HUS-323 DEGREE EXAMINATIONS NOVEMBER 2014 TIME: 3 HRS SUBJECT: HUMAN MOVEMENT SCIENCE MARKS: 100 PAPER: BIOMECHANICS INTERNAL EXAMINERS EXTERNAL EXAMINER MR. T. XOXO; DR LYOKA Prof. Travill INSTRUCTIONS: Answer All Questions Question 1 a. Explain the 5 goals of exercise and sport biomechanics. [33] (2 × 5 = 10) b. When a soccer ball is kicked into the air (and thus becomes a projectile): Explain the two things that act on it to change its motion. (2 × 2 = 4) c. Discuss the three different optimum angles of release or takeoff bearing in mind the purpose of the activity. (2 × 3 = 6) d. A cross-country skier travels 5Km due North, makes a right-angled turn, and travels a further 12Km due East. (a) If the distance is covered in 51 min, what is the skier average speed in Km/h? (b) What is the magnitude of the skier’s average velocity in Km/h? (8) e. A softball player sprints from first base to second base. If her instantaneous horizontal velocity is 9.3 m/s when she is 8 m from second base and 7.5 m/s when she is 5 m from second base 0.4 s later, what is her average acceleration over that 3 m interval? (Assume that she travels in the positive direction when moving from first to second.) (5) Question 2 [28] a. Contrast two volleyball players in terms of their velocity of the hand, ‘A’ with longer arms and ‘B’ with shorter arms using the following statement to prove your case: The velocity of the hand is equal to the product of the angular velocity of the arm and the distance from the hand to the axis of rotation. (10) b. A tennis ball is travelling at 25m/s meets a racket travelling in the opposite direction at 35m/s and then after impact, the racket and ball travel in the same direction with velocities of 10m/s and 30m/s, respectively. If the direction in which the ball was moving before impact is arbitrarily designated as positive, calculate the coefficient of restitution. (8) c. Explain in detail the affect of no-spin, topspin and backspin on the flight path and rebound of a tennis ball. Assume that the velocities of the balls at impact with the court are the same. (10) Question 3 a. Explain the type of levers by supporting with examples. [39] (2 × 3 = 6) b. Discuss the various types of stability, giving examples of each. (3 × 3 = 9) c. Explain the lift and drag on a javelin in flight by means of a sketch. (10) d. Explain the concept of density and explain why certain members of the community will float easier than others (8) e. Explain Newton’s laws of motion and their application in sports. (6) TOTAL 100 HUS-323 DEGREE EXAMINATIONS MEMORANDUM NOVEMBER 2012 TIME: 3 HRS SUBJECT: HUMAN MOVEMENT SCIENCE MARKS: 100 PAPER: BIOMECHANICS INTERNAL EXAMINERS MR. T. XOXO EXTERNAL EXAMINER INSTRUCTIONS: Answer All Questions Question 1 a. Explain the 5 goals of exercise and sport biomechanics. [2 × 5 = 10] 1. Performance improvement is the ultimate goal of exercise and sport biomechanics and a secondary goal is injury prevention. 2. Technique improvement is the most common method for improving performance and was highlighted as one motivation for studying biomechanics. 3. Equipment improvement, have an effect on performance directly or through injury prevention. 4. Training improvement; an analysis of the technique deficiencies of an athlete can assist the coach or teacher in identifying the type of training the athlete requires to improve. 5. Injury prevention and rehabilitation; biomechanics is useful to sports medicine professionals in identifying what forces may have caused and injury, how to prevent the injury from recurring and what exercises may assist with rehab from the injury. b. When a soccer ball is kicked into the air (and thus becomes a projectile): Explain the two things that act on it to change its motion. [2 × 2 = 4] The first is gravity, which pulls it toward the earth with a constant acceleration of 9.81m/s2, and secondly air resistance which serves to retard its forward motion and, under certain circumstances, to cause it to be lifted. c. Discuss the three different optimum angles of release or takeoff bearing in mind the purpose of the activity. [2 × 3 = 6] When the projectile lands at the same level as that from which it was released, the optimum angle is 450. When the projectile lands at a level below that from which it was released – as in the shot put and long jump - the optimum angle is always less than 450. Finally when the projectile lands at a level above that from which it was released- a rare occurrence in sport the optimum angle is greater than 450 and again depends on its velocity and relative height of release. d. A cross-country skier travels 5Km due North, makes a right-angled turn, and travels a further 12Km due East. (a) If the distance is covered in 51 min, what is the skier average speed in Km/h? (b) What is the magnitude of the skier’s average velocity in Km/h? [8] 51 min/60h = 0.85 h Average speed = distance covered/time = (12 Km + 5 Km) / 0.85 h = 20 Km/h Therefore the skier average speed is 20 Km/h. 