galileo, Kepler, and others

• Worked out the “three laws of motion” governing the movement of all objects at all times an in all circumstances.

• He published them in his book Philosophiae Naturalis Principia Mathematica (mathematic principles of natural philosophy) in 1687.

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Sir Isaac Newton

– 1st Law – an object at rest or in motion stays at rest or

in motion unless acted upon by an outside force

* Known as the “Principle of Inertia”

– 2nd Law – describes how an object accelerates or changes direction when a force is applied

to it * F = ma

– 3rd Law – for action there is an equal and opposite reaction

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What is a Force?

• Force can be defined as a push or a pull. – (Technically, force is something that can accelerate

objects.)

• Force is measured by N (Newton).

• A force that causes an object with a mass of 1 kg to accelerate at 1 m/s is equivalent to 1 Newton.

1 Newton = kg * m/s2

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What are the types of Forces?

Type of Force Force Symbol

Contact Friction Ffric or Ff

Contact Normal FN

Contact Applied FAppl

Contact Spring FS

Contact Tension FT

Contact Thrust Fthrust

Long-Range Weight Fgrav

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Contact Forces vs. Long-Range Forces

– Contact Force = acts on an object only by touching it. (ie. Books resting on a desk)

– Long-Range Force = forces that are exerted without contact or forces resulting from action-at-a-distance, (ie. force of gravity).

– Types of Forces = see Table 6-2 in your books!

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Forces have Agents

– Each force has a specific, identifiable, immediate cause called the agent.

– How to solve for agents:– 1. Create a pictorial model of situation.– 2. Circle the system and identify every place where the system touches the environment.– 3. It is at these places that contact forces are exerted.– 4. Then identify any long-range forces on the system, ie. Force of gravity (Fgrav).

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Example of Forces having Agents

A physics book resting on a desk.

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Newton’s 1st Law of Motion

• An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. (outside force)

The velocity of an object remains constant unless acted on by an unbalanced force.

““Law of Inertia”Law of Inertia”

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Newton’s 1st Law of Motion

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Newton’s 1st Law of Motion

• Inertia - the resistance an object has to a change in its state of motion. [Newton’s 1st Law]

• Equilibrium - if the net force on an object is zero, then it is said to be in equilibrium.– An object is in equilibrium when: at rest

or moving at constant velocity.

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Some Examples from Real Life

Two teams are playing tug of war. They are both exerting equal force on the rope in opposite directions. This balanced force results in no change of motion.

A soccer ball is sitting at rest. It takes an unbalanced force of a kick to change its motion.

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Net Force

• After you have added and subtracted all the forces you are left with the net force acting on the object.

• There are several common forces acting on objects that you need to memorize:

Force SymbolFriction Ffric or Ff

Normal FN

Applied FAppl or Fa

Net FNet

Weight Fgrav or Fg

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Free Body Diagrams

• To keep track of how all these forces are affecting a single object, it is a good idea to draw a free body diagram.

• A free body diagram is just a simple sketch of the object showing all the forces that are acting on it. – Draw a quick sketch of the object. – Draw an arrow showing every force acting on the object. – To calculate the net force, add any vectors acting on the

same axis (x and y), making sure to pay attention to the directions.

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Examples of Free Body Diagrams

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Examples of FNet

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Examples of FNet