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2 Kinetics Study of Motion Internal Forces:  generated by muscles pulling via their tendons on bones, and to bone-on-bone forces exerted across joint surfaces External Forces:  acting from without, such as the force of gravity or the force from any body contact with the ground, environment, sport equipment, or opponent  Focuses on the various forces that are associated with a movement

3 Calculating Moments of Force  Moment arm is the shortest (perpendicular) distance from the axis of rotation to the line of action of the force  Moment of force is influenced by the magnitude of moment arm and the magnitude of the force Moment of Force = Moment Arm x Force Moment of Force = Moment Arm x Force By grasping the wrench at the end (A) a greater torque is generated because the moment arm is greater than in (B)

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Torque n n In any object experiencing torque, the distance from the pivot point (the lug nut, in this case), to the area where force is being applied is called the moment arm. n n On the wrench, this is the distance from the lug nut to the place where the operator is pushing on the wrench handle. 5

Torque n n Torque is the product of force multiplied by moment arm, and the greater the torque, the greater the tendency of the object to be put into rotation. The fact that torque is the product of force and moment arm means that if one cannot increase force, it is still possible to gain greater torque by increasing the moment arm. 6

n n This is the reason why, when one tries and fails to disengage a stubborn lug nut, it is a good idea to get a longer wrench. Likewise with a lever, greater leverage can be gained without applying more force: all one needs is a longer lever arm. 7

n The same concept can be applied to your lab on Monday with the length of the hinge point away from the force of the weight. 8

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 A lever is referred to as the simplest mechanical device that can be called a “machine”  (an instrument for performing work)  Every moveable body, whether acting alone or with others, is part of a lever system that facilitates movement.  3 Classes of Levers:  Class 1 Lever: (Eg – The Teeter-totter)  Class 2 Lever: (Eg – The Wheelbarrow)  Class 3 Lever: (Eg – The Snow shovel)

Fulcrum: (or pivot) The point at which the lever rotates. Load Arm: The distance between the load and fulcrum Effort Arm: The distance between the effort and fulcrum The way in which a lever operates is dependant on the type of lever. CLASS 1 CLASS 2 CLASS 3 (Least Common) (Most Common)

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Everyday uses of Levers 13

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Factors affecting the moment of force A. Balanced teeter-totter C. Increasing the applied force by adding a friend B. Increasing the moment arm by leaning backwards D D 15

Ideal Mechanical Advantage of a Lever n IMA= Length of input arm Length of output arm Length of output arm 16

Pulleys n Consists of a grooved wheel with a rope or cable looped around it n Changes the direction of the force depending on whether it is fixed or movable n A fixed pulley has an ideal mechanical advantage of one 17

Movable Pulleys n If one end of the rope is fixed, and the pulley(s) is allowed to move, you now have a movable pulley 18

IMA of a Pulley n The IMA of a pulley system is equal to the number of support ropes 19

n Each segment of the rope that applies a force on the pulley is considered a support rope. n Example: If you pull a rope with an input force of 5 N, the rope applies this force to the movable pulley n In the example on the right, you simply multiply the input force by the number of strings. 20

n The last point of the previous slide can be summed up by this… n A fixed pulley will only output a mechanical advantage of 1, no matter how many fixed pulleys you have in a system n There must be a movable pulley present for an MA to exist. 21

n The mechanical advantage of a rigging that will require upward pull can be determined by counting the number of rope lengths running between engaged pulleys and those doing the work. n Likewise, if the assembly will require downward pull, count the ropes and subtract one to get the mechanical advantage number. n The subtraction is necessary because with the fixed pulley, the downward pull equals the load on the other length of rope so the last "pull" rope does not provide any mechanical advantage. 22

A B C D A B C D A= mechanical Advantage of 1 B= mechanical Advantage of 3 C= Mechanical Advantage of 4 D= Mechanical Advantage of 5 23

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Wheel and Axle n Consists of a shaft or axle that is attached to a larger disk, called the wheel n The effort force on the wheel magnifies the load force on the axle n Examples: screwdriver, steering wheel 25

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n Opposite occurs on a bicycle n A large effort of force is applied to the axle to overcome the smaller load force on the rim of the wheel n Advantage is that the wheel has to travel a farther distance in the same amount of time 27

Wheel and Axle IMA If the input force is applied to the axle, the IMA can be calculated by dividing the radius of the axle (r a) by radius of the wheel ( r w) n IMA = radius of axle radius of wheel radius of wheel If the input force is applied to the wheel, the IMA can be calculated by dividing the radius of the wheel (r w) by radius of the axle ( r a) n IMA = radius of wheel radius of axle radius of axle 28

Inclined Plane IMA= Length of Ramp Height of Ramp Height of Ramp 29

Wedge n Similar in shape to an inclined plane but used in a different way… n It is forced into an object to split it apart n The wedge increases the force applied to the object to help split it apart. 30

Screw n Is actually an inclined plane that winds around itself n It helps increase the force you use by converting rotational motion into a straight line very slowly. 31

Mechanism n Two or more machines working together. 32

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