Physics - Mechanics and Gravitation

Mass is an intrinsic property of matter that remains constant everywhere. Weight = mass × gravitational acceleration. In a satellite orbiting Earth, the astronaut and satellite are in free-fall, experiencing apparent weightlessness despite gravity still acting - this is why objects float inside spacecraft.

Nuclear force is the strongest of the four fundamental forces in nature, much stronger than electromagnetic, gravitational, and weak nuclear forces. It acts between nucleons (protons and neutrons) at very short ranges (~1 fm) and is responsible for holding the nucleus together despite the repulsive electromagnetic force between protons.

When a stone is thrown horizontally (parallel to Earth's surface), it follows projectile motion under gravity. The horizontal component of velocity remains constant (ignoring air resistance), while the vertical component follows accelerated motion due to gravity. The combination of these two perpendicular motions produces a parabolic trajectory.

Decreases

By a common balance, we measure mass.

Acceleration due to gravity (g) decreases with altitude because g = GM/r², where r is the distance from Earth's center. As altitude increases, r increases, causing g to decrease. This is why objects weigh slightly less at higher altitudes.

Weight (mg) is more at the poles because g is maximum at the poles and minimum at the equator. This happens because: (1) Earth is not a perfect sphere - it's flattened at poles (polar radius < equatorial radius), so you're closer to Earth's center at poles, and (2) centrifugal force at equator slightly reduces effective g.

In the absence of air resistance (negligible viscosity), all objects fall with the same acceleration due to gravity regardless of their mass. This is a consequence of Galileo's principle and Newton's second law: a = F/m = mg/m = g. Both balls will hit the ground simultaneously when dropped from the same height.

Weight on the moon is approximately 1/6th of weight on Earth because moon's gravitational acceleration (g_moon ≈ 1.62 m/s²) is about 1/6th of Earth's (g_earth ≈ 9.81 m/s²). A 12 kg weight on Earth would weigh approximately 12 × (1/6) = 2 kg on the moon. Note: the question uses 'kg' for weight when it should use 'N' for force - mass (12 kg) remains constant.

A geostationary satellite appears stationary from Earth's surface because its orbital period exactly matches Earth's rotational period (approximately 24 hours). This requires the satellite to be at a specific altitude (~35,786 km above equator) where its orbital period equals one sidereal day.

The Moon has no atmosphere because its escape velocity (2.38 km/s) is much lower than the root mean square velocity of gas molecules at lunar surface temperatures. Light gas molecules (like hydrogen and helium) easily exceed escape velocity and escape into space over geological time. Higher mass molecules might be retained, but the Moon lacks significant sources for these gases.

Hydrogen bombs (thermonuclear weapons) work on the principle of nuclear fusion, where light nuclei (like isotopes of hydrogen - deuterium and tritium) combine to form heavier nuclei, releasing enormous energy. This is the same process that powers the Sun and stars. A fission bomb is typically used as the trigger to create the extreme temperatures and pressures needed to initiate fusion.

Curie is the traditional unit of radioactivity, defined as 3.7 × 10^10 disintegrations per second (originally based on the activity of 1 gram of radium-226). The SI unit is becquerel (Bq), where 1 curie = 3.7 × 10^10 Bq. Einstein and Planck are units used in photochemistry and quantum mechanics respectively, not radioactivity.

 Planets are kept in orbit due to the gravitational attraction to the sun.

When a watch spring is compressed or wound, work is done on it, storing mechanical energy in the form of elastic potential energy. Upon release, this potential energy converts into kinetic energy to drive the watch mechanism.

When a raindrop falls at terminal velocity (uniform velocity), the net force is zero (gravity = air resistance). The raindrop's mechanical energy (KE + PE) decreases continuously because work is done against air resistance (friction). This energy is transferred to the air as heat and turbulence. The total energy of the system (raindrop + air) is conserved, but the raindrop's mechanical energy alone is not - it dissipates into the atmosphere.

 Work is a scalar quantity.

Work is defined as force multiplied by displacement in the direction of the force (W = F × d × cosθ). When pushing against a wall that doesn't move, the displacement is zero, so no work is done regardless of how hard you push. Option A is incorrect because no displacement means no work. Option C is wrong because the wall doesn't move. Option D is incorrect since B is the right answer.

The acceleration due to gravity (g) at Earth's surface is approximately 9.8 m/s². This is the maximum value because g decreases with increasing altitude or depth from the surface. Option A (980) would be in cm/s², not m/s². Option B (98) and D (0.98) are incorrect values.

According to Newton's law of universal gravitation, gravitational force (and thus g) decreases with the square of the distance from Earth's center. As altitude increases, distance from Earth's center increases, so g decreases. Option B is the opposite of what happens. Option C would mean g is constant regardless of height, which contradicts gravitational law.

At Earth's center, a body is equidistant from all surrounding mass. Gravitational forces from all directions cancel out completely, resulting in zero net gravitational force and thus g = 0. Option A is incorrect because g is maximum at the surface, not the center. Option B is wrong since g varies with depth. Option D is misleading - g is zero, not just minimum.

In a vacuum, all objects fall with the exact same acceleration due to gravity, regardless of their mass or composition, as established by Galileo. Speed and velocity change continuously during free fall, so they are not constant. Force depends on mass, so different objects experience different gravitational forces.

A gravitational field has both magnitude and direction - it points toward the mass creating the field. This makes it a vector quantity. Options A and C are incorrect because the field definitely has direction. Option D is wrong because the field strength is non-zero (it's g at that point).

During free fall, the only force acting on a body is gravity (mg). Since there's no support force (normal reaction), the body experiences apparent weightlessness - what we commonly call 'weightless'. This is standard physics terminology despite the body still having mass. Options B, C, and D are incorrect because free fall isn't about having minimum, maximum, or medium weight.

Weight is minimum (zero) at Earth's center because gravitational forces from all surrounding mass cancel completely. At the poles (A and B), weight is maximum due to no centrifugal reduction. At the equator (C), weight is slightly less than at poles due to centrifugal force from Earth's rotation, but still much more than zero at the center.