Review These For Information Regarding Newton’s Laws
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Newton's First Law
Newton’s First Law states that an object will remain in uniform motion unless it is acted upon by an external force. This means that it is currently moving, it will continue to move in that manner and if it is at rest, it will continue to remain at rest.
Newton's Second Law
Newton’s Second Law states that the acceleration of an object is directly proportional to the magnitude of the net force exerted on it, in the same direction as the net force, and inversely proportional to the mass of the object.
Newton's Third Law
Newton’s Third Law states that each force is met with an equal and opposite force. This means that the application of a force on an object results in an equal magnitude force applied in the opposite direction to the applicant.
View the Free Body Diagram Below:
Many forces are exerted on the body while running, the most important of these is the propulsion exerted by the foot onto the ground.
This results in a vector quantity with components in the x and y directions.
- x component allows runner to move forward
- y component allows runner to overcome gravity and move upwards
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x-Component of Force
The x-component of the propulsive force causes an acceleration in the horizontal direction (according to Newton’s Second Law). This allows the runner to move forward.
y-Component of Force
The y-component of the propulsive force causes a nearly instantaneous net force in the upwards direction as it overcomes the force due to gravity. This allows the runner to move up off the ground.
Note on Friction and Newton’s Third Law:
Many people believe that friction is only a force that opposes the motion of an object (as seen in this incorrect diagram:)
When running this is not the case:
Imagine running on a frictionless surface (e.g. ice). If no static friction were present, the foot would push back on the surface and just slide back. Nothing would resist this motion.
When running, we exert a force opposite the direction of motion. By Newton’s 3rd Law, the ground then exerts a force on us equal and opposite to our force on the ground, allowing forward motion.