FRICTION BALANCE & STABILITY Week 9

What is Friction? Friction ______________ motion Occurs when one body moves across the ______________ of another Friction is what causes a ball to ______________ down and eventually stop after it has rolled along the ground

Types of Friction Sliding friction Rolling friction

Sliding Friction Occurs when two objects slide or tend to slide on or over one another Sliding a box along a wooden floor (Fig 2.46)

Rolling Friction Ball rolls along a surface Much less friction than sliding friction Rolling friction depends on: Nature of the ball and surface Soft, hard, dry, wet, over-inflated, under-inflated Normal reaction The weight of the ball Diameter of the ball

Changing Friction Two ways of modifying friction between two bodies: Alter the nature of the two surfaces (oil, wax) Change the forces holding the surfaces together (lift a table to make a book slide) Examples Gymnasts using powder on hands to grip bars Waxed skis Lycra body suits for cyclists

Balance & Stability Balance: The ability of the body or an object to maintain ______________ or equilibrium When stationary or moving Stability: The ability of a body to ______________ being moved How difficult it is to disturb your balance

Types of Stability Static stability When an object is at rest, a set position has to be maintained for a set time Archery, diving off a platform Dynamic stability When an object is moving, it can be easily altered Dodging in netball

Factors that Affect Balance & Stability Mass of the object (weight) Area of the object’s base of support Height of the centre of gravity above the base of support Position of the line of gravity

Mass of the Object The ______________ the mass, the greater the stability

Area of the Object’s Base of Support It is the area that is bound by the body parts in contact with a surface (eg: ground) The greater the area of support, the ______________ the stability

Height of the Centre of Gravity of the Object Above the Base of Support Gravity is the force that acts upon the human body in a ______________ direction, pulling downwards towards the centre of the earth The line of gravity will always pass through the ______________ of an object’s mass This is known as the centre of gravity The centre of gravity of a human changes with movement and can even lie outside the body The closer the centre of gravity to the base of support, the ______________ the stability

Position of the Line of Gravity Relative to the Base of Support The line of gravity is an imaginary line passing downwards through the centre of gravity to the base of support The closer the position of the centre of gravity to being directly over the middle of the base of support, the greater the ______________ Movement can only occur when the line of gravity falls ______________ the object’s base of support This creates an imbalance and the object will move in the direction of the imbalance Athletes deliberately position the line of gravity close to the limits of their base of support so that only a small force is needed to move them Swimmers and runners on the blocks

Activity Activity 18, page 93

Equilibrium Define equilibrium Explain the three states of equilibrium