Kinesiology 201 Solutions
Free Body Diagrams
Tony Leyland
School of Kinesiology
Simon Fraser University
Free-Body Diagrams
Fbarbell & hand
1. If the barbell were moving upwards there would be
some air resistance although this would be negligible.
FAIR
Left Forearm
Fmuscles
FR
Fmuscles
FL
mg
Fjoint
2. The answer to this problem somewhat depends on the complexity of the
model you wish to develop in terms of how many muscles you would include in your
model of the forearm. The two smaller arrows represent a signal equivalent muscle
for forearm flexion and for forearm extension.
3. Since our system of interest is the
sailboard we must remove all other
objects from consideration and
replace them with by equivalent
external forces. This is a threedimensional problem as are all
bodies in reality, but you could
have done it in two dimensions.
Note that the sail and boom would
be pulling the surfer forward and
this would result in the foot friction
in the direction indicated.
drag on board is neglected.
2-dimensions
surfer
weight
foot
water drag friction
bouancy
force
Air
Generally for this type of question
I would give ½ mark for each
force and another ½ for the
vector
being
reasonably
represented.
surfer
weight
sail & mast
weight
sail thrust
board
weight
sail & mast
weight
sail thrust
foot
friction
water drag
bouancy
force
3-dimensions
lateral sail
force
board
weight
lateral
daggerboard
force
1
b. The system is defined as the sail, sailboard and the surfer. The table below gives
some examples of action and reaction forces and states whether they are internal to
the system or external. This ability to determine what force is acting on what object
and which is crucial. Knowledge of what is the initiating force is also important.
Int/Ext
Internal
Internal;
Action
Hands pull on sail boom
Feet try to slide along board
Internal
Sail, mast and boom push down
on board with their own weight
Earth attracts system mass
towards Earth's centre (gravity)
External
External
External
Wind pushes against sail
Board pushes water ahead of it
and pulls water along with it (the
water behind it and along its
wetted surface)
System pushes down on water
due to its desire to accelerate
towards centre of Earth
External
4. a) With this question it is not really
necessary to include all four ground
reaction forces and four frictional
forces at the wheels (I have shown
some but it gets messy showing
all). Again I have neglected air
resistance.
b) The diagram below should be selfexplanatory by now.
Fair
Fpull
Ffricton
Fjoint
Reaction
Boom pulls on hands
Board resists foot motion with
friction
Board pushes up on mast, sail and
boom.
System attracts Earth towards
system's centre of mass (also
gravity). Note the buoyancy force is
not the reaction to the Earth's pull.
Sail pushes against wind
Water resists motion with fluid drag
Water pushes back
(buoyancy force)
on
Fpassenger
Fpush
R
Fflexors
Fpull
Ffriction
mg
R
board
R
Question 5
Solution
Ffriction
R
mg
Fforearm
Fhand
2