Newton’s Laws Chapter 12

Newton's first law of motion Sometimes referred to as the law of inertia. Newton's first law of motion is often stated as “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 external force.”  

Applications of Newton's First Law Buckle up?

What is meant by inertia? Inertia is the tendency of an object to maintain it’s current state of motion….or to resist changes in its velocity….or to resist accelerations.   More mass = more inertia Ex.) Car vs. toy car

Newton's Second Law of Motion Pertains to the behavior of objects for which all existing forces are not balanced. The second law states that the acceleration of an object is dependent upon two variables the net force acting upon the object and the mass of the object. Summed up in this equation: F = ma

Newton’s Second Law a = F / m OR m=F/a Can also be written two other ways: a = F / m OR m=F/a The acceleration of an object depends directly upon the net force acting upon the object, and inversely upon the mass of the object.

a = m * a Newton’s Second Law As the force acting upon an object is increased, what must happen to the acceleration? the acceleration of the object is increased. = m * a

m = m * a Newton’s Second Law As the mass of an object is increased, what must happen to the acceleration of the object? The acceleration of the object is decreased m = m * a

Solve for the unknown value: . Solve for the unknown value:

Newton's third law of motion For every action, there is an equal and opposite reaction. Forces always come in pairs equal and opposite action- reaction force pairs

Check your understanding: While driving down the road, a firefly strikes the windshield of a car and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the car and the car hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the car?

Check your understanding… from the www.physicsclassroom.com Trick Question! Each force is the same size. For every action, there is an equal ... (equal!). The fact that the firefly splatters only means that with its smaller mass, it is less able to withstand the larger acceleration resulting from the interaction.

Check your understanding… from the www.physicsclassroom.com For years, space travel was believed to be impossible because there was nothing which rockets could push off of in space in order to provide the propulsion necessary to accelerate. This inability of a rocket to provide propulsion is because ... ... space is void of air so the rockets have nothing to push off of. ... gravity is absent in space. c. ... space is void of air and so there is no air resistance in space. d. ... nonsense! Rockets do accelerate in space and have been able to do so for a long time.

Check your understanding… from the www.physicsclassroom.com For years, space travel was believed to be impossible because there was nothing which rockets could push off of in space in order to provide the propulsion necessary to accelerate. This inability of a rocket to provide propulsion is because ... The answer is D.

Guns recoil when fired. result of action-reaction force pairs. A gunpowder explosion creates hot gases which expand outward allowing the gun to push forward on the bullet. Consistent with Newton's third law of motion, the bullet pushes backwards upon the gun. The acceleration of the recoiling rifle is ... greater than the acceleration of the bullet. smaller than the acceleration of the bullet. c. the same size as the acceleration of the bullet.

Guns recoil when fired. c. the same size as the acceleration of the bullet. Yet, acceleration depends on both force and mass. The bullet has a greater acceleration due to the fact that it has a smaller mass. Remember: acceleration and mass are inversely proportional. Consistent with Newton's third law of motion, the bullet pushes backwards upon the gun. The acceleration of the recoiling rifle is ...

Challenge Question: If the action force is opposite and equal why do they not cancel ….remember this…. …have a net force of zero… …so no motion! Action and Reaction forces do not cancel because they are acting on different objects!

Identifying Action and Reaction Force Pairs 1. Baseball pushes glove leftwards. Reaction? The glove pushes the baseball rightward.

Identifying Action and Reaction Force Pairs 2. Bowling ball pushes pin leftwards Reaction? Pin pushes bowling ball rightward

Identifying Action and Reaction Force Pairs 3. Enclosed air particles push balloon wall outwards Reaction? Balloon wall pushes enclosed air particles inwards.

Review Newton’s first law of motion- Objects in motion stay in motion and objects at rest stay at rest, unless acted upon by an outside force; aka Law of Inertia Newton’s seconds law of motion- F=ma, force is proportional to the mass and acceleration of an object Newton’s third law of motion- For every action, there is an equal and opposite reaction.