The relative velocity of the athlete relative to the ground is 5.2 m/s
The given parameters;
constant velocity of the athlete, V = 5.2 m/s
let the velocity of the ground = Vg = 0
The relative velocity concept helps us to determine the velocity of a moving object relative to a stationary observer.
The athlete is the moving object in this question while the ground is stationary.
The relative velocity of the athlete relative to the ground is calculated as follows;

Thus, the relative velocity of the athlete relative to the ground is 5.2 m/s
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The tennis ball that is hit the hardest would have the most kinetic energy.
Internal energy is B. The sum of the kinetic and potential energies of the particles in a system
Explanation:
The internal energy possessed by the particles in any substance has two origins:
- Part of the energy of the particles is due to the motion of the particles. In fact, for a substance at a temperature larger than absolute zero, the particles in the substance are always in motion (for a solid, they just vibrate around their fixed position, while in a liquid and in a gas, the particles are actually free to move). Because of this motion, the particles have kinetic energy. The temperature of a substance is a measure of the average kinetic energy of the particles in the substance.
- The rest of the energy of a substance is in the form of potential energy, and it refers to the energy contained in the bonds between the particles of the substance (for instance, in the intermolecular bonds between different atoms in a solid). For a gaseous substance, these intermolecular bonds are very weak, therefore the potential energy is not considered and the internal energy of a gas depends only on its kinetic energy (and therefore, only on the temperature of the substance).
Therefore, the internal energy of a substance is the sum of the kinetic and the potential energies of the particles in the substance.
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Electric Field Strength E = Force F/ Charge q
E = F/q
Force , F = E*q
But charge on an electron q = 1.6 * 10⁻¹⁹ C
E = 3 * 10³ N/C from the question.
Force F = 3* 10³ * 1.6 * 10⁻¹⁹
F = 4.8 * 10⁻¹⁶ N.
Electrostatic force F = 4.8 * 10⁻¹⁶ N