Unit 9 General Motion

What is translation?

Motion in which all points of an object have the same linear velocity at a given instant.

What is pure rotation?

Motion around a fixed axis in which all points share the same angular velocity.

What is general motion?

A combination of translation and rotation.

In general motion, is the axis of rotation stationary?

No. The axis moves.

What type of motion describes most human movement?

General motion.

A runner’s lower leg rotates while the knee moves forward. What type of motion is this?

General motion.

What is relative linear velocity?

The velocity of one point as observed from another point.

How are absolute and relative linear velocities related?

v₂ = v₁ + v₂ relative to ₁, using vector addition.

Two points have zero velocity relative to each other. Must both be stationary?

No. They can move together at the same velocity.

What is relative angular velocity?

The rate of change of the angle between two segments.

Is joint angular velocity an absolute or relative angular velocity?

Relative angular velocity.

How are absolute and relative angular velocities related in the deck’s planar model?

ω₂ = ω₁ + ω₂ relative to ₁.

Two linked segments have the same absolute angular velocity. What is their relative angular velocity?

Zero.

Can a joint angle stay constant while both segments rotate?

Yes. Both segments can rotate together.

What determines a rotating point’s absolute velocity when its axis moves?

The vector sum of axis velocity and tangential velocity relative to the axis.

Why can’t you always add axis speed and tangential speed directly?

Their directions may differ, so velocities must be added as vectors.

If axis velocity and tangential velocity point in the same direction, what happens?

Their magnitudes add.

If axis velocity and tangential velocity point in opposite directions, what happens?

Their magnitudes subtract, with the resultant pointing toward the larger contribution.

Can a rotating point have zero absolute velocity while its axis moves?

Yes, if its tangential velocity exactly opposes the axis velocity.

What is a kinematic chain?

A series of linked body segments.

What advantages can movement at multiple joints provide?

More degrees of freedom and greater distal endpoint speed.

What is the main disadvantage of a more complex kinematic chain?

More complex movement control.

How do throwing movements typically accelerate their segments?

Sequentially, with segments accelerating in a coordinated order.

How do pushing movements typically accelerate their segments?

More simultaneously.

What performance tendencies distinguish throwing from pushing?

Throwing favors high endpoint velocity. Pushing can favor greater accuracy.