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IMPORTANT!!!

Action force & reaction force NEVER balance each other because they act on different objects!

Repeat this to yourself over and over again

Slide 46

Walking, Running & Jumping

What forces accelerate us into motion when we walk, run, or jump?

Slide 47

Forces on a Dancer

For a dancer, the three main forces are:

Gravity (Downward)

Support of the floor (Upward)

Frictional force of the floor (Horizontal)

Only these forces can accelerate the dancer.

Gravity is constant but the force exerted by the floor can increase in reaction to the dancer exerting a force on the floor.

Slide 48

Walking Forward

Reaction

Action

When weight is on back foot it acts by pushing back on the floor. Reaction is the friction of the floor, which pushes your body forward

If there were no friction then dancer would fall straight down and could not walk forward

X (CG)

Slide 49

Jumping

Jumping is done by pushing downward on the ground (action) so the ground pushes upward on you (reaction).

How high you jump depends on the force and on the distance over which you apply that force.

Can only push while in contact with the ground so squatting helps by increasing distance.

Slide 50

Swinging Arms and Jumping

You swing your arms upward as you jump to increase the force pushing down on the ground.

Try jumping and swinging your arms upward after you leave the ground; you won’t jump as high.

Slide 51

Work & Force

The work done on an object is defined as

(Work done) = (Net Force) x (Distance)

The greater the work done, the greater the change in the object’s velocity.

Slide 52

Pushing Off on a Jump

Notice the orientation of the foot on the ground, which is pushing off on a jump

Slide 53

Push-Off with Turnout

The angle of flex is small so the distance over which the foot exerts a force is small.

Jump will not be very high.

Distance

Slide 54

Push-Off without Turnout

The angle of flex is larger so the distance over which the foot exerts a force is larger.

Jump will be higher.