Forces Flashcards
Define force?
(N)- a pushing or puling action that one object exerts upon another
1N force will accelerate at a 1kg mass at 1 m/s
What’s the difference between internal and external forces?
Internal- anything within the body
External- anything outside the body
How do we conduct linear force measurement?
-FORCE PLATFORM
-GRF in three different directions
Fz= vertical force
Fy= left side
Fx= right side
Use of an embedded force platform
Positives and negatives of embedded force platforms?
Positives:
Accurate and reliable 3D force measurement
Force in THREE DIRECTIONS and a centre of pressure
High sampling frequency= how many per second
High resolution= how many chunks is it split into- 10N, 1N, 0.01 N
Negatives:
Restricted to indoor, lab use- lab based
Possibly forces an unnatural gait due to force plate targeting
As an alternative to embedded force platforms, how do portable force plates compare?
Positives:
Not embedded and off the ground
They automate a lot of metrics as they’re in applied settings- can give you data such as jump height and asymmetry of legs
Negatives:
Slightly less accurate, lower resolution
Less reliable
Only vertical GRF, only one direction instead of three
What is an equation based way of calculating forces that doesn’t require a force plate?
F= m x a
If you know this equation then you could calculate a persons acceleration instead of force and use this as a proxy for what their force might be.
Use of a triaxial accelerometer
-Measures instantaneous acc
-A constant signal at every m/s
-Get acc in three directions
-Need to place device at COM
What are the positives of using a triaxial accelerometer?
Positives:
Cheaper than lab equipment
Easy to use
Often embedded in other devices
Can monitor site of interest
Negatives:
Valid for intended purpose?
Affect the movement being monitored?
Battery life and fitted comfort?
Define impulse?
Impulse= change in momentum
Impulse= force x time
Impulse= ft
Plot what the impulse would be during a race?
Early- positive> negative= speed up
Greater positive curve
Mid- equal curve
Late- negative> positive= slow down
Impulse example?
A 50kg participant was walking at 1m/s before contacting the force plate and at 1.5m/s on leaving the force plate-
Calculate horizontal net force?
“net force”= “net impulse”
Impulse= change in momentum
Momentum= mass x velocity
= mass x change in velocity
=50 (1.5m/s -1m/s)
=25N
Define the rate of force development?
=change in force/ time
The overall amount of force and how quickly can they produce this force ( CHANGE IN FORCE)
Looking at the rate of force development through diagrams:
VERTICAL GRF vs TIME (m/s) on a graph
Impact peak- when hit plate
Active peak- when pushing off
Vertical average/ instantaneous loading rate
VALR- average between 20-80% of impact peak
VILR: maximum gradient before peak impact
Define moment?
Also known as TORQUE
Rotation
=force x perpendicular distance
MEASURED- N/m
EG: a 5kg weight is suspended 2 metres away from a body, what is the torque?
5x 2= 10
Define friction?
Normal force- perpendicular to the surfaces in contact- the WEIGHT of this
=a force that acts in opposition to the movement of one surface over another
f= uN
Friction force= coefficient of friction x normal force
Describe the coefficient of friction?
A value between 0-1
A low coefficient= sliding motion, lack of friction
A high coefficient= no motion- rubber and tarmac, lots of friction
