- Chapter 4 - Newton’s Laws of Motion Conceptual Physics

Aristotle divided motion into two types: natural and violent. Stated a force was needed for motion to occur and continue

Galileo argued that only when friction is present is a force needed to keep an object moving Was the first to propose the idea of inertia Also the first to understand and study friction

Sir Isaac Newton Developed three important laws of motion Overturned Aristotelian ideas Wrote Principia, one of the most important books of all time that introduced the laws

First Law “Every object continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.” Simply stated “an object at rest tends to stays at rest and an object in motion tends to stay in motion unless acted upon by an unbalanced force.” Also known as the law of inertia Inertia – the tendency of things to resist changes in motion.

First Law cont. Every object posses inertia Mass - is a measure of an object’s inertia The more mass an object has, the greater its inertia and the more force it takes to change its state of motion. Ex.) Different sized vehicles accelerating and stopping Weight is the force of gravity on an object. https://www.youtube.com/watch?v=PcGIUZzWoVc

First Law cont. Common forces that interfere with an objects inertia: Friction Gravity Lifting a car on earth vs. on the moon Pushing a car on earth vs. on the moon

Inertia technology

First to Second The first law handles circumstances where the forces acting on an object are balanced (equilibrium). What happens when the forces are unbalanced? The cause of acceleration is unbalanced forces.

Second Law “The acceleration of an object depends directly upon the net force acting upon the object, and inversely upon the mass of the object.” Unbalanced forces acting on an object cause the object to accelerate. Equation stated: Force = mass x acceleration F = ma Units: Newton (N) = kg x m/s2

Examples If a car can accelerate at 2 m/s2, what acceleration can it attain if it is towing another car of equal mass? A car has a mass of 1000 kg. What is the acceleration produced by a force of 2000 N? If the force is 4000 N, what is the acceleration? How much force, or thrust, must a 30,000-kg jet plane develop to achieve an acceleration of 1.5 m/s2?

Second Law cont. When acceleration = 0 Object is in equillibrium Sum of the forces acting on the object = 0 This includes objects moving a constant velocity!!

Second Law cont. Friction – a force that occurs when surfaces slide over one another. Friction always acts in the opposite direction of the motion

Second Law cont. Does friction act on an object at rest? No! However, the force of friction is greater when you start to push something and less once you have it going Static friction vs. sliding friction

Second Law cont. Friction does not depend on speed Friction does not depend on the size of the contact area Friction does depend on the types of material in contact

Second Law cont. Rubber against concrete produces more friction than steel against steel, so concrete road dividers have replaced steel rails.

Second Law cont. Free falling objects- Does mass affect the acceleration of objects falling? Why?! More massive objects do have a stronger gravitational force acting on them BUT…Twice the mass = twice the inertia

Second Law cont. Non-free falling objects encounter air resistance The force of air resistance depends on: Size of object - (amount of air encountered) Speed of object – (faster = more air particles encountered in a second)

Second Law cont. As a falling object gains speed, the force of air resistance finally equals the weight of the falling object. Net forces = zero…this means no acceleration Hence terminal velocity is reached and falling objects reach a constant speed

Second Law cont. How do flying squirrels “fly”?

Third Law “Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.” Every action has an equal and opposite reaction.

Third Law Simply stated: a force is a push or a pull. A force is always part of a mutual action that involves another force

Third Law cont. In every interaction, forces always occur in pairs: You push against the floor, and the floor simultaneously pushes against you. The tires of a car interact with the road to produce the car’s motion. The tires push against the road, and the road simultaneously pushes back on the tires. When swimming, you push the water backward, and the water pushes you forward.

When action is A exerts force on B, the reaction is simply B exerts force on A.

When action is A exerts force on B, the reaction is simply B exerts force on A.

Think… What is the action- reaction force of a boulder falling?

Third Law cont. If all interactions have equal and opposite forces…Does a cannon and cannonball exert the same force on each other? Do they both accelerate equally? Why or why not?

Third Law cont. The cannonball undergoes more acceleration than the cannon because its mass is much smaller.

Third Law cont. How does a rocket propel into space? The rocket recoils from the “molecular cannonballs” it fires and climbs upward.

Review All https://www.youtube.com/watch?v=kKKM8Y-u7ds