FLUID MECHANICS AND PROJECTILE MOTION. CHAPTER 5: Fluid mechanics and projectile motion. Practice questions - text book pages 103 to 104

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1 PART 3 CHAPTER 5 CHAPTER 5: Fluid mechanics and projectile motion Practice questions - text ook pages 103 to 104 1) Which sentence est explains the of a projectile? a. the projectile travels further if air resistance is large compared with its.. a projectile ejected at 45 o to the horizontal will travel the furthest. c. the path of a projectile falls from its initial direction caused y gravity only.. d. and fluid friction are the only s acting on a projectile. c. 2) The effect causes a sideways on an oject moving through a fluid ecause: a. fluids flow in a laminar pattern past a moving oject.. the pressure exerted y a fast moving fluid is less than that exerted y a slow moving fluid. c. the pressure exerted y a fast moving fluid is greater than that exerted y a slow moving fluid. d. an unstreamlined oject will cause fluid flow to reak into vortices.. 3) If every particle of fluid has irregular flow, then flow is said to e: a. laminar flow.. turulent flow. c fluid flow. d oth a and.. 4) A racing car has a ody with: a. laminated design.. turulent design. c. flat design. d. streamlined design. d. 5) Which of the following reasons explains why the est take-off angle for an elite long jumper is much closer to 22 degrees than to the predicted optimum angle from projectile motion theory of aout 42 degrees? a. the jumper needs to generate rotation.. achieving a take-off angle close to the theoretical value would drastically reduce take-off speed. c. the theory ignores air resistance. d. the jumper does not understand projectile motion theory.. 42

2 QUESTIONS AND ANSWERS QUESTIONS AND ANSWERS 6) a) Using examples, explain how the shape of an oject can alter its path. for four of: The size of the air resistance (drag) depends on the size of the oject as it is viewed from the forward direction (the technical name for this is forward cross section). The igger this size the igger the air resistance. Also, air resistance depends on the streamlining effect. The more the shape allows laminar flow (flow in layers without vortices) of air past the oject, the less will e the air resistance (see figure Q5.1). figure Q5.1 air resistance laminar flow moving cycle helmet The effect of air resistance is to deviate the path from the symmetric shape known as a paraola. Air resistance acts in a direction opposite to the direction of motion of the oject. Therefore the oject would always e moving more slowly (than if there were no air resistance). The igger the air resistance compared to the, the igger the asymmetry of the path. ) Explain the effect of air resistance on the of two adminton shuttles, one of which has een struck hard and the other gently. 10 marks 5 marks for five of: A rapidly moving adminton shuttle will have a very large value for air resistance at the eginning of its. Compared to the of the shuttle. Therefore the resultant (see figure Q5.2a) is almost in the same direction as the air resistance. Later in the, the shuttle would have slowed consideraly. Hence the air resistance value will have dropped. Until the is much igger than the air resistance. Then the shuttle would fall as if under gravity only. Hence a path which differs markedly from the symmetric paraolic path which would e oserved if there were zero or very little air resistance. See figure Q marks for five of : Badminton shuttle struck gently - see figure Q5.3a. Here the is the predominant. Because the shuttle is moving slowly, air resistance is small. Hence resultant is almost that of the only. And the is almost paraolic. See figure Q5.3. a resultant air flow figure Q5.2 s acting on a adminton shuttle a air resistance air resistance resultant fast moving figure Q5.3 s acting on a adminton shuttle slow moving c) Briefly explain why the path of a shot in athletics is so different from the of a adminton shuttle. Shot in (see figure Q5.4) - resultant is almost in the same direction as the. Hence the of the shot is similar to gravity only. Badminton shuttle in - resultant is almost in the same direction as air resistance. Opposite to the direction of motion hence marked deceleration and asymmetric path. figure Q5.4 air resistance for shot shot air resistance resultant Answers 43

