Download Speed and Velocity

Speed and Velocity
• Speed is simply how fast something is
going .
• we measure it in metres per second
(written as m/s or ms -1 )
• Example: if I walk 6m and it takes me 3
seconds, what is my speed?
Answer: speed = distance divided by time,
= 6 divided by 3, = 2 metres per second
• Average speed is really easy to work out:
total distance divided by total time for
the journey.
• Avg speed = Total Distance
Total time
Velocity
• Velocity is speed in a particular
direction.
• Ex : a car has a speed of 10 m/s, and go
on to say that it's velocity is 10 m/s
northwards
(i) Over what distance was the race
run?
(ii) How long did the runner take to
cover this distance?
iii) How can we conclude from the graph that
the runner ran at a steady speed during the
race?
iv) What was the runner’s average
speed?
(v) On the same axes plot a graph for a
runner who ran 96 m in 12 seconds at a
steady speed.
Acceleration: changing velocity
• Acceleration (a) of a body is the rate of change
of velocity with respect to time
• Acceleration = Change in velocity
Time
Velocity is measured in metres per second (m/s),
so acceleration is measured in metres per
second per second (written as m/s/s or m/s2 )
• Example: a car accelerates from 8 m/s to
20 m/s, and takes 6 seconds to do it.
What is the acceleration?
• Answer: change in velocity is 20 minus 8
= 12 m/s. Time taken for change is 6
seconds.
Acceleration = 12 divided by 6 = 2 m/s/s
Forces : pushes and pulls
• A force is a push or a pull. We measure
forces in "Newtons" (N)
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Forces can change:the speed of an object
the direction that an object is moving in
the shape of an object.
Make a stationary body move
Gravity
• Gravity is a force that acts towards the centre
of the Earth
• The gravitational pull of the Earth is what gives
objects weight. Thus weight is a force - it's
how hard the Earth is pulling on an object.
• The Earth pulls on every kilogramme with a
force of ten Newtons.In other words, an object
with a mass of 1kg has a weight of 10N.
Gravity Questions
• How hard does the Earth's gravity pull on
a 2kg object?
The Earth pulls on every kilogramme with
a force of 10N. This force is the object's
weight.So the weight of a 2kg object is:2 x 10 = 20 Newtons.
2. What is the weight of a 4kg
rock?
The Earth pulls on every kilogramme with
a force of 10N. This force is the object's
weight.So the weight of a 4kg rock is:4 x 10 = 40 Newtons.
3. What is the weight of a 100
gramme plastic toy?
Firstly, we need to get the mass into
kilogrammes.
There are 1,000 g in a kg, so 100 g is 0.1
kg. So the weight of a 0.1kg toy is:- 0.1 x
10 = 1 Newton.
Friction: slowing things down
• Friction is the force that resists the motion
of one surface relative to another with
which it is in contact .
1. "Static" or "sliding" friction
• The frictional force depends only on:-
• the type of surfaces
• how hard the surfaces are pressed
together
2. "Fluid" friction
• Fluid friction is also known as "drag". On
aircraft it's also called "air resistance".
• t depends on:• how thick the fluid is
• the shape of the object
• the speed of the object
Aircraft and car designers want to reduce drag, so
that the vehicle can go fast without having to waste
too much fuel.
Terminal velocity
• When an object falls, it accelerates. As its
speed increases, the air resistance
increases .
• Eventually, the force from the air
resistance will equal the force from the
weight of the object
Reducing friction
• We can reduce friction by
oiling ("lubricating") the
surfaces. This means that
the surfaces no longer rub
directly on each other, but
slide past on a layer of oil.
It's now much easier to
move them.
• Air Hockey" tables
in amusement
arcades use the
same principle. The
puck goes a long
way when you hit it,
because there's
almost no friction to
slow it down.
Using friction or Increasing
Friction
• We use friction to help us grip. This means
that our shoes grip the floor, so we don't
fall over.
Right now you're using a mouse, which
works because of friction between the ball
and the mouse mat.
• If it wasn't for friction between the tyres
and the road, driving a car would be like
trying to drive on an ice rink. This would
make cornering and stopping very difficult!
1 What does dry friction depend
upon?
• The frictional force depends only on:• the type of surfaces
• how hard the surfaces are pressed
together.
2. What is another name for fluid
friction?
• Fluid friction is the force that tries to slow
objects down when they move through a
liquid or a gas.
• It's also known as "drag", or "air
resistance".
3.What does fluid friction depend
upon?
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Fluid Friction depends on:how thick the fluid is
the shape of the object
the speed of the object
Remember that "fluid" means a liquid (e.g.
water) or a gas (e.g.air).
4. Why do we oil machinery?
• We oil ("lubricate") machinery to reduce
friction.
• This reduces wear, reduces frictional
heating, increases efficiency and extends
the life of the machine.
5. Why does a falling object reach
a "terminal velocity"?
• As an object falls faster and faster, the air
resistance will get more and more.
• Eventually, the air resistance will be equal
to the object's weight, so the object will
stay at that speed, and stop accelerating.
• This speed is called the terminal velocity.
Balanced Forces
Unbalanced Force
Upthrust
• Upthrust is the force that pushes an
object up and makes it seem to lose
weight in a fluid.
• (Remember, a fluid means a liquid or a
gas).
Mass
• Mass (m) of a body is the measure of
amount of matter in the substance.
• SI unit of mass is Kilogram (Kg)
Weight
• The weight (w) of a body is the
gravitational force exerted on it by Earth
Gravity
• Gravity
Gravity is a force that for us is
always directed downwards. But
to say that gravity acts
downwards is not correct.
Gravity acts down, no matter
where you stand on the Earth. It
is better to say that on Earth
gravity pulls objects towards
the centre of the Earth.
Weightlessnes
• Feeling "Weightless" When You Go "Over
the Hump"
Circular Motion
Centrifugal force: The force which acts towards the
centre of the circular path .