Explanation:
On a frictionless surface, no net external force is required to maintain uniform velocity (Newton's first law). Only to change velocity force is needed. Thus, force required is zero. If friction present, force would be needed to balance friction.
Explanation:
Coulomb's force in a vacuum is F = (1/4piepsilon_0) * (q1q2/r^2). When a dielectric medium with relative permittivity (dielectric constant) 'K' is introduced, the new force is F' = F / K. Since K > 1 for all insulating materials (e.g., K for water is ~80), the electrostatic force between the charges is significantly reduced by the medium.
Explanation:
Kepler's Second Law states that the line joining a planet to the Sun sweeps out equal areas in equal intervals of time (areal velocity is constant). Because the gravitational force is a central force (acting strictly along the radius vector), it exerts zero torque on the planet. When net torque is zero, angular momentum is strictly conserved.
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