Forces and Motion
Discover how forces shape our world, from the gravity that keeps us grounded to the push and pull that moves objects around us.
In This Section
Introduction to Forces
A force is a push or pull that can cause an object to move, stop, or change direction. Forces are all around us—from the gravity that keeps us on the ground to the friction that helps us walk without slipping. Forces can act through direct contact (like pushing a shopping cart) or at a distance (like magnets attracting each other).
In physics, forces are measured in units called newtons (N), named after Sir Isaac Newton, who formulated the fundamental laws of motion. One newton is the force needed to accelerate a 1-kilogram mass at a rate of 1 meter per second squared.
Did You Know?
Forces always come in pairs! When you push on a wall, the wall pushes back on you with equal force in the opposite direction. This is known as Newton's Third Law of Motion.
Newton's Laws of Motion
Sir Isaac Newton's three laws of motion form the foundation of classical mechanics and help us understand how forces affect objects.
First Law: Inertia
An object at rest stays at rest, and an object in motion stays in motion with the same speed and direction, unless acted upon by an unbalanced force.
Second Law: F=ma
The acceleration of an object depends on the force applied and the object's mass. Force equals mass times acceleration (F = ma).
Third Law: Action-Reaction
For every action, there is an equal and opposite reaction. Forces always come in pairs.
Try This!
Experiment with Newton's laws using our interactive "Forces and Motion" simulation. Apply different forces to objects and observe how they accelerate based on their mass.
Try the simulation →Gravitational Forces
Gravity is a force of attraction that exists between any two objects with mass. The strength of gravity depends on two factors: the masses of the objects and the distance between them.
Newton's Law of Universal Gravitation states that every particle of matter in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
F = G × (m₁ × m₂) / r²
Where F is the gravitational force, G is the gravitational constant, m₁ and m₂ are the masses of the objects, and r is the distance between their centers.
On Earth, gravity causes objects to accelerate toward the ground at approximately 9.8 meters per second squared (9.8 m/s²). This acceleration due to gravity is often denoted as "g".
Real-World Connection
Gravity is what keeps planets in orbit around the Sun and the Moon in orbit around Earth. It's also what gives us weight. If you were to stand on the Moon, you would weigh about 1/6th of what you weigh on Earth because the Moon has less mass than Earth and therefore exerts less gravitational force.
Electromagnetic Forces
Electromagnetic forces are interactions between electrically charged particles. These forces include both electric forces (between charged particles) and magnetic forces (between moving charges or magnetic materials).
Electric Forces
Like charges repel each other, while opposite charges attract. The strength of the electric force depends on the magnitude of the charges and the distance between them.
Magnetic Forces
Magnetic forces are created by moving electric charges. Like magnetic poles repel each other, while opposite poles attract. Magnetic fields can exert forces on moving charges and other magnets.
Electromagnetic forces are much stronger than gravitational forces, but we don't usually notice them pulling on us because most objects have a balanced number of positive and negative charges, making them electrically neutral overall.
Try This!
Explore magnetic fields and forces with our "Magnets and Electromagnets" simulation. See how magnets interact with each other and how electric currents create magnetic fields.
Try the simulation →Balanced & Unbalanced Forces
When multiple forces act on an object, the object's motion depends on whether these forces are balanced or unbalanced.
Balanced Forces
When forces are balanced, they are equal in size but opposite in direction, resulting in no change in motion. An object at rest stays at rest, and an object in motion continues moving at a constant speed and direction.
Unbalanced Forces
When forces are unbalanced, there is a net force that causes acceleration in the direction of the greater force. An object at rest will start moving, and an object in motion will change its speed or direction.
The net force on an object is the vector sum of all individual forces acting on it. If the net force is zero, the forces are balanced. If the net force is not zero, the forces are unbalanced, and the object will accelerate according to Newton's Second Law (F = ma).
Real-World Connection
When you're sitting in a chair, the force of gravity pulling you down is balanced by the normal force of the chair pushing up on you. But when you jump off a diving board, the force of gravity is unbalanced, causing you to accelerate downward until you hit the water, where drag forces eventually balance with gravity, and you reach a terminal velocity.
Collisions & Momentum
Momentum is a measure of an object's motion, calculated as the product of its mass and velocity. When objects collide, momentum is transferred between them, but the total momentum of the system remains constant if no external forces are acting on it.
p = m × v
Where p is momentum, m is mass, and v is velocity. Momentum is a vector quantity, meaning it has both magnitude and direction.
Types of Collisions
Elastic Collisions
In elastic collisions, both momentum and kinetic energy are conserved. The objects bounce off each other with no loss of energy. Examples include collisions between billiard balls or gas molecules.
Inelastic Collisions
In inelastic collisions, momentum is conserved, but some kinetic energy is converted to other forms (like heat or sound). The objects may stick together or deform. Examples include car crashes or a ball of clay hitting a wall.
The Law of Conservation of Momentum states that in a closed system (no external forces), the total momentum before a collision equals the total momentum after the collision. This principle is fundamental in understanding everything from car crashes to rocket propulsion.
Try This!
Experiment with collisions in our "Collision Lab" simulation. Change the masses and velocities of objects and observe how momentum is conserved in different types of collisions.
Try the simulation →Check Your Understanding
1. What is Newton's First Law of Motion?
Answer: An object at rest stays at rest, and an object in motion stays in motion with the same speed and direction, unless acted upon by an unbalanced force.
2. If you push a 2 kg object with a force of 10 N, what will its acceleration be?
Answer: Using F = ma, a = F/m = 10 N / 2 kg = 5 m/s²
3. What happens to an object when the forces acting on it are balanced?
Answer: The object will either remain at rest or continue moving at a constant velocity (same speed and direction).