FORCES AND NEWTON’S LAWSFORCES AND NEWTON’S LAWS
2.1-2.22.1-2.2
What is a force?What is a force?
WHAT IS A FORCE?WHAT IS A FORCE?
A force is any push or pullA force is any push or pull
WHAT DOES FORCE DO?WHAT DOES FORCE DO?
A force changes the velocity of an object, because it A force changes the velocity of an object, because it changes its speed and/or directionchanges its speed and/or direction
Why is force hard to define?Why is force hard to define?
• It is something that you take for granted that It is something that you take for granted that exists – although you know about it, you exists – although you know about it, you have never had to be specific about ithave never had to be specific about it
• Also: there are many forces that are Also: there are many forces that are INVISIBLEINVISIBLE
• If you push/pull on an object, you know If you push/pull on an object, you know where it comes from because you can see it where it comes from because you can see it happeninghappening
• But there are some that you can’t seeBut there are some that you can’t see
Examples of “invisible” forcesExamples of “invisible” forces
• GravityGravity• Objects falling towards the earth are changing Objects falling towards the earth are changing
their speed – can you see what causes them to do their speed – can you see what causes them to do this?this?
• FrictionFriction• Rolling objects that are pushed come to a stop – Rolling objects that are pushed come to a stop –
but can you see what causes them to do this?but can you see what causes them to do this?
Enter NewtonEnter Newton
• Based on these basic observations, a scientist Based on these basic observations, a scientist named Sir Issac Newton came up with 3 laws named Sir Issac Newton came up with 3 laws that describe the behaviour of forcesthat describe the behaviour of forces
• These are known as NEWTON’S LAWS OF These are known as NEWTON’S LAWS OF MOTIONMOTION
Newton’s first law: ask yourself thisNewton’s first law: ask yourself this
• Collisions are a good way to start thinking about Collisions are a good way to start thinking about Newton’s first lawNewton’s first law
• When two objects collide – one object applies a When two objects collide – one object applies a force onto the otherforce onto the other
• When a car collides into a wall: what happens When a car collides into a wall: what happens to the car and the wall?to the car and the wall?
• When a truck collides into a car, what happens When a truck collides into a car, what happens to the truck and the car?to the truck and the car?
• http://www.consumerreports.org/cro/resourhttp://www.consumerreports.org/cro/resources/streaming/crashtestpopups/ToyotaSolarces/streaming/crashtestpopups/ToyotaSolara/TOYOTA-1.htmla/TOYOTA-1.html
What happens?What happens?
• Between the wall and the car:Between the wall and the car:• The wall doesn’t moveThe wall doesn’t 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
Why?Why?
• IS THE CAR APPLYING A FORCE IS THE CAR APPLYING A FORCE ONTO THE WALL?ONTO THE WALL?
• YesYes• WHY DOESN’T THE WALL MOVE?WHY DOESN’T THE WALL MOVE?• Because the wall is rooted to the ground – it Because the wall is rooted to the ground – it
is hard to move the wallis hard to move the wall
Why?Why?
• WHY DOES THE TRUCK MOVE WHY DOES THE TRUCK MOVE FORWARD EVEN AFTER SLAMMING FORWARD EVEN AFTER SLAMMING INTO THE CAR?INTO THE CAR?
• Because the truck has a heavier mass than Because the truck has a heavier mass than the car – and therefore applies a large force the car – and therefore applies a large force that can cause the car to move without that can cause the car to move without stopping itselfstopping itself
In both cases….In both cases….
• The force that is applied to the object will only The force that is applied to the object will only move the object if the force is great enough move the object if the force is great enough
• OBJECTS DON’T MOVE OR CHANGE OBJECTS DON’T MOVE OR CHANGE THEIR MOVEMENT UNLESS THEY ARE THEIR MOVEMENT UNLESS THEY ARE FORCED TO!FORCED TO!
• The amount of force required depends on the The amount of force required depends on the object’s:object’s:• Mass Mass • Friction keeping it in placeFriction keeping it in place
What did Newton call this?What did Newton call this?
