Physics Class: Newton's Laws of Motion

Weight on Earth and Moon

• Weight is calculated using the formula Weight = mass (m) * acceleration due to gravity (g).
• Weight on Earth for a mass of 100 kg is 981 N (100 kg * 9.81 m/s²).
• Weight on the Moon for the same mass is 163.5 N (100 kg * (9.81 m/s² / 6)) due to reduced gravitational pull.

Newton’s Third Law of Motion

• Action-reaction principle: for every action, there is an equal and opposite reaction.
• Forces exerted by two objects on each other are equal in magnitude and opposite in direction.

Ground Reaction Force (GRF)

• GRF is the reaction force provided by a surface during movement, derived from Newton’s third law.
• Measured using a force platform; consists of vertical, forward-backward, and side-to-side components.
• The contribution of a body segment to GRF depends on mass and the acceleration of the center of mass (COM).
• Estimates: trunk and head contribute 50%, legs 17% each, arms 5% each to the GRF.

GRF During Running

• During running, initial contact generates upward vGRF and backward hGRF.
• Mid-stance and toe-off involve reduced vGRF and hGRF aiding in forward propulsion.
• GRF counters the downward motion of the runner throughout the gait cycle.

Newton’s Laws: Angular Motion

• Moment of Inertia (I) reflects a body's resistance to rotational motion and depends on mass distribution from the axis of rotation.
• Units for moment of inertia are kg*m².
• Moment of inertia is influenced more by how mass is distributed than by mass alone.

Angular Momentum

• Angular momentum (H) is calculated as the product of moment of inertia and angular velocity (w).
• Formulated as H = I * w.

Newton’s Laws: Angular

• Law I: Angular momentum remains constant unless acted upon by an external moment.
• Law II: Angular acceleration is proportional to the moment causing it (T = I * a).
• Law III: Bodies exert equal and opposite moments on one another, crucial for understanding biomechanics in physical activities.

Kinetics and Linear Motion

• Kinetics involves forces causing movement and forms the basis of understanding human motion.
• Newton’s linear laws include:
  • Law I: Inertia - a body in motion stays in motion unless acted upon.
  • Law II: Acceleration - directly proportional to force and inversely proportional to mass (F = ma).

Mass vs. Weight

• Mass reflects the quantity of matter, measured in kg; weight is the gravitational force acting on a mass (Weight = m * g).
• Gravity accelerates bodies towards the Earth at 9.81 m/s², with weight varying on different celestial bodies (e.g., Moon’s gravity is 1/6th of Earth’s).
• Example calculation of weight on Earth (981 N) vs. Moon (163.5 N) for a 100 kg block.

Whole Body Center of Mass (COM)

• Center of Mass indicates where mass is evenly distributed, influencing rotational dynamics.
• Path of a body in flight can be traced by tracking the COM.
• Rotational motion occurs about axes that pass through the COM.

Somersaulting and Angular Motion

• Applying a force that does not act through the COM creates a turning moment (T) essential for somersaulting.
• The moment arm (d) contributes to the turning moment; if d = 0, T = 0, indicating no torque produced.

Key Topics Summary

• Covered principles of kinetics, linear motion, ground reaction force, and angular laws.
• Highlighted significance of moment of inertia, angular momentum, and the center of mass in understanding biomechanics and physical movement.

Test your understanding of Newton's Laws of Motion with this quiz focused on weight calculations and the action-reaction principle. Explore how weight differs between Earth and the Moon, and apply Newton's third law in various scenarios. Perfect for physics students aiming to solidify their grasp on these fundamental concepts.