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Newton’s Laws of Motion A journey into inertia, net force, and other topics….
Force is central to understanding Newton’s Laws A Force can be thought of as a push, like a quarterback throwing a football.
Force is central to understanding Newton’s Laws A Force can also be thought of as a pull, like a boat towing a skier.
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!!
Along with the idea of mass is Inertia.
More mass means more Inertia.
Less mass means less Inertia.
Inertia is the tendency of an object to stay at rest or move at a constant velocity What does that sound like?
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?
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!
When forces are equal there is NO Net Force!
What if the wind was pushing on the rock with greater force than the rock could push back?
There would be a Net Force in the direction of the wind!
The rock would fall over in the direction of the Net Force!
What is the Net Force in this picture? Contact of the bat on the baseball!!
What other forces are in action in this picture? Air Resistance Gravity
Initially the force of the ball coming off of the bat is stronger! Air Resistance Gravity It is the Net Force!!
Acceleration can also be thought of as a signal that a force has been applied!! Air Resistance Gravity Even changing directions requires force!!
Newton’s 2 nd Law F net =mama Net Force mass acceleration
Net Force has direction. We call this a vector !
We have already talked about velocity, acceleration, and momentum, which have direction and are also vectors !
Force is measured in Newtons (N) 1 N=1 kg∙ m s 2
Free-Body Diagrams Engine of truck exerting a force forward
Free-Body Diagrams Why will the truck not go on forever at constant velocity?
Free-Body Diagrams Air Resistance, a form of friction, pushes against the truck.
Free-Body Diagrams There is also friction between the road and the tires.
Free-Body Diagrams Gravity pulls down on the sewing machine
Free-Body Diagrams The desk pushes up on the sewing machine The desk is exerting what is called Normal Force!
Free-Body Diagrams Is there a Net Force in this case? No! The forces are equal and the desk is not moving!
Let’s Look at Types of Forces Gravity or Weight Normal force Friction Tension Thrust
Gravity or Weight Can be as simple as an object sitting on a table. Formula for Weight is W=mg.
Thrust Forces that move objects from rockets to cars to people.
Normal Force The force exerted by the surface the object is on. It is always perpendicular or at 90°.
Normal Force at an Angle FNFN FgFg F Net F Net is the sum of the vectors. More on that later….
Friction FNFN FgFg F Net Friction is the resistance of two surfaces sliding past one another FfFf
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
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
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
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.
Newton’s Third Law You probably learned this as “For every action there is an equal but opposite reaction.”
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