𝑎2 = 122 + 52 a = 13 Therefore the skier’s displacement is 13 Km due North. Velocity = displacement/time Velocity = 13 Km/0.85 h Therefore the magnitude of the skier’s average velocity is 15.29 Km/h e. A softball player sprints from first base to second base. If her instantaneous horizontal velocity is 9.3 m/s when she is 8 m from second base and 7.5 m/s when she is 5 m from second base 0.4 s later, what is her average acceleration over that 3 m interval? (Assume that she travels in the positive direction when moving from first to second.) [5] 𝑎= 𝑎= 𝑣−𝑢 𝑡 7.5 𝑚/𝑠 −9.3 𝑚/𝑠 0.4 𝑠 = -4.5m/s2 Therefore the player’s average acceleration over the 3 m interval is -4.5m/s2 Question 2 a. Contrast two volleyball players in terms of their velocity of the hand, ‘A’ with longer arms and ‘B’ with shorter arms using the following statement to prove your case: The velocity of the hand is equal to the product of the angular velocity of the arm and the distance from the hand to the axis of rotation. [10] Algebraic expression of “the velocity of the hand is equal to the product of angular velocity of the arm and the distance from the hand to the axis of the rotation,” followed by a practical sporting example. b. A tennis ball is travelling at 25m/s meets a racket travelling in the opposite direction at 35m/s and then after impact, the racket and ball travel in the same direction with velocities of 10m/s and 30m/s, respectively. If the direction in which the ball was moving before impact is arbitrarily designated as positive, calculate the coefficient of restitution. [8] 𝑒= 𝑣1 − 𝑣2 𝑢1 − 𝑢2 𝑒 = −( −30 − (−10) ) 25 − (−35) e = - (-20/60) e = 0.33 Therefore the coefficient of restitution is equal to 0.33 c. Explain in detail the affect of no-spin, topspin and backspin on the flight path and rebound of a tennis ball. Assume that the velocities of the balls at impact with the court are the same. [10] If no spin has been imparted to it, all parts of it will have the same horizontal velocity at the instant it lands on the court. Under such circumstances, the opposing friction will depend on the velocity of the ball and the nature of the surfaces in contact. When topspin is applied to the ball, the component velocities of that part of it that makes contact with the court tend to offset, or cancel, each other. As a result, the forward velocity with which this part strikes the court, the evoked frictional reaction, and the decrease in the ball’s forward velocity are all less than when no spin is imparted to the ball. The net effect is that the application of top spin causes the ball to come off the court faster and at a lower angle than it would in the absence of spin. It is for this reason that top spin is widely used in tennis, table tennis, and other games as an offensive weapon. If backspin is applied to the ball, the results obtained are essentially the reverse of those produced when topspin is applied. The translational and rotational components complement each other, the evoked frictional reaction is increased (compared with what it would be in the absence of spin), and the post impact horizontal velocity of the ball is markedly less than in either the topspin or no-spin cases. This effect is observed on occasion when a golfer plays a short, lofted shot to the green. The application of a backspin causes the ball to come off the court more slowly and at a higher angle than it would if no spin were applied; backspin is often used in defensive situations. Question 3 a. Explain the type of levers by supporting with examples. [2 × 3 = 6] First class levers, consists of the axis that lies between the force and resistance points: e.g., elbow extension. Second class levers, consists of the resistance point that lies between the force and axis points: e.g., heel rising. The third class levers consist of the force point that lies between the axis and resistance points as in elbow flexion. b. Discuss the various types of stability, giving examples of each. [3 × 3 = 9] If a body in equilibrium is given a small linear or angular displacement from its equilibrium position, one of three things will happen to it – it will keep going in the same direction, it will stop in the position to which it has been moved, or it will return to its original position. For example, consider a ball that is lying at rest on some fixed surface. If the surface is convex, and the ball is given a small displacement, the ball will keep moving in the direction of the displacement. It will simply roll farther and farther away from its original equilibrium position. If the surface is flat and