• INERTIA:INERTIA: the “desire” of an object to maintain its line the “desire” of an object to maintain its line of motionof motion
Only a force great enough to overcome an object’s Only a force great enough to overcome an object’s INERTIAINERTIA will cause it to change its direction will cause it to change its direction
That’s why:That’s why:• The wall stayed stillThe wall stayed still• The truck kept moving – the car didn’t apply a great The truck kept moving – the car didn’t apply a great
enough force to overcome its inertia to make it stopenough force to overcome its inertia to make it stop
NEWTON’S FIRST LAW: LAW NEWTON’S FIRST LAW: LAW OF INERTIAOF INERTIA
An object at rest tends to stay at rest and an An object at rest tends to stay at rest and an object in motion tends to stay in motion object in motion tends to stay in motion with the same speed and in the same with the same speed and in the same direction unless acted upon by an direction unless acted upon by an unbalanced force.unbalanced force.
What the heck?What the heck?
• An object at rest tends An object at rest tends to stay at restto stay at rest
• and an object in and an object in motion tends to stay in motion tends to stay in motion with the same motion with the same speed and in the same speed and in the same directiondirection
• unless acted upon by unless acted upon by an unbalanced force.an unbalanced force.
= An object won’t move = An object won’t move on its ownon its own
=something moving =something moving won’t change speed or won’t change speed or directiondirection
= unless a force comes = unless a force comes along to change italong to change it
What is an unbalanced force?What is an unbalanced force?
• Unbalanced = not equalUnbalanced = not equal• For example:For example:• If one person pushes in one direction on an If one person pushes in one direction on an
object:object: The object moves because there is only one force moving in one direction – IT’S NOT BALANCED
What happens when forces are What happens when forces are balanced?balanced?
• If two people push with the exact same If two people push with the exact same force but in different directions on the same force but in different directions on the same object:object:
The object won’t move because the two forces BALANCE EACH OTHER AND CANCEL EACH OTHER OUT!
Newton’s second lawNewton’s 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 We have to be able to MEASURE force somehow
– which means numbers– 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 How do we calculate movement? (We just studied
this)this)• DISPLACEMENT, VELOCITY OR DISPLACEMENT, VELOCITY OR
ACCELERATION ACCELERATION • WHICH ONE IS BEST?WHICH ONE IS BEST?
DISPLACEMENTDISPLACEMENT
• Difficult to tell – remember that just Difficult to tell – remember that just because an object travelled far doesn’t because an object travelled far doesn’t mean that it was pushed/pulled hard mean that it was pushed/pulled hard
• You have no idea how long it took to do it You have no idea how long it took to do it • If you have no idea how much time was If you have no idea how much time was
involved – then it could have been going involved – then it could have been going slow (little force) or fast (more force)slow (little force) or fast (more force)
VELOCITYVELOCITY
• Velocity overcomes the problem with Velocity overcomes the problem with distance – it tells you both time and distancedistance – it tells you both time and distance
• But – objects can have a speed even though But – objects can have a speed even though nothing is pushing it at the momentnothing is pushing it at the moment
SLIDING OBJECTS HAVE SPEED BUT NO APPARENT FORCE APPLIED TO THEM – SO SPEED IS NOT RELIABLE
THAT LEAVES ACCELERATIONTHAT LEAVES ACCELERATION
• Acceleration is the best way to measure Acceleration is the best way to measure forceforce
• Acceleration describes changes in speed Acceleration describes changes in speed and direction – both of which are important and direction – both of which are important to consider when studying forcesto consider when studying forces
• Slowing down, speeding up or changing Slowing down, speeding up or changing direction ALL REQUIRE FORCESdirection ALL REQUIRE FORCES
So what is force dependent on?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 truck A truck • LARGER ENGINE: DOES IT EQUAL MORE FORCE LARGER ENGINE: DOES IT EQUAL MORE FORCE
OR LESS 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 WHY DOES A TRUCK ACCELERATE SLOWER
EVEN THOUGH THE ENGINE IS LARGER?EVEN THOUGH THE ENGINE IS LARGER?• Because the truck is heavierBecause the truck is heavier
So acceleration of an object depends So acceleration of an object depends on:on:
• The amount of force The amount of force • The mass of the objectThe mass of the object
Newton’s second law:Newton’s second law:The acceleration of an object as produced by a net The acceleration of an object as produced by a net
force is directly proportional to the magnitude of force is directly proportional to the magnitude of the net force, in the same direction as the net the net force, in the same direction as the net force, and inversely proportional to the mass of force, and inversely proportional to the mass of the object.the object.
