Isaac Newton formulated the laws of motion! His first law is inertia, second law is F = ma, third law is action-reaction. The unit of force (newton, N) is named after him. A force of 1 N accelerates a 1 kg mass at 1 m/s². Forces can be balanced (net force = 0, no acceleration) or unbalanced (net force ≠ 0, acceleration). Unbalanced forces cause changes in motion. So the answer is Isaac Newton.

Inertia is the correct answer! The first law is also called the law of inertia. Inertia explains why a ball continues rolling until friction stops it, why a space probe can coast through the solar system without fuel, and why you feel a jolt when a car starts or stops. The more mass an object has, the more inertia it has (harder to change its motion). So inertia resists changes in motion.

F = ma is the second law! This equation is fundamental to physics. For example, a 1,000 kg car accelerating at 2 m/s² requires a force of 2,000 N. If you apply the same force to a 500 kg car, it will accelerate at 4 m/s² (twice as fast). The second law also explains why objects fall at the same rate (gravity acceleration is independent of mass). Weight (W = mg) is a special case: weight = mass × acceleration due to gravity. So the answer is F = ma.

Newton's third law explains rocket lift-off! The action and reaction forces are equal in magnitude and opposite in direction, but they act on different objects. The action acts on the exhaust gases, the reaction acts on the rocket. The third law also explains why you can walk: your foot pushes backward on the ground (action), and the ground pushes you forward (reaction). So the answer is Newton's third law.

Velocity includes direction! If you run in a circle at constant speed, your velocity is constantly changing because your direction changes. This means you are accelerating. The formula for speed is distance/time. Velocity is displacement/time (displacement is the straight-line distance from start to finish). So the answer is velocity.

g = 9.8 m/s²! This value varies slightly depending on altitude and latitude. The acceleration due to gravity is constant for all objects (ignoring air resistance). That is why a feather and a hammer fall at the same rate in a vacuum (as demonstrated on the Moon). The formula for distance fallen is d = ½gt². So the answer is 9.8 m/s².

Static friction is the correct answer! Kinetic (sliding) friction acts when surfaces slide past each other. Rolling friction (wheels) is usually much lower. Friction can be reduced by lubrication (oil, grease) or by using ball bearings. Friction can be increased by using rough surfaces (sand on icy roads). Friction generates heat (rubbing hands together). So static friction prevents motion.

p = mv is the formula! Momentum conservation explains collisions. In an elastic collision, both momentum and kinetic energy are conserved. In an inelastic collision, only momentum is conserved (kinetic energy is not). A large truck has more momentum than a small car at the same speed. Impulse is the change in momentum (F·Δt = Δp). So the answer is p = mv.

Centripetal force is the correct answer! Centrifugal force is a fictitious force (apparent force) felt by an object in a rotating reference frame. The formula for centripetal force is F_c = mv²/r. If the centripetal force disappears, the object will move off in a straight line (Newton's first law). So the answer is centripetal force.

W = Fd is the formula! Work is measured in joules (J). 1 J = 1 N·m. If you lift a 1 kg object (weight = 9.8 N) by 1 m, you do about 9.8 J of work. Kinetic energy (KE = ½mv²) is energy of motion. Potential energy (PE = mgh) is gravitational potential energy. The law of conservation of energy states that energy cannot be created or destroyed, only transformed. So the answer is W = Fd.