Presentation on theme: "Newton’s Laws of Motion"— Presentation transcript:
1Newton’s Laws of Motion 1st Law – Law of Inertia2nd Law – Force = Mass x Acceleration3rd Law – Action-ReactionEverything to do with motion, forces, gravity, speed, or acceleration is governed by these laws.
2Newton’s First Law of Motion “Law of Inertia” An object will stay at rest unless acted on by an unbalanced forceAn object in motion will stay in motion unless acted on by an unbalanced force
4InertiaInertia – the tendency of an object to resist change in motion (whether at rest or moving)Table Cloth InertiaEgg Drop Inertia
5What does this mean?An object will “keep doing what it was doing” unless acted on by an unbalanced forceIf the object was sitting still, it will remain stationaryIf it was moving at a constant velocity, it will keep movingIt takes force to change the motion of an object
6Mass and InertiaThe amount of inertia an object has depends on its massThe greater the mass the greater the inertiaMass is measured in grams or kilograms
7The soccer ball has a little mass and therefore a little inertia When kicked it is easy to put it into motion because it has low inertia
8The brick wall has a large mass and therefore a lot of inertia When kicked it is difficult to put it into motion because it has high inertia
9Objects at rest remain at rest unless acted on by a net force. Since the train is so heavy, it is difficult to change its speed.Since the baby carriage is so small, it is very easy to change its speed.
10Which was easier to put into motion? Flick a pencilFlick a binderWhich was easier to put into motion?
11Objects in motion remain in motion in a straight line (unless acted upon by an outside force). Since the train is so heavy, it is difficult to stop it once it is moving.Since the soccer ball is so small, it is very easy to stop it once it is moving.
12Which was easier to stop? Stop a pencilStop a binderWhich was easier to stop?
13Two crates, one heavy and one light, are at rest on a waxed floor Two crates, one heavy and one light, are at rest on a waxed floor. Which crate will need a greater force to provide the same acceleration? Use the concept of inertia to explain your answer.The heavy crate has more inertia, so it requires more force to make it accelerate.
14Section ReviewWhat is inertia? How is it related to Newton’s first law of motion?Inertia is the tendency of an object to resist any change in its motion. Newton’s first law is called the law of inertia. The law states that an object in rest stays in rest and an object in motion stays in motion unless acted on by an outside force.
15Friction and GravityNewton’s First Law of Motion states that an object in motion will stay in motion unless acted on by an outside force.Two forces that act on objects are friction and gravity.
16FrictionFriction acts in a direction opposite to the object’s direction of motionWithout friction, the object would continue to move at a constant speed foreverThe strength of the force of friction depends on two factors:the types of surfaceshow hard the surfaces push together.
17Types of Friction1. Sliding Friction – solid surfaces slide over each other2. Rolling Friction – when an object rolls over a surfaceForce of friction is less in rolling friction than with sliding friction3. Fluid Friction – when an object moves through a fluidForce of friction is less in fluid friction than with sliding friction
18Explain the role of friction in walking on a sidewalk versus walking on a sidewalk covered with ice. Friction between your shoe and the sidewalk allows you to push against the sidewalk. Ice decreases this friction.
19Gravity Gravity is the force that pulls objects toward each other Force and GravityGravity is the force that pulls objects toward each other
20Air ResistanceObjects falling through air experience a type of friction called air resistanceAs surface area increases, air resistance increases.Eventually, air resistance equals gravityThe greatest velocity an object reaches is called terminal velocityIn a vacuum, all objects fall at the same rateResultant Force Podcast
22Weight vs. MassWeight – a measure of the force of gravity on an object. (weight is a force) (Weight on the moon is 1/6 of your weight on Earth)Mass – a measure of the amount of matter in that object.Weight = Mass x Acceleration due to gravityUnits of Measure:Weight – measured in NewtonsMass – measured in KgAcceleration – measured in m/s2(50 Kg person weighs 50kg x 9.8m/s2 = 490 Newtons)Your Weight on Other WorldsNewton’s 1st Law of Motion
23Some Examples from Real Life A soccer ball is sitting at rest. It takes an unbalanced force of a kick to change its motion.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.