What the heck?What the heck?
• The acceleration of an object as The acceleration of an object as produced by a net forceproduced by a net force
• is directly proportional to the is directly proportional to the magnitude of the net forcemagnitude of the net force
• in the same direction as the net in the same direction as the net force, force,
• and inversely proportional to and inversely proportional to the mass of the object.the mass of the object.
=acceleration created by a net =acceleration created by a net forceforce
=increases if net or unbalanced =increases if net or unbalanced force increases, decreases if it force increases, decreases if it decreasesdecreases
=if you push/pull left, object will =if you push/pull left, object will accelerate towards the left - accelerate towards the left - therefore, an object will therefore, an object will accelerate in the same direction accelerate in the same direction as net forceas net force
=acceleration increases if mass =acceleration increases if mass decreases, decreases if mass decreases, decreases if mass increasesincreases
Again….Again….
• If force increases, acceleration increasesIf force increases, acceleration increases• The stronger the push, the faster it increases speedThe stronger the push, the faster it increases speed• If force decreases, acceleration decreasesIf force decreases, acceleration decreases• The weaker the push, the slower it increases speedThe weaker the push, the slower it increases speed• If mass increases, acceleeration decreasesIf mass increases, acceleeration decreases• The heavier the object, the slower it increases speedThe heavier the object, the slower it increases speed• If mass decreases, acceleration increasesIf mass decreases, acceleration increases• The lighter the object, the faster it increases speedThe lighter the object, the faster it increases speed
Now the math..Now the math..
• Mathematically:Mathematically:
Fnet = maFnet = ma• Fnet = net/unbalanced force acting on an Fnet = net/unbalanced force acting on an
object in N (Newtons)object in N (Newtons)• m = mass of object in kgm = mass of object in kg• a = acceleration of object in m/sa = acceleration of object in m/s22
Weight vs. massWeight vs. mass
• MASSMASS: the amount of mass in an object measured in g or kg: the amount of mass in an object measured in g or kg• WEIGHTWEIGHT: the force of gravity acting on an object: the force of gravity acting on an object• Every object on earth accelerates towards the earth at 9.8 Every object on earth accelerates towards the earth at 9.8
m/sm/s22
• Therefore, every object’s weight or force of gravity acting Therefore, every object’s weight or force of gravity acting on it is based on Newton’s second lawon it is based on Newton’s second law
• Since: F = maSince: F = ma• And: a = gAnd: a = g
• Then force of gravity (FThen force of gravity (Fgg): F): Fgg = mg = mg
The third and funniest lawThe third and funniest law
• We know that objects only move if a force We know that objects only move if a force is applied on them – by the 1is applied on them – by the 1stst and 2 and 2ndnd law law
• But there are some cases where this is But there are some cases where this is difficult to seedifficult to see
Pushing off a wall?Pushing off a wall?
• Think about someone wearing rollerblades Think about someone wearing rollerblades that pushes off the wallthat pushes off the wall
• What happens?What happens?
If the child pushes on the wall
She moves backwards
Does this make sense when you follow Newton’s second law?
Direction of force applied by girl
Direction of acceleration of girlNo! According to
the 2nd law, the direction of force and acceleration has to be the same
Direction of force applied on girl by the wall
Direction of acceleration of girl
Based on this fact, Newton came up with the idea that in order for the girl to be moving backwards – THE WALL MUST BE PUSHING ON HER!
This is the idea behind Newton’s 3rd law: that anything that is pushed pushes back
That’s weirdThat’s weird
• Does that sound strange?Does that sound strange?• If you think about it – if you apply a force, there has If you think about it – if you apply a force, there has
to be contact between two surfacesto be contact between two surfaces• When two objects touch, force is exchanged When two objects touch, force is exchanged
between the twobetween the two• Imagine holding two pieces of paper in both hands, Imagine holding two pieces of paper in both hands,
and pushing against each other – they both bend!and pushing against each other – they both bend!• And the force exchanged between the two are equalAnd the force exchanged between the two are equal
Then why doesn’t the wall move?Then why doesn’t the wall move?