3 PART TOPIC 32 CHAPTER 58 7) a) Identify three physical factors (not skill factors) which govern a swimmer s speed and explain how one of these occurs. Thrust on the water ( exerted y hands and feet on the water). Body shape (which est allows smooth flow of water past the ody). Surface effects (like hair or shiny swimsuits or skullcap etc) which change the water flow past the ody. Dive entry or thrust on ath side on turning. ) Descrie the factors which determine the amount of fluid friction acting on a swimmer. Speed of the swimmer. Forward cross section (size of the person as viewed from the forward direction). Body shape or surface effects (smooth flow). Surface area in contact with the water (ig person as opposed to small person). Avoidance of water surface effects (like for example swimming underwater after a turn). c) Explain how you would minimise turulent flow (high drag) of the water past the swimmer s ody. 2 marks By shaving ody hair. By wearing a swim cap. Swimwear surface (shiny or directionally flocked) and shape (high neck to avoid drag at neckline). d) Give three examples, each from a different sporting context, to show how fluid friction affects the sportsperson. for three of: Air resistance involving performer (athlete running, skier, cyclist, parachutist). Air resistance involving sports vehicle (racing car, glider, cyclist). Air resistance involving projectile (adminton shuttle, all). Water resistance involving performer (swimmer). Water resistance on sports vehicle (water skis, canoeing, rowing, sailing, speedoats). e) How would you attempt to reduce fluid friction? Reducing forward cross sectional area y crouching. Cyclist - special ike shape to help with this, crouching when skiing. Removing resistance from surface. Special clothing (lycra), shaving head, athing cap, removing protruding its of cycle or oat. Reduce area of contact. Boat, windsurfer, water skis (aquaplaning). f) Look at figure 5.20 showing the vertical s acting on a swimmer during a stroke. Explain why it is difficult for a swimmer to keep a horizontal floating position. The two s ( and uoyancy ) do not act through the same point. The s are eccentric (are not in line). This causes a turning effect, in this case the ody is turned y these s anticlockwise. With the swimmer s feet falling downwards relative to the head. figure 5.20 s acting on a swimmer uoyancy 44

4 QUESTIONS AND ANSWERS QUESTIONS AND ANSWERS 8) a) Fluid friction is a which acts on a osleigh once it is moving. Identify the nature of the fluid friction in this case and explain how this might limit the maximum speed of the o. The fluid friction in this case is air resistance or drag. This increases if turulent flow occurs, or if streamlining reaks down. This increases if the size of the o (and its occupants) is larger than it could e (y for example a oman putting his head out of the top of the o). As the speed increases, the fluid friction gets igger, until it matches the component down the track, then the o couldn t go any faster. ) Explain the term turulent flow, and how the osleigh is used to minimise this factor. for three of: At low speeds, air flow past a moving oject is laminar, which means the air flows in layers. When this flow is interrupted either y going too fast or y a protrusion (which upsets the streamlined shape), then the air is thrown out into vortices, the layers mix up, and this is turulent flow. When there is turulent flow, the air resistance is greater ecause the o is forcing this air to e thrown into the vortex patterns. In order to minimise turulent flow, the o has to e as streamlined as possile (so that flow is as laminar as possile). This is done y having a specially designed streamlined shape to the o. And y having no protrusions from the o while in motion - handles, heads of omen (this is why the omen crouch down once in the o after the start). 9) a) Sketch a diagram to show the path of the shot from the moment it leaves the putter s hand to the moment it lands. See figure Q5.5. Flight path a paraola not a circle. 2 marks figure Q5.5 path of a shot ) State and riefly explain three factors (excluding air effects) which should e used y the putter to optimise the distance thrown. 6 marks Speed of release. The faster the shot is released the further it will go. Angle of release. The optimum angle depends on height of release, ut would e etween 42 o and 45 o. Height of release aove the ground. The higher the release, the further the shot will travel. c) Explain why the turn in a discus throw produces greater horizontal range than the standing throw. Forces are applied to the discus over a larger distance. Since work = x distance, this means that more energy is given to the discus, which therefore will have more kinetic energy on release. Or, the s are applied over a longer time. Therefore the x time (impulse) is igger, and the change of momentum igger. So the discus has a higher speed at release. Or, the discus is accelerated to some degree efore the standing throw position is reached, and hence the turn produces extra velocity over and aove the standing throw. Answers 45

