Transcript 4. acceleration and terminal velocity

12 April 2016
Today’s Title: CW: Forces and acceleration
Learning Question:
How is acceleration related to the size of a force?
Who will drop to
the ground faster,
and why?
http://www.bbc.co.uk/learningzone/clips/acceleration-and-force/25.html
Aims
• Able to give the correct units for force, mass
and acceleration
• Able to state newton's second law.
• Able to use newton's second law.
Resultant force
• The single force that has the same effect on the
object as all the individual forces acting together.
Size of the force
• An object will accelerate in the direction of
the resultant force.
• The bigger the force, the greater the
acceleration.
• Doubling the size of the (resultant)
force doubles the acceleration.
The mass
• An object will accelerate in the direction of
the resultant force.
• A force on a large mass will accelerate it less
than the same force on a smaller mass.
• Doubling the mass halves the acceleration.
1.
Draw an arrow to show the direction of
the resultant force.
2.
How will drop to the ground slower?
3.
Explain why you have made your choice.
Force = Mass X Acceleration
F
mxa
Forces and acceleration calculations
• You should know the equation that shows the relationship between
resultant force, mass and acceleration, and be able to use it.
The equation
Resultant force (newton, N) = mass (kg) × acceleration (m/s2).
• You can see from this equation that 1 N is the force needed to give 1 kg an
acceleration of 1 m/s2.
• For example, the force needed to accelerate a 10 kg mass by 5 m/s2 is:
• 10 x 5 = 50 N
• The same force could accelerate a 1 kg mass by 50 m/s2 or a 100 kg mass
by 0.5 m/s2.
• Putting it simply, we can say that it takes more force to accelerate a larger
mass.
Your task
• Complete questions 1 – 5 from pages 234 –
235 from the textbook.
• You have 10 minutes to answer these
questions
12 April 2016
Today’s Title: CW: Terminal velocity
Learning Question:
What is terminal velocity?
http://www.bbc.co.uk/learningzone/clips/why-does-a-human-have-adifferent-terminal-velocity-to-a-mouse/8751.html
Lesson objectives
Know: what ‘terminal velocity’ means
Understand: why falling things reach a
terminal velocity
Be able to: describe and explain the stages in
a parachute jump (including the forces
involved and whether or not acceleration is
happening at each stage)
Terminal velocity extra questions
1.
Explain why a falling human reaches a higher terminal velocity than a falling cat.
2.
Explain why a falling human without a parachute reaches a higher terminal velocity
than a falling human with one.
3.
Parachutes on the moon? – Comment on their usefulness.
4.
Film of parachute jumpers often makes it look as though they shoot upwards when
their parachute opens. They don’t – so why does it look that way?
5.
Sketch a graph showing the speed of a skydiver from the moment she jumps to the
moment she lands.
6.
What possible reasons can you think of for why the exam board decided terminal
velocity was important enough to be part of the Physics 2 module?
Terminal velocity extra questions – outline answers
1.
The object accelerates until the air resistance matches the weight. A human has a
bigger weight, so has to reach a higher speed before the air resistance matches it.
2.
A falling human with a parachute has a greater air resistance at a given speed, so does
not have to reach as high a speed for air resistance to match weight.
3.
Parachutes are useless on the moon, as there is no air, so no air resistance. Everything
keeps on accelerating to the end of its fall.
4.
The camera operator remains in free fall while he films the parachutist opening his
parachute. At this point, the parachutist suddenly slows down relative to the camera
operator, who shoots past him. This makes the parachutist appear to move upwards.
Velocity-time graph for parachutist…
Parachute opens – diver
slows down
Velocity
Speed
increases…
Terminal
velocity
reached…
Time
New, lower terminal velocity
reached
Diver hits the ground
Your task
• Complete questions 1 – 7 from pages 236 –
237 from the textbook.
• You will be given a test on the motion and
forces topic tomorrow!
• Study support is TONIGHT! No excuses!
Q1.
What does displacement
mean?
a. the speed of something
b. The speed and size of
something
c. The total path travelled
d. The distance between the
start and finish
D
Q2.
What is the equation to work
out speed?
a.
Speed (m/s) = distance ÷
time
b. Speed (m/s) = time –
distance
c. Speed (m/s) = distance x
time
d. Speed (m/s) = distance +
time
A
Q3.
What information does
velocity give you?
a. How fast something
is going
b. The magnitude of
something
c. The speed and
direction
d. The path of
something
C