Chapter 12 Flashcards
Newton’s second law describes what condition?
Dynamics
Newton’s second law can be applied using three distinct theoretical approaches.
What are they?
1 - Continuously varying force
2 - Impulse-momentum relationship
3 - Work-energy theorem
In the 1500’s Leonardo DaVinci aimed to quantify the strength in the legs of man. What experiment does he describe?
Leonardo Da Vinci describes having a person stand on the sand. Their feet were found to sink in more with someone on their back. However, their feet sunk in EVEN MORE when they did a two foot vertical jump. Thus he concludes that man has more than twice the force/strength in his legs than he needs to support himself.
What force is responsible for propelling one in the air during a vertical jump?
The reaction force applied by the ground, against the body (Newtons 3rd Law).
Describe the relationship between the ground reaction force and mass * acceleration of the body during a vertical jump.
1 - Initially, when the jumper is simply standing, the Ground Reaction Force and the Force of their Mass *Acceleration will be equal
2 - As they perform the counter movement (unweighting phase) to their jump (bending knees), the Ground Reaction Force will be less than the force of their body weight.
3 - During the push off phase, the ground reaction force will exceed the force of their body weight, which will propel them into the air.
What is the benchmark of a good jumper?
They can generate a ground reaction force over 2 times their body weight.
Describe the equation for calculating propulsion force. How can the equation be rearranged to explain the data provided by the force platform. Interpret this second equation.
Propulsion force, otherwise known as the ground reaction force is calculated with the following equation:
Ma=Fgr+mg
Where:
ma = propulsion force
Mg = body weight
Fgr = ground reaction force
The equation can be reorganized into a second equation the explains the data provided by the force platform (-Fgr)
Fgr = ma-mg
This equation shows that the ground reaction force is a combination of reactions to the body weight as well as reactions to the bodies acceleration (either towards or away from the ground).
What process and steps are used to determine the height of a vertical jump, using force plate data?
What type of dynamics is this?
1 - Write the equations for dynamics
Eg. Fgr = ma - mg
Ma = Fgr + mg
need to find acceleration at takeoff
2 - Divide the propulsion force by body mass to get acceleration
Propulsion force = ma
A = propulsion force/mass
3 - Integrate acceleration to get a change in velocity
4 - Integrate Velocity to get a change in displacement
What is the expected take-off velocity of a vertical jump?
2-3.2m/s
Why would a jump with a higher peak force result in a lower displacement?
Jumps with a lower Fgr can result in a higher vertical displacement, if the force is applied over a longer period of time - known as the impulse-momentum relationship.
Thus, the highest force value, will not necessarily equate to the highest jump.
What are the limitations to the continuously varying force approach to determine vertical jump height?
The limitations of this approach are primarily conceptual, meaning they may affect how one interprets the information.
The data derived from the varying force approach focuses primarily on peak force values. This may lead to conceptual errors when trying to understand the causes of motion when viewing the plot of force against time.
How might the shape of the force-time graph during the weighting phase of a vertical jump give an indication about the level of performance of the athlete?
Poor vertical jumpers typically demonstrate the inverted V shaped curve,
while high level jumpers typically display the inverted U curve.
Describe why an inverted U on a force-time graph during the weighting phase of a vertical jump would lead to proportionately more displacement?
An inverted U means that the person is applying ground reaction force over a longer period of time, which generated a faster takeoff velocity and this vertical displacement.
For example; if two people were to generate the same peak force during a vertical jump, the more time spent generating this force would result in a greater takeoff velocity which can be described by the impulse-momentum relationship.
How much force would you need to generate to reach the same vertical jump height displacement as someone simply applying force over time?
Nearly twice as much - this is calculated by integration to find the area under a curve (larger area with a rectangle versus a triangle).
What are the two movement strategies of completing a vertical jump? Describe both.
1 - The Simultaneous strategy
This strategy is achieved by extending the trunk, thigh, leg and foot segments all at once.
2 - The sequential (Proximal-to-distal) strategy
This strategy is achieved by first extending the trunk, followed by the thigh and so-on.
The sequential strategy creates less force, as only one extensor is moving at a time, however, it generates a larger impulse, takeoff velocity and thus vertical displacement
What are two ways you can assess the vertical jump strategy of an athlete?
1 - video/digitization
2 - force platform data
What is the relationship between the vertical impulse and stride length when running?
The vertical impulse determines the flight time and thus the amount of time available to fly horizontally through the air.
What would the graph of a vertical ground reaction force looks like for someone running?
What shift to this curve would indicate an increase in performance?
An inverted U curve. A shift upward in the inverted U curve would indicate a greater ground reaction force, more air-time and thus better performance.
What would the graph of horizontal ground reaction forces look like for someone running?
What shift to this curve would indicate an increase in performance?
This graph would look like a sine-wave.
During the first half (strike phase), there will be a ground reaction force in the backwards direction, and the line will curve into the negative area.
During the second half (propulsion phase), the line will become positive, indicating a forward propulsion force.
Both will be equal and opposite to each other indicating a constant speed phase.
Better performing runners, will have higher horizontal ground reaction forces during acceleration, allowing them to propel further during their flight time. However, at top speed their ground reaction forces will become moderate, allowing them to maintain efficiency.
What will be the net horizontal impulse of a sprinter during their acceleration phase?
What would it be in the deceleration phase?
Net horizontal impulse will be positive during acceleration and negative during deceleration.
How will the vertical impulse of a sprinter change in the acceleration phase versus at top speed?
In the acceleration phase, the vertical will be smaller as the athlete is mostly generating force in the horizontal direction. However, at top speed their body becomes upright. As they get faster, vertical impulse increases in an effort to extend flight time and stride length.
The amount of posterior (negative) horizontal ground reaction force applied during the striking phase of a running stride is dependent on what??
It is dependent on the placement of the foot relative to the body’s centre of mass.
Negative ground reaction forces will be greater with overstriding and may cause injury.
How can you maximize ground reaction forces during the acceleration phase?
To maximize anterior-directed forces, lean the body forward
What are the limitations of taking the impulse-momentum approach when analyzing jumping/projectile motions?
This approach is conceptually easier to use to explain dynamics than the continuously varying force approach, however, it is mathematically incomplete.
Because the impulse-momentum relationship only describes changes in velocity, it fails to explain changes in COM displacement throughout the movement.
