Transcript Momentum
Momentum • We have already discussed the concept of inertia and how it is a measure of an object’s desire to resist movement. It was a product of its mass. • Momentum is moving inertia. It is a product of not only an objects mass but also how fast it is moving (velocity) • Newton’s 1st law rewritten to include momentum • An object that is moving will continue to move in the direction the force was applied until another force is applied Controlling Momentum Eccentric forces are also used to help cancel out rotation (momentum) when the body lands in sports such as gymnastics and long jump How does this happen? By landing in such a way that force is applied in front of the COG, backward rotation is initiated thereby ‘cancelling out’ some of the forward rotation Angular Momentum When an object rotates, it has momentum In angular momentum inertia has an important role to play In rotation movements, inertia is due not only to mass but how that mass is distributed about the axis of rotation http://www.youtube.com/watch?v= GMsdCsNz7L4 In a golf context…. When setting up for a golf shot you are trying to position yourself away from the ball so you have to reach a little bit during your shot. This helps you to hit the ball straight instead of slicing the ball or hooking the ball. Doing this also helps the golfer to be in control of their swing by increasing rotary inertia and decreasing the angular velocity. Question Consider an ice skater spinning. How do they speed themselves up in the spin? How do they slow themselves down? Answer To speed up, they bring their body parts closer to the axis of rotation. Then move them out again to slow down (achieved answer) Doing this increases the angular velocity by decreasing the rotary inertia (Merit +) Important words- axis of rotation, angular velocity, rotary inertia Controlling rotation Once the body is rotating, control of the rotation is achieved through the redistribution of mass about the centre of gravity or axis of rotation a) When the mass is distributed closer to the axis of rotation or the COG: a) b) Angular velocity increases Rotary Inertia decreases b) When the mass is distributed further away from the axis of rotation, or the COG a) angular velocity decreases b) rotary inertia increases Tennis Why do tennis players bring their non striking arm closer to the body when looking to return the ball in the forehand side? This decreases the rotary inertia of the system allowing for a faster swing of the racquet (angular velocity) Question In diving if an athlete wishes to complete more somersaults on the descent, what should they do? Answer Either they tuck tighter increase angular velocity and decreasing rotary inertia, or generate greater eccentric forces at take off • http://www.youtube.com/watch ?v=YTh4Pk-8mRM In a golf context…….. Momentum is moving inertia. So the golf ball has momentum and will move in the direction this momentum (force) is applied until it is acted on by another force. Force such as a tree, gravity, air, the ground can slow momentum.