the ball is given a small displacement, it will remain in the position to which it has been displaced. Finally if the surface is concave and the ball is displaced, the ball will return to its initial equilibrium position. On the basis of these three different outcomes, the ball is said to have been initially in a state of unstable equilibrium, neutral equilibrium, or stable equilibrium. c. Explain the lift and drag on a javelin in flight by means of a sketch. [10] If a javelin is mounted on a wind tunnel and air is made to flow past it, the resultant force that the air exerts on the javelin – the air resistance – usually acts at some angle to the direction from which the air approaches the javelin. The component of the air resistance that acts in the direction of the approaching flow of air is called the drag and, in an actual throw, serves to oppose the forward motion of the javelin. The component that acts in a direction at right angles to that of the approaching air is called the lift and in practice serves to lift the javelin above the path it would otherwise follow. In short air resistance tends to simultaneously oppose the forward motion of the javelin and to lift it higher in the air. d. Explain the concept of density and explain why certain members of the community will float easier than others [8] Mass = Volume × density Demineralisation Fat v/s muscle e. Explain Newton’s laws of motion and their application in sports. (6) Law of inertia Law of acceleration Law of reaction HUS-323 SUPPLEMENTARY EXAMINATIONS FEBRUARY 2013 TIME: 3 HRS SUBJECT: HUMAN MOVEMENT SCIENCE MARKS: 100 PAPER: BIOMECHANICS INTERNAL EXAMINERS EXTERNAL EXAMINER MR. T. XOXO; DR LYOKA Prof. Travill INSTRUCTIONS: Answer All Questions Question 1 a. Explain the 5 goals of exercise and sport biomechanics. [31] (2 × 5 = 10) b. Explain the three forms of human motion with sporting examples. (2 × 3 = 6) c. Discuss the three forms of motion with practical examples. (2 × 3 = 6) d. A swimmer covers the first 50 m length of a 100 m backstroke race at an average speed of 1.5 m/s and the second length at an average speed of 1.2 m/s. (a) What were the times for the first and second length, respectively? (b) What was the average speed for the whole race? (c) What was the average velocity for the whole race? (9) Question 2 [30] a. Contrast two volleyball players in terms of their velocity of the hand, ‘A’ with longer arms and ‘B’ with shorter arms using the following statement to prove your case: The velocity of the hand is equal to the product of the angular velocity of the arm and the distance from the hand to the axis of rotation. (12) b. A tennis ball is travelling at 25m/s meets a racket travelling in the opposite direction at 35m/s and then after impact, the racket and ball travel in the same direction with velocities of 10m/s and 30m/s, respectively. If the direction in which the ball was moving before impact is arbitrarily designated as positive, calculate the coefficient of restitution. (8) c. Explain in detail the affect of no-spin, topspin and backspin on the flight path and rebound of a tennis ball. Assume that the velocities of the balls at impact with the court are the same. (10) Question 3 a. Explain the type of levers by supporting with examples. [39] (2 × 3 = 6) b. Discuss the various types of stability, giving examples of each. (3 × 3 = 9) c. Explain instances in sport where the Magnus effect is applied and further more explain its effect on a projectile. (12) d. Explain how surface drag can affect a javelin thrown in the air. TOTAL 100 (12) HUS-323 SUPPLEMENTARY EXAMINATIONS MEMORANDUM FEBRUARY 2013 TIME: 3 HRS SUBJECT: HUMAN MOVEMENT SCIENCE MARKS: 100 PAPER: BIOMECHANICS INTERNAL EXAMINERS MR. T. XOXO EXTERNAL EXAMINER INSTRUCTIONS: Answer All Questions Question 1 [31] a. Explain the 5 goals of exercise and sport biomechanics. [2 × 5 = 10] 1. Performance improvement is the ultimate goal of exercise and sport biomechanics and a secondary goal is injury prevention. 2. Technique improvement is the most common method for improving performance and was highlighted as one motivation for studying biomechanics. 3. Equipment improvement, have an effect on performance directly or through injury prevention. 4. Training improvement; an analysis of the technique deficiencies of an athlete can assist the coach or teacher in identifying the type of training the athlete requires to improve. 