Physics Fundamentals: Forces, Energy and Motion Explained

Physics Fundamentals: Forces, Energy and Motion

The following questions and answers review essential concepts of forces, motion, gravitation and energy, using the International System of Units (SI).

Forces and Newton's Laws

The fourth law of motion states that the force of a body's action on another is equal in magnitude to the force exerted by the second body in the opposite direction, because it opposes the direction of motion. This is consistent with Newton's third law, which states that forces always act in pairs.

• Newton's Third Law: Forces always act in pairs, with equal magnitude and opposite direction.
• Newton's First Law: If the resultant of all forces acting on a body is zero, the body is at rest or in uniform rectilinear motion.
• Forces are vector quantities: They are defined by four characteristics: magnitude, direction, sense, and point of application.

Forces are represented by vectors. The nuclear force, which is the strong force, keeps the nucleus of the atom's particles together.

Gravitational Force

The gravitational force between two bodies depends on their masses and the distance between them. The force with which the Earth attracts an apple is equal to the force with which the apple attracts the Earth. The gravitational attraction increases when the distance between the bodies decreases.

When two charges are placed at a distance of 1 km and attract each other with a force, the value of that force can be calculated using Coulomb's law applied to the given data.

Using Newton's law of universal gravitation with the Sun's mass of 6.1030 kg (corrected from the original data), the Earth's mass of 6.1024 kg (corrected from the original data), and a distance of 150,000,000 km, the gravitational force between them is approximately 1.067 × 10²³ N.

The gravitational acceleration (g) is the acceleration with which bodies fall near the Earth's surface.

Motion and Trajectories

The trajectory of a mobile is the imaginary line that the mobile follows as it moves.

A body that moves with rectilinear uniform motion follows a trajectory at constant velocity. A body with an acceleration of 1 m/s² increases its velocity by 1 m/s every second.

Any object is in motion when it changes position relative to a reference frame.

Rectilinear uniformly accelerated motion has constant acceleration.

Energy

Energy is the capacity of a body to produce changes. This statement is true.

• Kinetic energy is the energy a body possesses due to its motion. A body located at a height above the ground does not have kinetic energy simply because of its height; this statement is false.
• Potential energy does not depend on velocity; the statement that a body with potential energy has velocity is false.
• When a body falls from a height, its gravitational potential energy decreases while its kinetic energy increases. The total mechanical energy is conserved, so the statement that falling bodies lose energy in violation of the conservation principle is false.

The unit of energy in the International System is the joule (J), not the second; this statement is false.

Kinetic Energy Calculation

To calculate the kinetic energy of a 1,800 kg car traveling at 110 km/h, first convert the velocity to SI units:

v = 110 km/h × (1000 m / 1 km) × (1 h / 3600 s) ≈ 30.56 m/s

Then apply the formula:

E_k = ½ m v² = ½ × 1,800 kg × (30.56 m/s)² ≈ 841,000 J (≈ 8.41 × 10⁵ J)

SI Units

The unit of mass in the International System is the kilogram (kg).