5 TOPIC PART 32 CHAPTER 75 10) a) The Magnus effect (the effect applied to spinning alls) states that a faster flowing liquid or gas exerts less pressure than a slower moving liquid or gas. Using figure 5.21, show how the Magnus effect explains the swerve of a spinning all. The all moving through the air causes the air flow to separate, with air flowing further past the lower half of the all. More air is sent on the lower route, since as the air arrives at the nearest edge of the all, it is dragged down y the downward spinning surface of the all. In the same time as the air flow over the top of the all (the air is a fixed entity - the all moves through it). Therefore the air flows faster past the lower half of the all. Therefore there is less pressure on the ottom of the all. Hence the all will experience a (the Magnus effect or effect) downwards. figure 5.21 Magnus effect on a spinning all direction of air flow ) Use diagrams to show how your explanation relates to the of a tale tennis all with side, ack and top spin. Side spin, see figure Q5.6a. Back spin see figure Q5.6. Note tendency to travel straighter. Or even lift. Top spin see figure Q5.6c. Note pronounced curve downwards (dip). a side spin original direction figure Q5.6 path of a spinning all ack spin normal soaring normal c top spin dipping c) Sketch a vector diagram of all s acting on a tale tennis all in with ack spin, and explain how the resultant on the all predicts the actual acceleration of the all. See figure Q5.7. Note that the effect is upwards, downwards, and air resistance (fluid friction) in a direction opposite to the direction of motion of the all. effect is equal or igger than the. Resultant would therefore e upwards and to the left of the diagram. Acceleration of the all (deceleration) is in the same direction as the resultant, i.e. upwards and to the left. Hence the path upwards and to the left of original direction. figure Q5.7 s acting on a spinning all ack sp in B er n o ulli f orc e air d i re ct ion resistance o f t rav el w e igh t d) Identify one sport other than a all game, in which the effect plays a part. 1 mark Ground effects in racing cars. Wing effects on gliding or flying or kites. Effects on ski jumping. Effects on hand shape in swimming (enales greater thrust on water). 46

6 QUESTIONS AND ANSWERS QUESTIONS AND ANSWERS 11) What do you understand y the Magnus effect? Explain how a knowledge of Magnus s can assist a tennis player to execute different types of spins. The Magnus effect (the on a spinning all) is caused y a spnning all moving through air as shown in figure Q5.8. The all moving through the air causes the air flow to separate, with air flowing further past the lower half of the all. More air is sent on the lower route, since as the air arrives at the nearest edge of the all, it is dragged down y the downward spinning surface of the all. In the same time as the air flow over the top of the all (the air is a fixed entity - the all moves through it). Therefore the air flows faster past the lower half of the all. Therefore there is less pressure on the ottom of the all. Hence the all will experience a downwards. 10 marks figure Q5.8 Magnus effect on a spinning all direction of air flow Side spin, see figure Q5.9a. Back spin see figure Q5.9. Note tendency to travel straighter. Or even lift. Top spin see figure Q5.9c. Note pronounced curve downwards (dip). The tennis player will impart these different spins to the all to achieve side spin, ack spin, ut usually top spin. And hence control the direction of the all in. a side spin original direction figure Q5.9 path of a spinning all ack spin normal soaring normal c top spin dipping 12) Compare and contrast the use of side spin in footall, and the hook in golf. Both use the Magnus effect to impart a side-spin to footall or golf all. Both apply a to the all to the side of its centre of mass (eccentric ). The golfing hook is caused y a applied to the right of a all s centre of mass which will spin anticlockwise (viewed from aove). And will therefore swing to the left of a straight line defined y the initial direction of motion. The same will apply to a footall struck to the side of its centre of mass. Most footallers can apply such a on either side of the all s centre of mass to create a leftward or a rightward swerve during. Answers 47

7 PART TOPIC 32 CHAPTER 58 13) a) Descrie how a lift is generated y a discus in. As the discus moves forward, the angle presented y the lower surface of the discus to the direction of motion (called the angle of attack). This can cause the air molecules through which the oject is moving to e deflected downward. Hence would cause a downward on the air through which the oject passes (figure Q5.10). This downward on the air would cause an upward on the moving discus in reaction to the downward on the air (y Newton s third law). This is the lift. figure Q5.10 lift on a discus ) Explain how a high angle of attack will affect the distance travelled y a discus. A low angle of attack will cause the discus to skip on the air (a it like a flat stone on water). If the angle of attack is increased there will come a point at which the air will e d forward. And hence the reaction on the discus will e ackward, which will slow the discus down and drastically shorten its distance. 48