Force in physics

Main thing

Force is any influence that can change the motion, speed, or shape of an object. It is a vector quantity, meaning it has both magnitude and direction. The SI unit of force is the newton (N). Forces can be balanced, resulting in no change in motion, or unbalanced, resulting in acceleration according to Newton's second law: F = ma.

Forces arise from interactions between objects. These interactions can be categorized as contact forces (objects physically touching) or field forces (objects interacting at a distance). Examples of contact forces include friction, tension, and normal force. Examples of field forces include gravity, electromagnetism, and the strong and weak nuclear forces.

Forces play a crucial role in various aspects of physics and everyday life:

• Mechanics: Forces cause objects to move, change speed, or change direction. Examples include pushing a cart, hitting a ball, or sliding down a ramp.

• Fluid dynamics: Forces acting on liquids and gases lead to pressure, buoyancy, and lift. Examples include water pressure in a pool, a helium balloon floating, or an airplane wing generating lift.

• Thermodynamics: Forces between particles cause pressure in gases and influence heat transfer. Examples include air pressure in a tire or the force of expanding gas in an engine cylinder.

• Electromagnetism: Electric and magnetic forces govern the behavior of charged particles and the interaction of magnets. Examples include the force between two electric charges or the magnetic force acting on a compass needle.

• Quantum mechanics: Fundamental forces govern the behavior of subatomic particles. Examples include the strong force binding quarks in a proton or the weak force responsible for radioactive decay.

Terms

• Force: Any influence that can change the motion, speed, or shape of an object. A push or pull.

• Vector: A quantity having both magnitude and direction.

• Newton (N): The SI unit of force. 1 N is the force needed to accelerate 1 kg of mass at 1 m/s².

• SI unit: The International System of Units, the standard metric system used in science.

• Balanced forces: Equal forces acting in opposite directions on an object, resulting in no change in motion.

• Unbalanced forces: Forces that do not cancel each other out, resulting in a change in motion.

An analogy

Force is like a person pushing a shopping cart. The person (one object) exerts a force on the cart (another object), causing it to move or change speed. The harder the person pushes (magnitude) and the direction they push determine how the cart moves.

A main misconception

Many people think force always results in motion. However, forces can be balanced, resulting in no change in motion. For example, a book sitting on a table has the downward force of gravity balanced by the upward normal force from the table, so the book remains stationary.

The history

1. 4th century BC: Aristotle's ideas about forces shaping motion were widely accepted.

2. 1687: Isaac Newton published his three laws of motion, mathematically defining force and revolutionizing physics.

3. 1798: Henry Cavendish measured the weak gravitational force between masses in a laboratory.

4. 1915: Albert Einstein's general relativity showed gravity as curvature of spacetime.

5. 1970s-present: Physicists search for a theory unifying the fundamental forces of nature.

Three cases how to use it right now

1. Understand your weight is a force due to gravity pulling you down. Scales measure this force. Calculate your mass using the equation F = mg, where F is your weight, m is your mass, and g is the acceleration due to gravity (9.8 m/s²).

2. Push heavy furniture by exerting a steady force to overcome friction and change its motion. Minimize friction by using sliders or lifting the furniture slightly. Apply the force in the desired direction of motion.

3. Throw a ball by applying a force with your arm. The force determines the ball's speed and direction. Adjust the magnitude and angle of the force to control the ball's trajectory. The greater the force, the faster the ball will travel.

Interesting facts

• The gravitational force between you and Earth is over 1,000,000,000,000,000,000 times weaker than the electromagnetic force holding atoms together.

• Light exerts a tiny force when reflecting off objects, known as radiation pressure. This force is used in solar sails to propel spacecraft.

• The four fundamental forces are the gravitational, electromagnetic, strong nuclear, and weak nuclear forces. They govern all known interactions in the universe.

• Centripetal force causes an object to follow a curved path and is always perpendicular to velocity. It is not a separate force but a result of other forces like tension or gravity.

• Friction is a force that opposes motion between two surfaces in contact. It converts kinetic energy into heat and can be reduced using lubricants or smooth surfaces.