5. Injury prevention and rehabilitation; biomechanics is useful to sports medicine professionals in identifying what forces may have caused and injury, how to prevent the injury from recurring and what exercises may assist with rehab from the injury. b. Explain the three forms of human motion with sporting examples. (2 × 3 = 6) Linear motion /translation occur when all parts of a body/system move the same distance, in the same direction, in the same time. A ski jumper experiences linear motion as he begins his downward approach to the takeoff. Angular motion / rotation occurs when a body moves along a circular path about a central line so that all parts of the body move through the same angle, in the same direction, in the same time. The central line which lies at right angles to the plane of motion is known as the axis of rotation. A body-builder performing biceps curl. Combinations of linear and angular motion are common in sport and are referred to as general motions. For example, pole vaulting during the run up. c. Discuss the three forms of motion with practical examples. (2 × 3 = 6) • Linear motion (Translation) occurs when all parts of a body move the same distance, in the same direction in the same time. Angular motion (Rotation) occurs when a body moves on a circular path about a central line so that all parts of the body move through the same angle, in the same direction, in the same time. • The central line, which lies at right angles to the plane of motion, is known as the axis of rotation (it is important to emphasize here that an axis of rotation is a line and not a point. This distinction is important because although axes can generally be treated as points when dealing with motion that are confined to one plane, the extension of this practice to more complex motions often leads to erroneous conclusions.) Linear and angular motions are much less common in physical education, athletics, and rehabilitation than are motions that are either a combination of these two or a combination of a several linear or angular motions. Such motions are referred to here as general motions d. A swimmer covers the first 50 m length of a 100 m backstroke race at an average speed of 1.5 m/s and the second length at an average speed of 1.2 m/s. (a) What were the times for the first and second length, respectively? (b) What was the average speed for the whole race? (c) What was the average velocity for the whole race? (9) (a) First length t = d/s t = 50m/1.5m.s-1 = 33.33 s (2 marks) The time for the first length is 33.33 seconds. Second length t = d/s t = 50m/1.2m.s-1 = 41.47 s (2 marks) The time for the second length is 41.47 seconds. (b) Average speed for the whole race: S =d/t time for whole race = (33.33s + 41.47s) = 74.8s (1 mark) s = 100m/74.8s = 1.34m/s (2marks) The average speed for the whole race is 1.34m/s (c) The average velocity for the whole race: v = displacement/time v = 0m/74.8s = 0m/s (2 marks) The average velocity for the whole race is 0m/s Question 2 [30] a. Contrast two volleyball players in terms of their velocity of the hand, ‘A’ with longer arms and ‘B’ with shorter arms using the following statement to prove your case: The velocity of the hand is equal to the product of the angular velocity of the arm and the distance from the hand to the axis of rotation. [12] Algebraic expression of “the velocity of the hand is equal to the product of angular velocity of the arm and the distance from the hand to the axis of the rotation,” followed by a practical sporting example. b. A tennis ball is travelling at 25m/s meets a racket travelling in the opposite direction at 35m/s and then after impact, the racket and ball travel in the same direction with velocities of 10m/s and 30m/s, respectively. If the direction in which the ball was moving before impact is arbitrarily designated as positive, calculate the coefficient of restitution. [8] 𝑒= 𝑣1 − 𝑣2 𝑢1 − 𝑢2 𝑒 = −( −30 − (−10) ) 25 − (−35) e = - (-20/60) e = 0.33 Therefore the coefficient of restitution is equal to 0.33 (8 marks) c. Explain in detail the affect of no-spin, topspin and backspin on the flight path and rebound of a tennis ball. Assume that the velocities of the balls at impact with the court are the same. [10] If no spin has been imparted to it, all parts of it will have the same horizontal velocity at the instant it lands on the court. Under such circumstances, the opposing friction will depend on the velocity of the ball and the nature of the surfaces in contact. When topspin is applied to the ball, the component velocities of that part of it that makes contact with the court tend to offset, or cancel, each other. As a result, the forward velocity with which this part strikes the court, the evoked frictional reaction, and the decrease in the ball’s forward velocity are all less than when no spin is imparted to the ball. The net effect is that the application of top spin causes the ball to come off the court faster and at a lower angle than it would in the absence of spin. It is for this reason that top spin is widely used in tennis, table tennis, and other games as an offensive weapon. If backspin is applied to the ball, the results obtained