Forces Affecting Movement

1. Forces Affecting Movement Gravity – The acceleration that produces weight. Muscle Tension – Pulling force produced by muscle contraction. Creates torque and affects the stability of joints. Impact – The force registered when an object strikes or is struck by another object or surface. (Force produced by a collision.)

2. Forces Affecting Movement - Spring Tension • Energy which is stored when a material is deformed and then released when it returns to shape. Fs = -kDx where: k is a constant value called the spring constant (it is specific to the material acting as the spring) DX is the change in shape or deformation • Examples of spring tension: • The bounce or recoil of a tennis ball, football, golf ball, etc. when struck • The spring effect in the tissues of the arch of the foot (the calcaneonavicular or spring ligament and the plantar fascia)

3. Forces Affecting Movement - Fluid Drag (air, water, etc.) FD = ½ CD * AP * r * v2 where: CD = coefficient of drag (a unitless number that gauges an object’s resistance to fluid flow) AP = area exposed • (the Greek letter rho) = fluid density V2 = the square of the velocity of the object relative to the fluid Fluid drag can slow down an object (as in projectile flight) or can propel an object (like a sailboat)

4. Reduction of Fluid Drag on Competition Bicycles

5. For a swimmer, the fluid drag on the body can impede forward motion while fluid drag on the hands while they push backwards against the water will help to produce forward thrust.

6. Forces Affecting Movement • Friction - A resisting force that is normal or perpendicular to the reaction force of an object. applied force weight friction reaction force

7. Forces Affecting Movement – Friction • Static friction – The friction present when a force attempts to slide an object on a surface but is not strong enough to actually cause movement • As the force attempting to move the object increases, the magnitude of friction increases equally as long as the object doesn’t move. • The magnitude of the maximum force that can be applied to the object before it moves is equal to the maximum static friction (Fm).

8. Text page 390 Fa = Applied force parallel to surface FS = Static friction R = Reaction force wt = Object weight

9. Forces Affecting Movement – Friction • Dynamic friction – The friction present once an object starts to move or slide along a surface. • Also called kinetic friction (symbol: Fk) • Unlike static friction, dynamic friction remains constant once an object starts to slide. • The magnitude of dynamic friction is always less than the magnitude of maximum static friction for a given object

10. Text page 390 Fa = Applied force parallel to surface Fm = Maximum static friction Fk = Kinetic friction R = Reaction force wt = Object weight Once movement begins, we go from a static system to a dynamic system.

11. Forces Affecting Movement – Friction • Rolling friction – The friction that resists the movement of an object rolling on a given surface.

12. Factors Governing the Magnitude of Friction • The coefficient of friction • SYMBOL: The Greek letter mu (m) • A unitless number indicating the relative ease of an object sliding across a surface • Affected by the roughness of the sliding surfaces and the molecular interaction between them • The normal (perpendicular) reaction force • The force which is equal in magnitude but opposite in direction to the object’s weight plus any other downward forces acting on the object

13. Friction Calculations • General: F = mR • Maximum Static Friction: Fm = msR where ms is the coefficient of static friction • Dynamic (Kinetic) Friction: Fk = mkR where mk is the coefficient of dynamic or kinetic friction. SEE SAMPLE PROBLEM ON TEXT p. 396