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### PEShare.co.uk Shared Resource

1. 1. Application of Forces A2 Sports Studies Mr Jennings
2. 2. Projectile motion <ul><li>Imagine the path a ball takes when it is thrown </li></ul><ul><li>Sketch the path in your booklet </li></ul><ul><li>Flight path = </li></ul>Parabolic curve
3. 3. <ul><li>On your curve label: </li></ul><ul><li>The highest point of the ball’s flight </li></ul><ul><li>The point of release </li></ul><ul><li>The point at which you think gravity begins to overcome the horizontal component </li></ul><ul><li>The point at which you think gravity has totally overcome the horizontal component </li></ul>
4. 4. 3 factors determine how far an projectile can travel
5. 5. Components <ul><li>Motion of a projectile has 2 components: </li></ul><ul><ul><li>Vertical component </li></ul></ul><ul><ul><ul><li>The upward motion of the object </li></ul></ul></ul><ul><ul><li>Horizontal component </li></ul></ul><ul><ul><ul><li>The horizontal motion of the object </li></ul></ul></ul>
6. 6. Projectile motion Vertical motion is affected by gravity – Horizontal motion is affected by friction – Vertical component Horizontal component it is continually decreasing which can be negligible
7. 7. Variations in vertical and horizontal components This causes the observed parabolic flight and affects the motion components as follows: A B C D E
8. 8. Impulse <ul><li>Force is not applied to objects instantaneously </li></ul><ul><li>When we run, our feet are in contact with the ground for a period of time (milliseconds) </li></ul><ul><li>This means ground reaction force is applied over e period of time </li></ul><ul><li>Impulse = force x time </li></ul><ul><li>Impulse is also a change in momentum (mass x velocity) </li></ul><ul><li>Mass stays constant so equates to acceleration </li></ul><ul><li>Shown on force-time graphs </li></ul>
9. 9. <ul><li>The bigger the area </li></ul><ul><li>The bigger the impulse and the greater the change of momentum of the runner </li></ul><ul><li>The greater the acceleration </li></ul>
10. 10. Negative impulse generated as footfall Positive impulse generated from push off time force negative positive
11. 11. Exam tip <ul><li>Exam questions usually refer to impulse in relation to a race </li></ul><ul><li>They usually ask what you understand by the term impulse (2 marks) </li></ul><ul><li>And then to draw or interpret/explain some impulse graphs (5/6/7 marks) </li></ul>
12. 12. Start of a race time force neg pos Small negative impulse Large positive impulse Net impulse is positive – performer is accelerating
13. 13. Middle of race Landing - negative impulse Push-off - positive impulse Positive = negative impulses Zero impulse No acceleration Runner at constant velocity time force neg pos
14. 14. End of race Large negative impulse time force neg pos Small positive impulse Net impulse is negative – performer is decelerating
15. 15. Exam Question <ul><li>June 05 Question 2 </li></ul><ul><li>(c) (i) As a sprinter accelerates along the track at the beginning of a race, they generate a large impulse . What do you understand by the term ‘impulse’? ( 2 marks) </li></ul><ul><li>(ii) Sketch and label a graph to show the typical impulse generated by the sprinter at this stage of a race (6 marks) </li></ul>
16. 16. Mark Scheme <ul><li>Jun 2005 Question 2 </li></ul><ul><li>(c) </li></ul><ul><li>Impulse is force x time/force applied in unit of time; </li></ul><ul><li>Equates to change in momentum; </li></ul><ul><li>If mass constant equates to change in acceleration; max of 2 marks </li></ul><ul><li>(d) positive clearly larger than negative; </li></ul><ul><li>x axis – time; </li></ul><ul><li>y axis - force; </li></ul><ul><li>units of force shown as Newtons; </li></ul><ul><li>units of time shown as milliseconds/less than 1 second </li></ul><ul><li>time intersecting at zero on force axes; </li></ul><ul><li>positive and negative force axes labelled; </li></ul><ul><li>shape of graph - negative and positive components of force shown with negative first; </li></ul><ul><li>negative and positive components of force labelled; max of 6 marks </li></ul>
17. 17. The body in rotation <ul><li>Uses familiar terms to linear motion (straight line) </li></ul><ul><li>Uses the word angular – meaning rotating or spinning around an axis </li></ul><ul><li>E.g. Angular velocity or angular momentum </li></ul>
