Newton's FIRST LAW
"An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force." - Physics Classroom
"In the Blocks"
When an athlete is in the blocks, they are not moving and therefore at rest. An athlete has not applied any force in the X direction, resulting in the motion in the X direction being zero ( ΣFx=0). There is no external forcing acting upon the hurdler when in the blocks. Similar to the X direction, the motion in the Y direction is also zero ( ΣFy=0). The reason is that the even though there are external forces acting on the hurdler, the sum of the forces is equal. The external forces are the Force of Earth on the Hurdler, and the Normal Force (force of hurdler on earth). The force the earth exerts on the hurdler is equal to the force that the hurdler exerts on the earth (normal force). This shows that when an athlete is in blocks, they are at rest and that the Net Force (ΣF) is equal to zero. "The Race" A hurdler runs for the majority of the race at a constant speed. The hurdler will continue at this consistent speed because of inertia. It would require the hurdler to exert more force in order to change their velocity. It is important for a hurdler especially to start the race strong because there are limited opportunities to change the velocity at which they are traveling. A hurdler is only on the ground for a few strides, therefore the lack of time and inertia acting on them makes it harder to change speed. Newton's Second Law“The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.” – Physics Classroom Newton's Second Law can also be written as ΣF=m*a. This means that the Net Force is equal to the hurdlers mass times their acceleration. This equation can be changed to say that acceleration is inversely proportional to mass and directly proportional to the net force. a=ΣF/m When thinking about any track and field event, the mass of the athlete greatly affects their performance. In a hurdling race mass is a key component to success. A hurdler with more mass will have to exert more force in order to accelerate. While a hurdler with less mass will have to exert a lesser force to match the acceleration of the more massive hurdler. The Free Body Diagram to the right, shows the 4 things that affect a hurdler in their race. In the y direction the forces that act on a hurdler are Normal and F of Earth on the hurdler. Throughout the race, these forces stay almost equal. Therefore concluding that the ΣFy = 0. In the x direction, the forces are not equal. The Force of Air Resistance on the Hurdler and the Force Friction on Hurdler both act on the hurdler. Seen in the Free Body Diagram above, the Force of Friction is greater than the force of Air Resistance. This means that the ΣFx > 0. This also allows for a hurdler to accelerate in the positive x direction.
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The Free Body Diagram above, shows the forces acting upon a hurdler when they are in the blocks and during their race. Their is no external forces acting upon the hurdler in the x direction. The forces in the y direction are Normal and Force of Earth on the Hurdler, as seen above. However these forces are equal in magnitude and opposite in direction resulting in ΣFy=0. Therefore the total net force for both the x and y components can be displayed as ΣF=0.
The clip above shows the blocks to the first hurdle in the 2012 London Olympics. All of the athletes in the clip forcefully push out of the blocks, allowing for a strong start. |