forces and newton’s laws 2.1-2.2. what is a force? what is a force? a force is any push or pull...

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FORCES AND NEWTONS LAWS 2.1-2.2 Slide 2 What is a force? WHAT IS A FORCE? A force is any push or pull WHAT DOES FORCE DO? A force changes the velocity of an object, because it changes its speed and/or direction Slide 3 Why is force hard to define? It is something that you take for granted that exists although you know about it, you have never had to be specific about itIt is something that you take for granted that exists although you know about it, you have never had to be specific about it Also: there are many forces that are INVISIBLEAlso: there are many forces that are INVISIBLE If you push/pull on an object, you know where it comes from because you can see it happeningIf you push/pull on an object, you know where it comes from because you can see it happening But there are some that you cant seeBut there are some that you cant see Slide 4 Examples of invisible forces GravityGravity Objects falling towards the earth are changing their speed can you see what causes them to do this?Objects falling towards the earth are changing their speed can you see what causes them to do this? FrictionFriction Rolling objects that are pushed come to a stop but can you see what causes them to do this?Rolling objects that are pushed come to a stop but can you see what causes them to do this? Slide 5 Enter Newton Based on these basic observations, a scientist named Sir Issac Newton came up with 3 laws that describe the behaviour of forcesBased on these basic observations, a scientist named Sir Issac Newton came up with 3 laws that describe the behaviour of forces These are known as NEWTONS LAWS OF MOTIONThese are known as NEWTONS LAWS OF MOTION Slide 6 Newtons first law: ask yourself this Collisions are a good way to start thinking about Newtons first lawCollisions are a good way to start thinking about Newtons first law When two objects collide one object applies a force onto the otherWhen two objects collide one object applies a force onto the other When a car collides into a wall: what happens to the car and the wall?When a car collides into a wall: what happens to the car and the wall? When a truck collides into a car, what happens to the truck and the car?When a truck collides into a car, what happens to the truck and the car? Slide 7 http://www.consumerreports.org/cro/resour ces/streaming/crashtestpopups/ToyotaSolar a/TOYOTA-1.htmlhttp://www.consumerreports.org/cro/resour ces/streaming/crashtestpopups/ToyotaSolar a/TOYOTA-1.htmlhttp://www.consumerreports.org/cro/resour ces/streaming/crashtestpopups/ToyotaSolar a/TOYOTA-1.htmlhttp://www.consumerreports.org/cro/resour ces/streaming/crashtestpopups/ToyotaSolar a/TOYOTA-1.html Slide 8 What happens? Between the wall and the car:Between the wall and the car: The wall doesnt moveThe wall doesnt move The car reboundsThe car rebounds Between the truck and the car:Between the truck and the car: The truck moves forwardThe truck moves forward The car moves forwardThe car moves forward Slide 9 Why? IS THE CAR APPLYING A FORCE ONTO THE WALL?IS THE CAR APPLYING A FORCE ONTO THE WALL? YesYes WHY DOESNT THE WALL MOVE?WHY DOESNT THE WALL MOVE? Because the wall is rooted to the ground it is hard to move the wallBecause the wall is rooted to the ground it is hard to move the wall Slide 10 Why? WHY DOES THE TRUCK MOVE FORWARD EVEN AFTER SLAMMING INTO THE CAR?WHY DOES THE TRUCK MOVE FORWARD EVEN AFTER SLAMMING INTO THE CAR? Because the truck has a heavier mass than the car and therefore applies a large force that can cause the car to move without stopping itselfBecause the truck has a heavier mass than the car and therefore applies a large force that can cause the car to move without stopping itself Slide 11 In both cases. The force that is applied to the object will only move the object if the force is great enoughThe force that is applied to the object will only move the object if the force is great enough OBJECTS DONT MOVE OR CHANGE THEIR MOVEMENT UNLESS THEY ARE FORCED TO!OBJECTS DONT MOVE OR CHANGE THEIR MOVEMENT UNLESS THEY ARE FORCED TO! The amount of force required depends on the objects:The amount of force required depends on the objects: MassMass Friction keeping it in placeFriction keeping it in place Slide 12 What did Newton call this? INERTIA: the desire of an object to maintain its line of motionINERTIA: the desire of an object to maintain its line of motion Only a force great enough to overcome an objects INERTIA will cause it to change its direction Thats why: The wall stayed stillThe wall stayed still The truck kept moving the car didnt apply a great enough force to overcome its inertia to make it stopThe truck kept moving the car didnt apply a great enough force to overcome its inertia to make it stop Slide 13 NEWTONS FIRST LAW: LAW OF INERTIA 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 unbalanced force. 