Lecture 15 - Linear Kinetics - Reaction Forces

Question 1

What is the net force?

Answer 1

The force you have acting on you after adding up all the forces acting on a body

Question 2

What does the net force determine?

Answer 2

If there is an acceleration and in what direction

Question 3

What causes an acceleration on the system?

Answer 3

A net force

Question 4

What is the effective force?

Answer 4

The net sum of all force vectors acting on a body

Question 5

How can you keep tract of all the forces on a system?

Answer 5

By drawing a free body diagram

Question 6

How do you draw a free body diagram?

Answer 6

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

Question 7

What is a contact force?

Answer 7

The force created when two bodies are touching each other

Question 8

What is a reaction force?

Answer 8

A force of equal magnitude and opposite direction created on an opposing body

Question 9

What is a ground reaction force?

Answer 9

The equal and opposite force the ground applies back on a person

Question 10

How can you determine the vertical effective force (vertical ground force)?

Answer 10

By subtracting the force from gravity (weight) from the ground direction force

Question 11

What occurs if the ground reaction force is greater than the force of gravity

Answer 11

The body center of mass will accelerate upward

Question 12

What occurs if the ground reaction force equals the force from gravity?

Answer 12

Then there will be no acceleration in the y direction.

Question 13

What is the always true of the vertical ground reaction force?

Answer 13

It it always positive or equal to zero

Question 14

What is the horizontal ground reaction force?

Answer 14

The force of friction between the ground and the shoe preventing slipping in the x direction

Question 15

What study did Ashby and Heegaard conduct in 2002?

Answer 15

“Role of Arm Motion in the Standing Long Jump” - Journal of Biomechanics

Question 16

What did Ashby and Heegaard investigate in their study?

Answer 16

The role of arm motion on the performance of a standing long jump

Question 17

How many subjects did Ashby and Heegaard use in their study?

Answer 17

3 males

Question 18

What types of jumps did Ashby and Heegaard use in their study?

Answer 18

Jumps with free arm motion and jumps with restricted arm motion

Question 19

What question did Ashby and Heegaard test in their study?

Answer 19

Does arm swing improve jumping distance?

Question 20

What had previous studies shown about standing vertical jumps?

Answer 20

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

Question 21

What had previous studies shown about the downward force of arm swings?

Answer 21

Arm swing may create additional downward forces on the body that slows the contraction of hip and knee extensors, enabling greater muscle force development

Question 22

What have previous studies shown to be the reason why maintaining balance is more difficult during a standing long jump?

Answer 22

Because the center of gravity does not remain over the lower body throughout the motion

Question 23

What was Ashby and Heegaard’s hypothesis?

Answer 23

Standing long jump performance is improved when arm movements is employed in the standing long jump

Question 24

What kinds of subjects were used in Ashby and Heegaard’s study?

Answer 24

Untrained males

Question 25

How were the subjects prepared for the long jump during Ashby and Heegaard’s study?

Answer 25

They warmed up by doing light running, jumping, and stretching

Question 26

How did Ashby and Heegaard instruct the subjects?

Answer 26

The subjects were instructed to stand on a force platform and jump as far as possible when given a verbal cue

Question 27

What kind of force platform used in Ashby and Heegaard’s study?

Answer 27

A split belt force treadmill

Question 28

What hertz was used to measure ground reaction force in Ashby and Heegaard’s study?

Answer 28

1000 Hz

Question 29

How many jumps did each subject perform in Ashby and Heegaard’s study?

Answer 29

6 long jumps with free arm movement and 6 long jumps with restricted arm movement

Question 30

How did the subjects in Ashby and Heegaard’s study perform the restricted arm jumps?

Answer 30

With their hands in front of their abdomen throughout the jump

Question 31

What constituted a successful jump in Ashby and Heegaard’s study?

Answer 31

A jump completed without falling on the landing

Question 32

How was the motion of the jump captured in Ashby and Heegaard’s study?

Answer 32

It was tracked in three dimensions with a passive marker motion capture system

Question 33

What did Ashby and Heegaard use the position of the markers to measure?

Answer 33

The kinematics of the body segments

Question 34

What equipment was used in Ashby and Heegaard’s study?

Answer 34

4 cameras and a force platform

Question 35

How was the data from the markers used in Ashby and Heegaard’s study?

Answer 35

The 7 markers were used to create a 2D, 6 segment model

Question 36

What segments were used in Ashby and Heegaard’s study?

Answer 36

Foot, calf, thigh, head, neck, trunk, upper arm, and forearm/hand

Question 37

How did Ashby and Heegaard apply projectile motion principles?

Answer 37

Projectile motion principles were applied to the center of gravity from take off until touch down