Transcript Forces and acceleration - science

20 July, 2015
Terminal Velocity
Objectives
Be able to describe why things fall at different rates,
define terminal velocity and calculate weight.
HSW: AF2 – Understanding the applications and implications of science
Used before in: lesson 5 braking forces
Will use again in:
PLTS: Team workers - collaborate with others to work towards common goals
Used before in: lesson 3 – speed
velocity acceleration
Will use again in:
Keywords
Terminal velocity, Drag, Surface area, Air resistance, Gravity, Acceleration,
Weight, Newton’s 2nd Law.
Outcomes
• All Must know that: The faster a body
moves through a fluid the greater the
frictional force which acts on it.
• Most Will explain what terminal velocity is
and be able to use the equation:
weight = mass × gravitational field strength.
• Some will remember the equation
weight = mass × gravitational field strength
and see how it links to Newton’s 2nd Law.
Amazing Jump…
Amazing jump

Why do we fall?

Why do we fall at different rates?

How can we change how fast we fall?

Does gravity matter?
Weight
Weight = Mass  acceleration due to gravity
The force due to gravity depends on the
masses of the two objects being attracted
to each other.
On Earth the force is 10N/kg
On the moon it is 1/6th of this – why?
Falling
objects.
acceleration = 10N/1kg
acceleration = 20N/2kg
10m/s2
1kg
g=10N/kg
ALL OBJECTS
What
ratefall
will
they
fall?
Force
= mass
x acceleration
They
at
the
FALL
AT
THE
This will be their
So acceleration.
2kg
Howsame
do we work
that/ mass
out?
acceleration
= Force
rate!
SAME
RATE.
In our case,
acceleration
= weightfall
/ massat What is the weight?
They
both
REALLY?
2 even
10m/s
THAT DOESN’T
thoughRIGHT…
their
SOUND
masses
are IT.
LET’S
PROVE
different!
20N
10N
What is the weight?
w=mxg
w = 1 x 10 = 10N
10m/s2
But…
• This is only true if there are no other
forces acting on the object except weight.
• What other force is there?
• Air Resistance (Drag/Friction)
Falling objects.
acceleration
acceleration =≠ 10N/1kg
10N/1kg
2
10m/s
It is less
1kg
1kg
1kg
g=10N/kg
Initially forces are unbalanced –
Object accelerates in direction of force
(down)
As the object goes faster it hits into
more air – the air resistance increases –
The resultant downward force is less.
So it speeds up less quickly –
The acceleration decreases.
The air resistance increases even more
until it is the same as the weight.
The resultant = 0N
There is no acceleration
TERMINAL VELOCITY
Terminal velocity of a skydiver
Velocity–time graph of skydiver
parachute
Terminal Velocity – Practical Time
Results:
Aim
To observe how objects fall
at different rates when dropped
through different fluids.
Total dist
travelled
/m
Distance
interval /m
Time to fall through
distance interval/ s
Mean velocity
over distance
interval / m s-1
Set up:
Tube 1
ball bearing
insulating
tape
viscous fluid
0.1
0.1
0.2
0.1
0.3
0.1
0.4
0.1
0.5
0.1
0.6
0.1
0.7
0.1
0.8
0.1
0.9
0.1
Tube 2
Tube 1
Tube 2
We will draw a velocity – time graph for each tube and compare
1) Copy and Complete (hint – keywords below in blue box):
Terminal Velocity
As objects move they have……………….
When they move ………… they have more air resistance.
When the air resistance (and friction) ……….. the force that is pushing them they will go at a
constant speed.
This is called the ……… ………...
Weight is the force on an object due to………...
Weight = …………. x gravity. (Gravity = 10m/s2)
equals
air resistance
Terminal Velocity
gravity
mass
quicker
2a) Copy the graph and then explain (i) why a sky diver reaches a maximum
speed and (ii) why the maximum speed is a lot lower when the parachute opens.
2b) (i) If the skydiver has a mass
of 60kg what will her weight be?
Don’t forget to write the units.
2b) (ii) If there is a resultant force of
450N on the skydiver, how long does
it take to reach 55.4m/s (124 mph)
(Hint – you will need to use F=ma
and equation for acceleration)
Outcomes Quiz
All :
• What happens to the frictional force on the body
when it moves through a fluid faster?
Most:
• What is terminal velocity?
• How can I make my terminal velocity increase?
• If weight = mass × gravitational field strength,
• What is my weight if I am 80Kg? (g=10N/kg)
Some:
• What is weight?
• What is Newton’s 2nd Law?