Biomechanics Questions part 2
Question 1: Defining forces
The propulsive force causes acceleration or speeding up the movement of the body while breaking force slows down the speed or decelerates the body. Therefore, propulsive force is the inverse of braking force; it increases the speed of an object by causing the acceleration. Two bodies in touch with each other experience a contact force between them. The forces are opposite directions and equal in magnitude. According to Newton’s third law, the two bodies in contact exert a reaction force acting opposite in direction to each other. Effective force is the difference between gravitation force or weight (Fg) and ground reaction force denoted as FGRF (Flanagan, 2019). Ideally, a body will move in upward direction if the gravitational force exceeds ground reaction force. The frictional force is parallel contact area force opposing the motion between two objects in contact and is proportional to the normal force. The normal force, on the other hand, holds two surfaces together, and the harder the normal force, the higher the frictional force. Finally, net force refers to the sum of all vector forces acting on a particle/body.
Question 2: Types of forces
Typically, for any force to be effective, it must be applied over time or via a distance. The magnitude of these forces changes in magnitude over time. If the size of the force is graphically plotted against time, the peak force is at the highest point on the curve – force with the largest magnitude. The rate of force development (RFD) is the positive slope on the curve, while the rate of force fatigue (RFF) is the negative slope on the force-time curve (Flanagan, 2019). Therefore, RFD and RFF represent a change in force overtime. Impulse is the area under the curve – basically a change in momentum due to force applied over time. It defines the magnitude and direction of momentum change during the period of interest.
Source: Flanagan, (2019).
Question 3: Newton’s First and second laws of motion
Newton’s first and second law of motion describes the momentum and how it changes. Accordingly, a system’s motion will never change unless an external force is applied to it. A force is defined as a push or a pull on an object. The external force is from an external system.
The first law states that a body will remain still or continue to move at a constant speed and the same direction unless an outside force is applied to it. The second of newton describes the force as the product of mass and acceleration. Force is measured in Newton, and one Newton is the force required to accelerate a mass of 1kg at a speed of 1M/sec2 (Flanagan, 2019). Since force is a vector, it has both direction and magnitude. Given that mass is a scalar, the force will take the direction of acceleration, causing that force. Therefore, larger acceleration can be attained by either increasing the force or reducing the mass of the body.
Question 4: Resolving forces into x and y components
- For the 450 slope angle
Fg = Mag x Correction factor
y = Mag x COS (900-450)
= Mag x COS450
x = Mag x SIN (900-450)
= Mag x SIN450
- For 600 slope angle
Fg = Mag x Correction factor
y = Mag x COS (900-600)
= Mag x COS300
x = Mag x SIN (900-600)
= Mag x SIN300
Q 5: Frictional force
No, increasing the surface area of a shoe with the same mass does not increase frictional force (Flanagan, 2019). The reason is that the frictional force is independent of the contact area between the surfaces. Doubling the surface area means that less weight is above each square centimetre to exert downward pressure. Therefore, having different shoes of the same mass but the different surface area, they will still have the same frictional forces.
- 6. Effect of negative force on a negative slope
Negative force means a decreasing force with time (Flanagan, 2019). Therefore, if the force is negative, and the slope is negative, the negative slope indicates the rate of force fatigue (RFF). This means that momentum is declining with time until the body eventually comes to a stop. The contrary is when a positive force is applied on a positive slope, then both the force and momentum are increasing, and the slope indicates a rate of force development (RFD).
Q6. Average force and momentum
The average force is assumed if the magnitude of the force were to remain constant over the whole period of interest. It is the ratio of an impulse to the time the force was exerted (Flanagan, 2019). Therefore, if the average force is increased, the momentum would also increase. The reason is that increased cause acceleration to increase the velocity of an object/body. Note that momentum is the product of mass and acceleration.
References
Flanagan, S.P. (2019). Biomechanics: A Case-Based Approach (2ndEd).Burlington, MA: Jones and Bartlett Learning. (Lesson 7, pages 113-142)