After reading through the revision material, attempt the following 4 practice questions. Each question has worked solutions of the method to solve the question, but we recommend that you try the questions without looking at the solutions first.
1. Bilblo’s door to his hobbit hole was blown down in a storm created by Smaug (it’s peak). This needs fixing. NOW. Frodo is helping Bilbo carry a new door from the specialist circular door shop. However it is usual for hobbits to skip leg day and so they can only withstand a wind blowing on the door of force 350N. The wind is blowing at 30mph, the door is 2m in diameter and 10cm thick. Will they be blown over by the wind?
Answer: 380.7 N (Therefore Bilbo and Frodo get blown over.)
2. Aladdin’s rug is going on an absolute bender. The rug is stiff and traveling at 0.7ms-1 into a headwind of 0.3ms-1. The kinematic viscosity of the air is 15.11x10-6 m2s-1 and density of the air is 1.225kgm-3. The rug is 2m long by 1.5m wide.
a) Calculate drag force on the rug.
b) Calculate boundary layer thickness at 1m.
Part A Answer: 13.4x10-3 N
Part B Answer: 13.7mm
3. Eminem throws up a ball of Mom’s spaghetti on his sweater already – his sweater is on the floor. While it’s flying through the air it forms a smooth ball which has a drag coefficient of 0.43. At the instant that it is falling at 0.8ms-1 it has a drag force of 0.0005N applied to it. Using this information, calculate what volume of spaghetti Eminem ate given that he threw up 80% of the spaghetti he ate.
Answer: 1.52x10-4 m3
4. Bart Simpson catapults a marble at Chief Wiggum’s son for eating all the ice cream. The marble has a relative roughness of 1.5x10-3 and has diameter of 1cm. It misses and pierces a container of mercury and travels at 1.2ms-1 through the liquid. The mercury has a kinematic viscosity of 0.1x10-6 m2s-1 and a density 13.534 gcm-3. Calculate the drag force on the marble.
Answer: 0.38 N