• This goes back to understanding Newton’s This goes back to understanding Newton’s first law – and also the collision between first law – and also the collision between the car and the wallthe car and the wall
• The girl pushes with a certain amount of The girl pushes with a certain amount of force against the wall – but because the wall force against the wall – but because the wall has a lot of inertia, the amount of force is has a lot of inertia, the amount of force is not great enough to move itnot great enough to move it
Then why does the girl move?Then why does the girl move?
• The girl has less inertia than the wallThe girl has less inertia than the wall• She is on wheels – which decreases the She is on wheels – which decreases the
amount of force needed to move somethingamount of force needed to move something• So even a little force – the amount that she So even a little force – the amount that she
is pushing off the wall – will cause her to is pushing off the wall – will cause her to movemove
Direction of force applied by girl
Direction of acceleration of girl
Direction of force applied on girl by the wall
Newton’s third lawNewton’s third law
• For every action, there is an equal and For every action, there is an equal and opposite reaction.opposite reaction.
What the heck?What the heck?
• For every actionFor every action • For every push or pull For every push or pull on an objecton an object
• There is an opposite There is an opposite and equal reactionand equal reaction
• The object pushes/pulls back in The object pushes/pulls back in the opposite direction on whatever the opposite direction on whatever it was that pushed/pulled on itit was that pushed/pulled on it
Newton’s 3Newton’s 3rdrd law explains why… law explains why…
• You don’t fall through the floor when you stand on You don’t fall through the floor when you stand on it!it!
• Think about it: when you stand on something, you Think about it: when you stand on something, you are essentially pushing down on it because gravity are essentially pushing down on it because gravity pulls down on you, and that pushes on the floorpulls down on you, and that pushes on the floor
• The reason why you don’t push through the floor is The reason why you don’t push through the floor is because the floor pushes back on youbecause the floor pushes back on you
• Since the forces are balanced – you stay on the Since the forces are balanced – you stay on the groundground
Force of gravity pushing on floor
Force of floor pushing back up on boy
If force of gravity is greater than the force of the floor…
If the force of the floor is greater than the force of gravity…
If both are equal…the boy stays on the ground!
Normal forceNormal force
• Normal force is the force that a surface Normal force is the force that a surface pushes back on an object that bears weight pushes back on an object that bears weight on iton it
• For example: your textbook sitting on your For example: your textbook sitting on your desk pushes down on your desk, and the desk pushes down on your desk, and the desk pushes back on the bookdesk pushes back on the book
• The force pushing back on the book is The force pushing back on the book is known as the normal force (Fknown as the normal force (FNN))
Communicating forcesCommunicating forces
• Free body diagrams are used to show all the Free body diagrams are used to show all the forces acting on an objectforces acting on an object
• You’ve seen how they have worked already You’ve seen how they have worked already in describing the boy on the ground or the in describing the boy on the ground or the girl pushing on the wallgirl pushing on the wall
Rules for free body diagramsRules for free body diagrams
• The object you are drawing is represented as a boxThe object you are drawing is represented as a box• Arrows are used to represent each force on the boxArrows are used to represent each force on the box• The tail end of the arrows attach to the boxThe tail end of the arrows attach to the box• The arrows move in the direction of the force acting The arrows move in the direction of the force acting
on the objecton the object• Don’t forget to add forces that you can’t see like Don’t forget to add forces that you can’t see like
friction, gravity, and normal forcefriction, gravity, and normal force
Draw a free body diagram of the forces on the cart
Force of man pushing cartForce of
friction on cart
Force of gravity on cart
Normal force on cart
Notice that you don’t have to worry about drawing the man in the free body diagram
Draw the free body diagram of the Draw the free body diagram of the forces on the garbage bagforces on the garbage bag
Force applied by the boy holding the bag (TENSION)
Force of gravity on the bag
Notice: since the object is hangingand is not touching the floorthere is no normal force since there is no surface contacting the bag!
