Lecture 15 - Linear Kinetics - Reaction Forces Flashcards
What is the net force?
The force you have acting on you after adding up all the forces acting on a body
What does the net force determine?
If there is an acceleration and in what direction
What causes an acceleration on the system?
A net force
What is the effective force?
The net sum of all force vectors acting on a body
How can you keep tract of all the forces on a system?
By drawing a free body diagram
How do you draw a free body diagram?
Draw the system isolated from the environment
Label the positive axes of your reference system
Identify the center of mass
Draw and label all the external forces acting on the system
What is a contact force?
The force created when two bodies are touching each other
What is a reaction force?
A force of equal magnitude and opposite direction created on an opposing body
What is a ground reaction force?
The equal and opposite force the ground applies back on a person
How can you determine the vertical effective force (vertical ground force)?
By subtracting the force from gravity (weight) from the ground direction force
What occurs if the ground reaction force is greater than the force of gravity
The body center of mass will accelerate upward
What occurs if the ground reaction force equals the force from gravity?
Then there will be no acceleration in the y direction.
What is the always true of the vertical ground reaction force?
It it always positive or equal to zero
What is the horizontal ground reaction force?
The force of friction between the ground and the shoe preventing slipping in the x direction
What study did Ashby and Heegaard conduct in 2002?
“Role of Arm Motion in the Standing Long Jump” - Journal of Biomechanics
What did Ashby and Heegaard investigate in their study?
The role of arm motion on the performance of a standing long jump
How many subjects did Ashby and Heegaard use in their study?
3 males
What types of jumps did Ashby and Heegaard use in their study?
Jumps with free arm motion and jumps with restricted arm motion
What question did Ashby and Heegaard test in their study?
Does arm swing improve jumping distance?
What had previous studies shown about standing vertical jumps?
Arm swing improves jump height by:
Increasing the height and velocity of the body’s center of gravity at take off
Contributing to the total body vertical momentum
Increasing the peak magnitude of the vertical ground reaction force
What had previous studies shown about the downward force of arm swings?
Arm swing may create additional downward forces on the body that slows the contraction of hip and knee extensors, enabling greater muscle force development
What have previous studies shown to be the reason why maintaining balance is more difficult during a standing long jump?
Because the center of gravity does not remain over the lower body throughout the motion
What was Ashby and Heegaard’s hypothesis?
Standing long jump performance is improved when arm movements is employed in the standing long jump
What kinds of subjects were used in Ashby and Heegaard’s study?
Untrained males
How were the subjects prepared for the long jump during Ashby and Heegaard’s study?
They warmed up by doing light running, jumping, and stretching
How did Ashby and Heegaard instruct the subjects?
The subjects were instructed to stand on a force platform and jump as far as possible when given a verbal cue
What kind of force platform used in Ashby and Heegaard’s study?
A split belt force treadmill
What hertz was used to measure ground reaction force in Ashby and Heegaard’s study?
1000 Hz
How many jumps did each subject perform in Ashby and Heegaard’s study?
6 long jumps with free arm movement and 6 long jumps with restricted arm movement
How did the subjects in Ashby and Heegaard’s study perform the restricted arm jumps?
With their hands in front of their abdomen throughout the jump
What constituted a successful jump in Ashby and Heegaard’s study?
A jump completed without falling on the landing
How was the motion of the jump captured in Ashby and Heegaard’s study?
It was tracked in three dimensions with a passive marker motion capture system
What did Ashby and Heegaard use the position of the markers to measure?
The kinematics of the body segments
What equipment was used in Ashby and Heegaard’s study?
4 cameras and a force platform
How was the data from the markers used in Ashby and Heegaard’s study?
The 7 markers were used to create a 2D, 6 segment model
What segments were used in Ashby and Heegaard’s study?
Foot, calf, thigh, head, neck, trunk, upper arm, and forearm/hand
How did Ashby and Heegaard apply projectile motion principles?
Projectile motion principles were applied to the center of gravity from take off until touch down