18. 18. <ul><li>Angular momentum = amount of motion a body has during rotation </li></ul><ul><li>Angular velocity = rate of movement in rotation </li></ul><ul><li>Angular acceleration = the rate of change of velocity </li></ul><ul><li>Moment of inertia = resistance of body to a change of state when rotating </li></ul>Key Terms
19. 19. Principle of moments <ul><li>Moment = force x distance from pivot to line of action of force </li></ul><ul><li>Moments tend to turn a lever arm: clockwise or anticlockwise </li></ul><ul><li>To keep the lever balanced the clockwise + anticlockwise forces must be equal </li></ul><ul><li>The muscle must generate enough force to overcome the moment of inertia (mass x distance from resistance to fulcrum) </li></ul><ul><li>Shot putt? </li></ul><ul><li>The further from the fulcrum, the greater force required to overcome the moment of inertia </li></ul>
20. 20. <ul><li>Moment of inertia is dependent on the mass of the object and how the mass is distributed from the fulcrum </li></ul><ul><li>The further the mass from the fulcrum, the greater the moment of inertia and therefore the greater the force required to make an object spin or stop spinning </li></ul>Gymnast tries to reduce moment of inertia by tucking up tightly
21. 21. <ul><li>Newton’s first law? </li></ul><ul><li>Objects rotating with large MI require large moments of forces / torque to change their angular velocity </li></ul><ul><li>Objects with small MI require small moments of force / torque to change their angular velocity </li></ul>
22. 22. Angular movement <ul><li>Questions on angular movement require you to discuss angular velocity, angular acceleration and angular momentum </li></ul><ul><li>Questions usually focus on a gymnast, diver or ice skater </li></ul><ul><li>They are quite straight forward questions which we will practice! </li></ul>
23. 23. <ul><li>Angular momentum = amount of motion a body has during rotation (angular velocity x moment of inertia) </li></ul><ul><li>Newton’s first law? </li></ul><ul><ul><li>A rotating body will continue to turn about its axis with constant angular momentum unless an external force acts upon it </li></ul></ul><ul><li>Air resistance, friction and gravity </li></ul><ul><li>Many sports require performers to attempt the conservation of angular momentum and minimize impact of external forces </li></ul>
24. 24. Conservation of angular momentum <ul><li>A body which is spinning / twisting / tumbling will keep its value of angular momentum once the movement has started </li></ul><ul><li>Therefore if Moment of Inertia changes by changing body shape: </li></ul><ul><li>Then angular velocity must also change to keep angular momentum the same </li></ul><ul><li>If MI increases (body spread out more) then angular velocity must decrease (rate of spin gets less) </li></ul>
25. 25. <ul><li>If the mass moves closer to the axis of rotation then the moment of inertia decreases </li></ul><ul><li>If the moment of inertia decreases , angular velocity must increase to conserve momentum </li></ul><ul><li>Increased angular velocity = increased speed of rotation </li></ul>
26. 29. A bit more complicated….. <ul><li>DANCER - SPIN JUMP </li></ul><ul><li>The movement is initiated with arms held wide - highest possible MI </li></ul><ul><li>Once she has taken off, angular momentum is conserved </li></ul><ul><li>Flight shape has arms tucked across chest - lowest possible MI </li></ul><ul><li>Therefore highest possible rate of spin </li></ul><ul><li>To stop spin – arms out to reduce angular velocity and in crease MI </li></ul>
27. 30. Exam question <ul><li>Jun 2002 Qu 1 </li></ul><ul><li>(c) Figure 1 shows a diver performing a tucked backward one-and-one–half somersault. </li></ul><ul><li>Figure 1 </li></ul><ul><li>Use figure 1 to explain why performing this dive in a tucked position is easier than performing it in an extended position (5 marks) </li></ul>
28. 31. Mark scheme <ul><li>(c) </li></ul><ul><li>1 (In air / during flight) angular momentum remains constant (may be shown as straight line on graph); </li></ul><ul><li>2 Because there are no net external forces acting; </li></ul><ul><li>3 Angular momentum = angular velocity x moment of inertia; </li></ul><ul><li>4 A change in moment of inertia results in a change in angular velolcity (may be shown on graph); </li></ul><ul><li>5 Tucked somersault has smaller moment of inertia than extended; </li></ul><ul><li>6 Hence rotation / angular velocity is quicker in tucked somersault; </li></ul><ul><li>7 The problem of somersaulting is the need to complete the movement quickly / lack of time – hence tucked somersault easier to do. </li></ul><ul><li>Any 5 for 5 marks </li></ul>