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 unbalanced force. Slide 14 What the heck? An object at rest tends to stay at restAn 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 directionand an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.unless acted upon by an unbalanced force. = An object wont move on its own =something moving wont change speed or direction = unless a force comes along to change it Slide 15 What is an unbalanced force? Unbalanced = not equalUnbalanced = not equal For example:For example: If one person pushes in one direction on an object:If one person pushes in one direction on an object: The object moves because there is only one force moving in one direction ITS NOT BALANCED Slide 16 What happens when forces are balanced? If two people push with the exact same force but in different directions on the same object:If two people push with the exact same force but in different directions on the same object: The object wont move because the two forces BALANCE EACH OTHER AND CANCEL EACH OTHER OUT! Slide 17 Newtons second law Physics is still about numbersPhysics is still about numbers So how do we put numbers into studying forces?So how do we put numbers into studying forces? We have to be able to MEASURE force somehow which means numbersWe have to be able to MEASURE force somehow which means numbers So: we know force causes an object to moveSo: we know force causes an object to move How do we calculate movement? (We just studied this)How do we calculate movement? (We just studied this) DISPLACEMENT, VELOCITY OR ACCELERATIONDISPLACEMENT, VELOCITY OR ACCELERATION WHICH ONE IS BEST?WHICH ONE IS BEST? Slide 18 DISPLACEMENT Difficult to tell remember that just because an object travelled far doesnt mean that it was pushed/pulled hardDifficult to tell remember that just because an object travelled far doesnt mean that it was pushed/pulled hard You have no idea how long it took to do itYou have no idea how long it took to do it If you have no idea how much time was involved then it could have been going slow (little force) or fast (more force)If you have no idea how much time was involved then it could have been going slow (little force) or fast (more force) Slide 19 VELOCITY Velocity overcomes the problem with distance it tells you both time and distanceVelocity overcomes the problem with distance it tells you both time and distance But objects can have a speed even though nothing is pushing it at the momentBut objects can have a speed even though nothing is pushing it at the moment SLIDING OBJECTS HAVE SPEED BUT NO APPARENT FORCE APPLIED TO THEM SO SPEED IS NOT RELIABLE Slide 20 THAT LEAVES ACCELERATION Acceleration is the best way to measure forceAcceleration is the best way to measure force Acceleration describes changes in speed and direction both of which are important to consider when studying forcesAcceleration describes changes in speed and direction both of which are important to consider when studying forces Slowing down, speeding up or changing direction ALL REQUIRE FORCESSlowing down, speeding up or changing direction ALL REQUIRE FORCES Slide 21 So what is force dependent on? Think about accelerating a truck vs. a carThink about accelerating a truck vs. a car WHICH ONE ACCELERATES FASTER?WHICH ONE ACCELERATES FASTER? A carA car WHICH ONE HAS A STRONGER ENGINE?WHICH ONE HAS A STRONGER ENGINE? A truckA truck LARGER ENGINE: DOES IT EQUAL MORE FORCE OR LESS FORCE?LARGER ENGINE: DOES IT EQUAL MORE FORCE OR LESS FORCE? Larger engines generate more force to push a vehicleLarger engines generate more force to push a vehicle WHY DOES A TRUCK ACCELERATE SLOWER EVEN THOUGH THE ENGINE IS LARGER?WHY DOES A TRUCK ACCELERATE SLOWER EVEN THOUGH THE ENGINE IS LARGER? Because the truck is heavierBecause the truck is heavier Slide 22 So acceleration of an object depends on: The amount of forceThe amount of force The mass of the objectThe mass of the object Newtons second law: The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object. Slide 23 What the heck? The acceleration of an object as produced by a net forceThe acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force is directly proportional to the magnitude of the net force in the same direction as the net force,in the same direction as the net force, and inversely proportional to the mass of the object.and inversely proportional to the mass of the object. =acceleration created by a net force =increases if net or unbalanced force increases, decreases if it

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