Download Forces - produce motion (change the motion) - in body segments

Forces
- produce motion (change the motion) - in body segments
- in the body as a whole
- maintain a balance while stationary
Need to understand:
- how to find the
when several forces act
- how to balance in order to maintain
- how to “resolve into components”
Defining Forces:
- A push or a pull (descriptively)
Definition: That which produces an acceleration.
i.e. Something which starts or
, stops or decelerates, or changes the
direction - i.e. in some way changes the motion of the object
Measuring the Force:
Magnitude: Measured in Newtons (N)
Since a force is that which changes the motion (accelerates). We define:
1N of force = the
required to
a 1kg mass at 1m/s2
1N = 1 kg·m/s2
Direction: Push or pull is always in a particular direction.
Thus force is a Vector
Vectors and Scalars:
To record/express vectors like forces we must specify both magnitude and
direction. E.g.
A force also:
- has a line of action – an imaginary line along which the force
- has a point of application - point where it is
.
- is exerted by something - has an “
” which pushes or pulls
- acts on something - always important to recognize which object it acts on.
- has a reaction force. Forces always come in pairs – Action and reaction forces
(Newton’s 3rd Law)
Internal and External Forces
System = Two or more objects interacting
e.g. Two or more
of the body’s link system
Internal Forces = forces exerted by objects within the system on other objects
within the same system
e.g. Forces which 2 segments
on each other.
e.g. Muscle forces stretching
or
Tendons pulling on
External Forces = Forces exerted by “outside agents” on the objects of the
system or on the system as a whole
e.g. gravity or friction acting on the body or segments
Note:
Internal forces cannot change the motion of the body’s
.
External forces are required to change the motion of the body’s center of mass or
the motion of the body as a
.
i.e. a gymnast or diver in air
- cannot produce a longer projectile
e.g.
- connot suddenly produce a rotation/spin
(without internal compensation)
e.g.
Contact and “Distance” Forces
Contact Forces
- exerted by making contact (macroscopiclly)
- push, pull,
Action at a Distance Forces - exerted without actual contact
- gravity, electric,