are essentially the reverse of those produced when topspin is applied. The translational and rotational components complement each other, the evoked frictional reaction is increased (compared with what it would be in the absence of spin), and the post impact horizontal velocity of the ball is markedly less than in either the topspin or no-spin cases. This effect is observed on occasion when a golfer plays a short, lofted shot to the green. The application of a backspin causes the ball to come off the court more slowly and at a higher angle than it would if no spin were applied; backspin is often used in defensive situations. Question 3 [39] a. Explain the type of levers by supporting with examples. [2 × 3 = 6] First class levers, consists of the axis that lies between the force and resistance points: e.g., elbow extension. Second class levers, consists of the resistance point that lies between the force and axis points: e.g., heel rising. The third class levers consist of the force point that lies between the axis and resistance points as in elbow flexion. b. Discuss the various types of stability, giving examples of each. [3 × 3 = 9] If a body in equilibrium is given a small linear or angular displacement from its equilibrium position, one of three things will happen to it – it will keep going in the same direction, it will stop in the position to which it has been moved, or it will return to its original position. For example, consider a ball that is lying at rest on some fixed surface. If the surface is convex, and the ball is given a small displacement, the ball will keep moving in the direction of the displacement. It will simply roll farther and farther away from its original equilibrium position. If the surface is flat and the ball is given a small displacement, it will remain in the position to which it has been displaced. Finally if the surface is concave and the ball is displaced, the ball will return to its initial equilibrium position. On the basis of these three different outcomes, the ball is said to have been initially in a state of unstable equilibrium, neutral equilibrium, or stable equilibrium. c. Explain instances in sport where the Magnus effect is applied and further more explain its effect on a projectile. [12] When topspin is applied to a tennis ball, the air that comes into direct contact with the ball is carried around with it. The layer of air that becomes effectively attached in this process is called is called the boundary layer. As the ball travels forward, the air on the top of the ball at any given instant has a forward velocity whereas that on the bottom of the ball has a backward velocity relative to the centre of gravity of the ball. The difference in the velocity with which the air in different parts of the boundary layer meets the air through which the ball passes creates a corresponding difference in the pressure exerted on the two halves of the ball. The head-on collision of the air on the top of the ball with the air through which the ball is passing causes an increase in pressure in the area immediately above the ball. The motion of the air on the bottom of the ball in the same direction as the relative flow causes a decrease in pressure in the area immediately below the ball. As a result of these two changes in pressure, the ball experiences a downward force that diverts it from the path it would otherwise follow. The ball thus returns to the court much more quickly than it would if no topspin were applied to it. Since this naturally decreases the time that an opponent has to react to the shot, the topspin drive is widely used in tennis – and in table tennis – as an offensive stroke. In baseball a skilled pitcher can apply spin to the ball to cause it to move disconcertingly in the air. d. Explain how surface drag can affect a javelin thrown in the air. (12) When air is driven past the javelin, the layers of air resistance that come into direct contact with the javelin are slowed down. These layers tend to slow those next to them – these in turn tend to slow those next to them, and so on. This process of slowing the air in close proximity to the javelin is brought about because the javelin exerts forces on the air. The reaction to these forces – known as surface drag because it arises as a result of the passage of the air across the surface of the body – contributes to the drag which the body experiences. The magnitude of the surface drag depends, among other things, on the velocity of the flow of relative to that of the body, the surface area of the body and the smoothness of the surface – the higher the relative velocity, the greater the surface area; the rougher the surface, the greater the surface drag. This form of drag is esp. important in activities like sailing, rowing & canoeing.
