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Newton’s Laws of Motion A journey into inertia, net force, and other topics….

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Force is central to understanding Newton’s Laws A Force can be thought of as a push, like a quarterback throwing a football.

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Force is central to understanding Newton’s Laws A Force can also be thought of as a pull, like a boat towing a skier.

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Another idea central to Newton’s Laws is MASS. Anything which is made of matter, or atoms, has mass. Anything you can touch or see has mass, including students at Central!!

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Along with the idea of mass is Inertia.

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More mass means more Inertia.

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Less mass means less Inertia.

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Inertia is the tendency of an object to stay at rest or move at a constant velocity What does that sound like?

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Newton’s 1 st Law Objects tend to either remain at rest or move at constant velocity unless acted on by a Net Force. What is a Net Force?

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When an object is at rest the forces are all equal or in equilibrium. Gravity pulls down on the rock But the rock is pushing up with equal force!

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When forces are equal there is NO Net Force!

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What if the wind was pushing on the rock with greater force than the rock could push back?

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There would be a Net Force in the direction of the wind!

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The rock would fall over in the direction of the Net Force!

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What is the Net Force in this picture? Contact of the bat on the baseball!!

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What other forces are in action in this picture? Air Resistance Gravity

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Initially the force of the ball coming off of the bat is stronger! Air Resistance Gravity It is the Net Force!!

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Acceleration can also be thought of as a signal that a force has been applied!! Air Resistance Gravity Even changing directions requires force!!

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Newton’s 2 nd Law F net =mama Net Force mass acceleration

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Net Force has direction. We call this a vector !

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We have already talked about velocity, acceleration, and momentum, which have direction and are also vectors !

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Force is measured in Newtons (N) 1 N=1 kg∙ m s 2

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Free-Body Diagrams Engine of truck exerting a force forward

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Free-Body Diagrams Why will the truck not go on forever at constant velocity?

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Free-Body Diagrams Air Resistance, a form of friction, pushes against the truck.

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Free-Body Diagrams There is also friction between the road and the tires.

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Free-Body Diagrams Gravity pulls down on the sewing machine

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Free-Body Diagrams The desk pushes up on the sewing machine The desk is exerting what is called Normal Force!

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Free-Body Diagrams Is there a Net Force in this case? No! The forces are equal and the desk is not moving!

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Let’s Look at Types of Forces Gravity or Weight Normal force Friction Tension Thrust

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Gravity or Weight Can be as simple as an object sitting on a table. Formula for Weight is W=mg.

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Thrust Forces that move objects from rockets to cars to people.

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Normal Force The force exerted by the surface the object is on. It is always perpendicular or at 90°.

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Normal Force at an Angle FNFN FgFg F Net F Net is the sum of the vectors. More on that later….

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Friction FNFN FgFg F Net Friction is the resistance of two surfaces sliding past one another FfFf

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Tension Tension is force being applied to a rope or string. It applies when an object is hanging by a string or being pulled by a rope. FTFT

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Tension FNFN FgFg F Net When someone is pulling a box up hill by a rope we now have thrust going up the ramp. FfFf FTFT F Thrust

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Tension FNFN FgFg F Net Notice when someone is pulling a box up hill by a rope Net Force and Friction reverse directions. FfFf FTFT F Thrust

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Newton’s Third Law Whenever one body exerts a force on another body, the second body exerts an oppositely directed force of equal magnitude on the first body.

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Newton’s Third Law You probably learned this as “For every action there is an equal but opposite reaction.”

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The End

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