lessons 7 newtons laws
TRANSCRIPT
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Newtons laws of motion
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Newtons Laws of Motion
Thats
me!
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Newtons 1st Law
If there is no resultant force acting on an
object, it will move with constant velocity.
(Note the constant velocity could be zero).
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Newtons 1st Law
If there is no
resultant force acting
on an object, it willmove with constant
velocity. (Note the
constant velocity
could be zero).
Does this make
sense?
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Newtons 1st Law
If there is no
resultant force acting
on an object, it willmove with constant
velocity. (Note the
constant velocity
could be zero).
Can you copy it
whilst you think
about it?
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Newtons 1st law
Newtons first law was actually discovered by
Galileo.
Newton nicked it!
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Newtons first law
Galileo imagined a marble rolling in a very
smooth (i.e. no friction) bowl.
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Newtons first lawIf you let go of the ball, it always rolls up theopposite side until it reaches its original height
(this actually comes from the conservation of
energy).
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Newtons first lawNo matter how long the bowl, this alwayshappens
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Newtons first lawNo matter how long the bowl, this alwayshappens.
constant velocity
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Newtons first lawGalileo imagined an infinitely long bowl wherethe ball never reaches the other side!
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Newtons first lawThe ball travels with constant velocity until its reachesthe other side (which it never does!).
Galileo realised that this was the natural state of objects
when no (resultant ) forces act.
constant velocity
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Another example
Imagine Mr Porter cycling at
constant velocity.
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Newtons 1st law
He is providing a pushing force.
Constant velocity
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Newtons 1st law
There is an equal and opposite
friction force.
Constant velocity
Pushing force
friction
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Newtons second law
Newtons second law concerns examples
where there is a resultant force.
I thought of this
law myself!
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Lets go back to Mr Porter on his
bike.Remember when the forces are balanced
(no resultant force) he travels at constant
velocity.
Constant velocity
Pushing force
friction
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Newtons 2nd law
Now lets imagine what happens if
he pedals faster.
Pushing force
friction
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Newtons 2nd law
His velocity changes (goes faster).
He accelerates!
Pushing force
friction
acceleration
Remember that acceleration is rate of
change of velocity. In other words
acceleration = (change in velocity)/time
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Newtons 2nd law
Now imagine what happens if he stops pedalling.
friction
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Newtons 2nd law
He slows down (decellerates). This is a negative
acceleration.
friction
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Newtons 2nd law
So when there is a resultant force,
an object accelerates (changes
velocity)
Pushing force
friction
Mr Porters Porche
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Newtons 2nd law
There is a mathematical relationship
between the resultant force and
acceleration.
Resultant force (N) = mass (kg) x acceleration (m/s2)
FR = maIts physics,
theres always a
mathematical
relationship!
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Newtons 2nd law
There is a mathematical relationship
between the resultant force and
acceleration.
Resultant force (N) = mass (kg) x acceleration (m/s2)
FR = maCan you
copy thistoo?
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An example
What will be Mr Porters
acceleration?
Pushing force (100 N)
Friction (60 N)
Mass of Mr Porter and bike = 100
kg
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An example
Resultant force = 10060 = 40 N
FR= ma
40 = 100aa = 0.4 m/s2
Pushing force (100 N)
Friction (60 N)
Mass of Mr Porter and bike = 100 kg
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Newtons 3rd lawIf a body A exerts a force on body B, body B will exert an equal but
opposite force on body A.
Hand (body A) exerts force ontable (body B)
Table (body B) exerts force on
hand (body A)
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Dont worry!
We wont do Newtons 3rd law until next year!
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Thats all folks!