Assessment Schedule 2011 Physics: Demonstrate understanding of mechanical systems (90521)

Transcription

1 NCEA Level 3 Physics (9052) 20 page of 5 Assessment Schedule 20 Physics: Demonstrate understanding of mechanical systems (9052) Evidence Statement Q Evidence Achievement Merit Excellence ONE (a) f = = 75 Hz! = 2!f! = 2! " 75! = 47 rad s # 2 Correct. (b) = 2 I! 2 2 Correct kinetic energy. 2 Correct power. = 2 ".20 " 0#3 " 47 2 = 33 J P = t P = = 08 W he electric motor exerts a force on the axle holding the blade. he wood exerts a force on the teeth of the blade. he 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. Both forces described. One torque described. orques on the blade stated to be balanced. Both torques described. Merit plus the torques are stated to be balanced.

2 NCEA Level 3 Physics (9052) 20 page 2 of 5 WO (a) = 2! L g 2 Correct working. his is a show question. = 2! = s! 4.70 s (b)(i) v max = A!! = 2" 4.70! =.337 rads # v max = A! = 0.90 $.337 =.20 m s # 2 Correct ω. 2 Correct answer. (ii) Equilibrium position shown. Correct. 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. Recognition that the tension force provides the centripetal force. Recognition that tension force must provide both the centripetal force and must balance gravity. 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. Explanation for why the centripetal force AND the gravity component that tension must balance are both greatest at the equilibrium position. (d) = 2! l g = 2!! Angular frequency / frequency decreases because the time period is increases. v decreases because increases and angular frequency decreases.! = v r " l So increasing length makes v max decrease.

3 NCEA Level 3 Physics (9052) 20 page 3 of 5 (e) 2 Correct diagram. 2 Correct θ tot. Correct time consistent with using r = Correct time.! tot = sin " sin" 0.9 =.68 rad t =! tot # =.26 s $ 4.70' =.68 % & 2! ( ) HREE (a) Correct single force vector with correct label. (b) F g = GMm r 2 = 6.67! 0"! 5.974! 0 24!.08! 0 3 ( )! 0 7 = 60 N ( ) 2 2 Correct answer consistent with radius of earth not added. Correctly substituted equation. 2 Correct answer.

4 NCEA Level 3 Physics (9052) 20 page 4 of 5 F = mv2 r = 080v2 2.66! 0 7 v 2 =.498! 0 7 v = m s v = 2!r 2 Correct velocity. 2 Correct period in seconds. Correct period in hours consistent with incorrectly calculated velocity. 2 Calculated period compared with 2 hours. 2!! 2.66! = = 4382 s = 4382 (60! 60! 24) = days (d) 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. F g = F C plus reason why mass has no effect. 2 Correct formula for v 2. Correct formula for v 2 plus reason why mass has no effect. 2 Correct relationship developed. Correct relationship developed plus reason why mass has no effect. F = mv2 r = GMm r 2 v 2 = GM r but v = 2!r! so " # 2!r $ % & 2 = GM r r 3 = GM 4! 2 ' 2 (e) 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. Satellites must have this orbital period to be geostationary. Achievement plus either same direction or above the equator.

5 NCEA Level 3 Physics (9052) 20 page 5 of 5 Judgement Statement Achievement Achievement with Merit Achievement with Excellence 2 A + 2 A2 + 2 A M + M2 + 2 M + 3 A E + E2 + M + M2 + M + 2 A Note: where the criterion is not specified, the required grade(s) can be from either.