24Check Your Understanding Ben Tooclose is being chased through the woods by a mooseIf Ben takes a zigzag pattern through the woods, he will be able to use the large mass of the moose to avoid being caughtExplain this in terms of inertia and Newton's first law of motion
25AnswerThe large mass of the bull moose means that the bull moose has a large inertia.Thus, Ben can more easily change his own state of motion (make quick changes in direction) while the moose has extreme difficulty changing its state of motion.Physics for better living!
27Newton’s Second Law of Motion The acceleration of an object is inversely proportional to the mass of the object and directly proportional to the forceUnits of measure:Acceleration = m/s2Mass = KgNewton = Kg x m/s2Newton - is the force needed to accelerate one kg of mass at 1 meter/sec 2 (1 N = 1kg x 1m/s2)Force = mass x accelerationF = maNewton’s 2nd Law
32Graphing Newton’s 2nd Law For the same force:more mass causes less accelerationless mass causes more acceleration
33Graphing Newton’s 2nd Law For the same mass:more force causes more accelerationless force causes less acceleration
34Graphing Newton’s 2nd Law For the same acceleration:more mass needs more forceless mass needs less force
35What does F = ma say?F = ma basically means that the force of an object comes from its mass and its acceleration.Something very massive (high mass) that’s changing speed very slowly (low acceleration), like a glacier, can still have great force.Something very small (low mass) that’s changing speed very quickly (high acceleration), like a bullet, can still have a great force.Something very small changing speed very slowly will have a very weak force.
36A Little PracticeHow much force will you need to accelerate a 200kg wagon at 5m/s2?1000 NHow much force will you need to accelerate a 10,000kg train at 5m/s250,000 NNewton’s Second Law Summary
37Question 1When the net force on an object increases, how does the object’s acceleration change?The acceleration increases.
38Question 2Suppose you know the acceleration of a shopping car as it rolls down a supermarket aisle. You want to find the net force with which it was pushed. What other information do you need in order to find the force?You need to know the mass of the shopping cart.
39Question 3Suppose you doubled the force acting on an object. In what way could you change its mass to keep its acceleration?If you double the net force acting on an object, you have to double the mass of the object to keep the acceleration unchanged.Newton’s Second Law
40Newton’s 3rd LawAction-ReactionAction and Reaction
41Newton’ s 3rd Law of Motion All forces occur in pairs, and these two forces are equal in strength and opposite in directionFor every action there is an equal and opposite reactionReactionAction
42What does this mean?Right now, gravity is pulling you down in your seat, but Newton’s Third Law says your seat is pushing up against you with equal forceThis is why you are not moving. There is a balanced force acting on you– gravity pulling down, your seat pushing up
43Equal but OppositeIce Skaters (push off of each other with equal force in opposite directions):The speed at which the two skaters move depend on their masses. (the greater the mass the slower the speed)
44Think about itWhat happens if you are standing on a skateboard and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force.Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts).
45Check Your Understanding While driving down the road, an unfortunate bug strikes the windshield of a busThe bug hits the windshield and the windshield hits the bug.Which of the two forces is greater:the force on the bug orthe force on the bus?
46Answer Trick Question! Each force is the same size. For every action, there is an equal ... (equal!).The fact that the bug splatters only means that with its smaller mass, it is less able to withstand the larger acceleration resulting from the interaction.
47Check Your Understanding A gun recoils when it is fired.The recoil is the result of action- reaction force pairs.As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards.The acceleration of the recoiling gun is ...
48AnswerThe force on the gun equals the force on the bullet (Newton’s 3rd Law)Yet, acceleration depends on both force and mass (Newton’s 2nd Law)The bullet has a greater acceleration due to the fact that it has a smaller mass.Remember: acceleration and mass are inversely proportional (Newton’s 2nd Law)
49Question 3Suppose you and a friend, who has exactly twice your mass, are on skates. You push away from your friend. How does the force with which you push your friend compare to the force with which your friend pushes you? How do your accelerations compare?The force with which you push your friend is equal and opposite to the force with which your friend pushes you. You accelerate faster than your friend and roll back further because your mass is less.Newton’s 3rd LawLego - Newton’s Laws of MotionLaws of Motion Rap
50Review (F = ma). Newton’s First Law (Inertia): Objects in motion tend to stay in motion and objects at rest tend to stay at rest unless acted upon by an unbalanced force.Newton’s Second Law (F=ma):Force equals mass times acceleration(F = ma).Newton’s Third Law:For every action there is an equal and opposite reaction.