Unit 8 Quiz #2 Targets 3-6

Explanation
Objects become electrically charged as a result of the transfer of electrons. When electrons are transferred from one object to another, it creates an imbalance in the number of positive and negative charges, leading to the object becoming charged. Electrons are negatively charged particles, so when they are transferred, the object either gains or loses electrons, resulting in a net charge. This transfer of electrons is what causes objects to become electrically charged.

Explanation
The nucleus of an atom has a positive electric charge because it contains protons, which are positively charged particles. The protons are balanced by an equal number of negatively charged electrons in the electron cloud surrounding the nucleus, resulting in an overall neutral charge for the atom.

Explanation
Rubbing a balloon through dry hair transfers electrons from the hair to the balloon. Electrons have a negative charge, so when the balloon gains electrons, it becomes negatively charged. Therefore, the correct answer is negative.

Explanation
When the charge on one object is doubled from q to 2q, the force between the objects becomes 2 F. This is because the electric force between two charged objects is directly proportional to the product of their charges. Therefore, when the charge is doubled, the force also doubles.

Explanation
When the distance between two objects decreases to 0.5r, the force between them becomes 4 times greater than the original force (4F). This is because the electric force is inversely proportional to the square of the distance between the objects. When the distance is halved, the denominator in the equation becomes 0.5^2 = 0.25, resulting in a force that is 4 times stronger.

Explanation
Both electric force and gravitational force are field forces because they act on objects even when they are not in direct contact. These forces can act over a distance, without the need for physical contact between the objects involved. This is in contrast to forces like friction or tension, which require objects to be in contact for the force to be exerted. Therefore, the correct answer is that both electric force and gravitational force are field forces.

Explanation
The given answer is correct because electric force is unique in that it can both attract and repel objects. This is due to the fact that electric charges can be positive or negative, and like charges repel while opposite charges attract. In contrast, gravitational force only attracts objects and does not have the ability to repel. Therefore, the ability of electric force to both attract and repel is a key difference between electric force and gravitational force.

Explanation
The time it takes for the letter to make one swing across the river can be determined using the formula for the period of a simple pendulum. The period is given by the equation T = 2π√(L/g), where T is the period, L is the length of the pendulum, and g is the acceleration due to gravity. In this case, the length of the pendulum is the sum of the height of the bridge (130.0 m) and the width of the river (16.0 m), so L = 130.0 m + 16.0 m = 146.0 m. The acceleration due to gravity is approximately 9.8 m/s^2. Plugging these values into the equation, we get T = 2π√(146.0/9.8) ≈ 11.43 sec. Therefore, the correct answer is 11.43 sec.

Explanation
The mass of the pearl can be determined using the formula for the period of a mass-spring system, which is T = 2π√(m/k), where T is the period, m is the mass, and k is the spring constant. Rearranging the formula to solve for m, we get m = (T^2 * k) / (4π^2). Plugging in the values given in the question, we have T = 1/1.20 = 0.833 s and k = 362 N/m. Substituting these values into the formula, we find m = (0.833^2 * 362) / (4π^2) ≈ 6.37 kg. Therefore, the mass of the pearl is 6.37 kg.