Transcript Newton`s 1st Law

VELOCITY
Remember we mentioned the word
‘Velocity’ before?
What does ‘Velocity’ mean?
Is this runner leaning? Which way?
Leaning means Acceleration.
So this runner is accelerating.
But is he ‘speeding up’ or ‘slowing down’?
Neither.
He is ‘changing direction’.
A runner leans forwards to speed up:
A runner leans backwards to slow down:
A runner leans sideways to CHANGE DIRECTION:
So,
acceleration is
a change of speed
and/or
a change of direction.
Can we simplify this definition?
We can.
If we use a new quantity: VELOCITY,
that specifies a speed in a particular direction
These are speeds:
50 mph, 60 mph
These are velocities:
50 mph northbound, 60 mph eastwards
(speed AND direction)
Would a police car travelling at 120 mph catch a
speeding granny going at 71 mph?
Would a police car travelling at 120 mph
northbound from Junction 7 catch a speeding
granny going at 71 mph southbound from
Junction 7?
Direction can matter.
That is why we use the quantity velocity as
well as the quantity speed.
You have met other quantities that use
direction…
Other quantities that use direction…
Force.
The direction of a force matters as much
as the strength of a force.
Imagine trying to push open a door
marked ‘pull’…
Other quantities that use direction…
Acceleration
Acceleration in the direction you are
travelling speeds you up…
Acceleration in the direction you are
NOT travelling slows you down… its
called deceleration.
So, direction matters.
That is why we use the quantity velocity
as well as the quantity speed, because
when we use velocity we are saying that
we are interested not just ‘how fast’ but
also ‘in what direction’.
And if you use velocity, the definition of
acceleration become a lot simpler…
Acceleration is
a change of velocity
(this could be a change in the value of
the velocity or a change in the direction
of the velocity)
A 200 metre runner
First they accelerate away
from the start (leaning
forward) : changing speed but
not direction.
Then they accelerate around the
bend (leaning sideways) :
changing direction but not speed.
Now, from Newton’s 2nd Law we know that
accelerations are caused by the application of
Forces.
And depending on the direction of the
force they can cause speeding up,
slowing down or a change of direction.
Take Jonah.
He’s running at top speed.
Other players could apply
force to him to:
•Help push him over the line
•Slow him down
•Push him into touch
Oops…
If there is no force,
there is no acceleration,
so the velocity DOES NOT CHANGE.
Does that make sense?
It made sense to Newton as well.
It’s Newton’s 1st Law.
If there is no force
The velocity does not change
But stays at whatever value (and direction)
it started at.
It makes perfect sense when the starting
velocity is 0 (standing still).
But can you believe it when the starting velocity
is some value like 8 mps rightwards?
Moving objects stop eventually… don’t they?
They don’t keep going for ever… do they?