Download Level 3 Physics (90521) 2011 Assessment Schedule

NCEA Level 3 Physics (90521) 2011 — page 1 of 5
Assessment Schedule – 2011
Physics: Demonstrate understanding of mechanical systems (90521)
Evidence Statement
Q
Evidence
ONE
(a)
(b)
1 2
Iw
2
1
= ´ 1.20 ´ 10-3 ´ 4712
2
= 133 J
Achievement
2
Correct.
2
Correct kinetic energy.
2
Correct power.
1
Both forces described.
OR
One torque described.
OR
Torques on the blade stated to be
balanced.
1
Both torques described.
Ek (rot) =
Ek (rot)
Ek (rot)
P=
Merit
Excellence
Ek (rot)
t
133
P=
= 1108 W
0.12
(c)
The electric motor exerts a force on the axle holding the blade.
The wood exerts a force on the teeth of the blade. The torque
from this force balances the torque from the motor. Because the
force from the wood acts at a much greater radius, the size of
this force is much smaller.
1
Merit plus the torques are
stated to be balanced.
NCEA Level 3 Physics (90521) 2011 — page 2 of 5
TWO
(a)
(b)(i)
vmax = Aw
2
Correct working.
This is a show question.
2
Correct .
2
Correct answer.
2p
4.70
w = 1.337 rads -1
w=
vmax = Aw = 0.90 ´ 1.337
= 1.20 m s -1
(ii)
Equilibrium position shown.
1
Correct.
(c)
At all points the tension force has to provide the centripetal
force required to keep the bag moving in a circle and balance a
component of the force of gravity. At the equilibrium point, the
tension is greatest because the speed is greatest and the gravity
component is the full gravity force. At the extremes, the speed
is zero, so there is no centripetal force, and as one component of
the gravity force is providing the restoring force the other
component, which must be balanced by the tension force is
therefore less than the full gravity force. For both these reasons
the tension force at the end positions has decreased.
1
Recognition that the tension force
provides the centripetal force.
1
Recognition that tension force
must provide both the
centripetal force and must
balance gravity.
OR
Explanation for why the
centripetal force is greatest at
the equilibrium position. or
why the gravity component that
tension must balance is greatest
at the equilibrium position.
1
Angular frequency / frequency
decreases because the time period
is increases.
1
v decreases because T increases
and angular frequency
decreases.
(d)
So increasing length makes vmax decrease.
1
Explanation for why the
centripetal force AND the
gravity component that
tension must balance are
both greatest at the
equilibrium position.
NCEA Level 3 Physics (90521) 2011 — page 3 of 5
(e)
2
Correct diagram.
THREE
(a)
1
Correct single force vector with
correct label.
2
Correct answer consistent with
radius of earth not added.
OR
Correctly substituted equation.
(b)
Fg =
=
GMm
r2
6.67 ´ 10-11 ´ 5.974 ´ 1024 ´ 1.08 ´ 103
((0.636 + 2.02) ´ 10 )
7
= 610 N
2
2
Correct tot.
OR
Correct time consistent with
using r = 5.5.
2
Correct answer.
2
Correct time.
NCEA Level 3 Physics (90521) 2011 — page 4 of 5
2
Correct velocity.
2
Correct period in seconds.
OR
Correct period in hours
consistent with incorrectly
calculated velocity.
2
Calculated period compared
with 12 hours.
• Gravitational force must be equal to the Centripetal force
required at that radius.
• Centripetal acceleration of any object must equal the
acceleration due to gravity at that radius, regardless of its
mass.
1
Fg = FC plus reason why mass has
no effect.
Correct formula for v2.
1
Correct formula for v2 plus
reason why mass has no effect.
Correct relationship developed.
1
Correct relationship
developed plus reason why
mass has no effect.
For a satellite to be geostationary, it must orbit in the direction
of the Earth’s rotation, in the plane of the equator, and with an
orbital period of one Earth day.
1
Satellites must have this orbital
period to be geostationary.
1
(c)
(d)
(e)
2
2
Achievement plus either same
direction or above the equator.
NCEA Level 3 Physics (90521) 2011 — page 5 of 5
Judgement Statement
Achievement
Achievement with Merit
Achievement with Excellence
2 A1 + 2 A2 + 2 A
1 M1 + 1 M2 + 2 M + 3 A
1 E1 + 1 E2 + 1 M1 + 1 M2 + 1 M
+2A
Note: where the criterion is not specified, the required grade(s) can be from either.