Physics Answers

More documents

Recommendations

Info

QUESTION FIVE New car models are crash tested under laboratory conditions to ensure that safety standards are met. A 1500 kg car is accelerated to a speed of 30 ms -1 from stationary in 6 seconds, before hitting a brick wall without braking. (a) Calculate the force the causes the car to accelerate. Modern cars are built with crumple zones, this means that the cars are designed to fold up (crumple) as they hit an object. This results in a lot more damage to the car, but reduces injury to the passengers by increasing the TIME the car takes to stop completely during a crash. (b) Crumple zone Discuss the energy and forces involved as the car drives towards the wall, then crashes into it. In your answer: Describe the energy transformations that take place Calculate the energy the car has the instant before it hits the wall Explain how the crumple zone works to reduce the force on impact and reduce injury QUESTION FIVE ANSWERS (a) Achievement requires correct acceleration calculation (a) a = 30/6 = 5 ms -2 Merit requires correct force calculation from acceleration (m) F = ma = 1500 x 5 = 7500 N (b) Achievement requires ONE correct statement (a) Eg: Kinetic energy heat energy (as friction) + sound (a) (sound not required) OR Kinetic energy calculated Energy E k = 1/2mv 2 = ½ x 1500 x 30 2 = 675000 J (675kJ) Merit requires Energy transformation AND kinetic energy calculation OR explanation of crumple zone (m) Eg: Kinetic energy heat energy (as friction) + sound AND Energy E k = 1/2mv 2 = ½ x 1500 x 30 2 = 675000 J (675kJ) (m) OR Crumple zone increases stopping time, ie. deceleration is less which decreases the forces (F = ma) (m) Excellence needs discussion energy transformation, with kinetic energy or force linked to explanation of crumple zone (e) Eg: The car has a large amount of kinetic energy, ie. E k = 1/2mv 2 = ½ x 1500 x 30 2 = 675000 J (675kJ) just before it hits the wall. All of this energy must be transformed as the car stops, mostly into heat through friction, and some sound. As the car hits the wall, it decelerates rapidly, giving a very large force. A crumple zone allows the car to stop more slowly, ie. it decreases the deceleration by allowing the energy to be releases as heat over a longer time. Using F = ma, if a is smaller due to longer stopping time, then force must be less. If a passenger’s body decelerates more slowly with less force, less damage/injury will result. A car with no crumple zone will stop completely more quickly, ie. large deceleration so a larger force and greater damage. (e)
QUESTION SIX: IN THE GYM Two men were lifting weights in the gym. They were lifting a bar with weights that had a total mass of 80 kg. (a) State the total weight they were lifting and explain the difference between mass and weight. The two men were bench pressing the weights (pushing the weights upwards from their chest as far as their arms could extend). One man’s arms were 0.65 m long and the other man’s were 0.60 m long. Both men took exactly 2.5 seconds to fully extend their arms. The man with the longer arms said that he had to do more work than the man with the shorter arms but the man with the shorter arms was more powerful. (b) Discuss the statement (in bold above). In your answer, you should: calculate the work done by each man calculate the power exerted by each man compare the results with the statement in bold and explain your findings QUESTION SIX ANSWERS: IN THE GYM a) Calculation of weight: F w = mg = 80 x 10 = 800 N Explanation of difference: Mass is the amount of material / matter in an object. Weight is the gravitational force on an object (not the amount of gravity). b) Calculation of work: Long arms W = F x d = 800 x 0.65 = 520 J Short armsW = F x d = 800 x 0.60 = 480 J Calculation of power: Long arms P = W/t = 520 / 2.5 = 208 W Short arms P = W/t = 480 / 2.5 = 192 W OR statement that both took the same time to lift weights therefore the man that did the most work also exerted the most power. Comparison with statement: Man with longer arms did the most work as he exerted the same force over a greater distance. However, man with the longer arms was also exerting more power as he did the greater amount of work in the same time. For Achievement your answer should have two of: in (a), calculates the weight correctly OR makes an accurate statement about mass or weight in (b), one calculation is undertaken by selecting and substituting into the correct formula and solving it in (b), long armed man identified as either doing the most work or as exerting the most power For Merit your answer should have both of: in (a), calculates the weight correctly AND distinguishes between mass and weight in (b), correctly calculates work for both men AND calculates power correctly for one man OR makes a statement comparing work over the same time with power. For Excellence your answer should have both of: in (b), correctly calculates work for both men AND calculates power correctly for both men OR makes a statement comparing work over the same time with power for both men. in (b), compares results with statement and explains either the difference in work Or difference in power
Page 1 and 2: QUESTION ONE A car is moving at con
Page 3 and 4: QUESTION THREE While a 1200 kg car
Page 5: (iii) Compare the forces as the stu
Page 9 and 10: QUESTION EIGHT: CONSERVATION OF ENE

Physics Answers

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?