What Causes Something To Keep Moving?

Motion is the act of an object changing its position over time. For something to be in motion, it must have kinetic energy, which is the energy associated with movement. There are several key concepts that explain why objects continue moving once set in motion. These include inertia, Newton’s Laws of motion, momentum, energy, friction, and gravity.

Inertia is the tendency of an object to resist changes in its motion. It is related to an object’s mass – the more mass an object has, the more inertia it has. Inertia causes objects to continue moving in their current direction unless acted on by an outside force.

Isaac Newton’s Three Laws of Motion describe how inertia works and the relationship between forces and motion. Newton’s First Law states that 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. This is also known as the Law of Inertia.


Inertia is the tendency of an object to resist any change in its motion. It is related to an object’s mass – the more mass an object has, the more inertia it has. Inertia is what keeps a moving object moving and a stationary object at rest, unless an outside force acts on it.

Inertia of motion refers to the tendency of a moving object to keep moving at the same speed and in the same direction, unless a force causes it to change. For example, a car driving down the highway will keep going at the same speed unless the driver brakes or accelerates. The more mass the car has, the more inertia it has, and the harder it is to change its motion.

Inertia of rest refers to the tendency of objects at rest to remain at rest. A book sitting on a table will not move unless something pushes or pulls on it. The book resists changes to its stationary state due to its inertia. Again, the more mass an object has, the greater its inertia and resistance to changing its stationary state.

Newton’s First Law

Newton’s First Law states that an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

This means that an object will continue doing what it’s currently doing unless a force causes it to change. For example, a ball sitting on the ground will not start rolling unless something like a person kicking it or the wind blows on it to create an unbalanced force. Similarly, a moving object like a car driving down the highway at 60 mph will continue at that same speed and direction unless forces like friction or air resistance slow it down or the driver applies the brakes or turns the steering wheel to change direction.

Newton’s First Law is also known as the law of inertia. Inertia is the resistance of any physical object to any change in its velocity. Massive objects have more inertia, meaning it takes more force to change their motion. Newton’s First Law implies that true inertial motion is always straight line motion.

This tendency of objects to resist changes in motion was not fully understood before Newton’s laws. Newton’s First Law made inertia a central concept of physics and showed that objects only respond to net external forces rather than just forces themselves.

Newton’s Second Law

Newton’s Second Law states that the acceleration of an object depends directly upon the net force acting upon the object, and inversely upon the mass of the object. In equation form, Newton’s Second Law is expressed as:

F = ma

Where F is net force, m is mass, and a is acceleration.

What this means is that acceleration is produced when a force acts on a mass. The greater the mass of the object, the greater the force needed to accelerate it. For example, it takes much more force to accelerate a car than it does to accelerate a bicycle. This is because the car has much greater mass than the bicycle.

Newton’s Second Law is extremely important because it allows us to quantitatively determine how an object will accelerate in response to an applied force. Many applications rely on this mathematical relationship, including the design of vehicles, athletic equipment, and amusement park rides.

Newton’s Third Law

Newton’s Third Law states that for every action, there is an equal and opposite reaction. This means that when two objects interact, they exert equal and opposite forces on each other. For example, when you push on a wall, the wall pushes back on you with an equal force. The wall’s push back on you is the reaction force to your push on the wall. Another example is when a rocket pushes gases out of its exhaust nozzle, the gases push the rocket upward with an equal force. This reaction force propels the rocket through space.

Newton’s Third Law is a symmetry law – the forces between two interacting objects are always equal in magnitude and opposite in direction. Action and reaction forces are simultaneous because they stem from mutual interactions. This law can explain many interactions we observe and experience in everyday life. It helps describe the physics behind recoil, rocket propulsion, sailing, exercising, collisions, and more. The Third Law is also essential in explaining macroscopic forces like tension, friction, and normal forces.

To summarize, Newton’s Third Law states that all forces arise between two objects as a mutual pair. Whenever an object exerts a force on another object, the second object exerts an equal and opposite reaction force. This law describes the nature of forces and is a fundamental principle in physics.


momentum depends on an object's mass and velocity.
Momentum is defined as the quantity of motion that an object has. It is calculated by multiplying the mass of an object by its velocity. The more mass an object has and the faster it is moving, the more momentum it has.

The law of conservation of momentum states that in a closed system, the total momentum before a collision or explosion will equal the total momentum after the event. Momentum is conserved because the net external force on the system is zero. During collisions between objects, momentum is transferred between the objects. The momentum lost by one object is gained by the other, so the total momentum stays the same.

For example, imagine two billiard balls colliding on a pool table. Before the collision, ball 1 is moving rapidly to the left while ball 2 is at rest. After the collision, ball 1 slows down and begins moving to the right, while ball 2 gains some momentum and also moves to the right. The momentum gained by ball 2 is equal to the momentum lost by ball 1. The total momentum of the two-ball system remains constant.


Energy plays a key role in allowing things to keep moving. There are two main types of energy that are relevant for motion – kinetic energy and potential energy. Kinetic energy is the energy of motion that an object has due to its velocity. The faster an object moves, the more kinetic energy it has. Potential energy is stored energy that an object has due to its position or shape. For example, a ball held at a height above the ground has potential energy due to gravity. As the ball falls, this potential energy gets converted into kinetic energy, allowing the ball to move faster and faster.

The transfers between kinetic and potential energy allow motion to continue in the absence of an external force. For example, a pendulum swings back and forth between kinetic energy at the bottom of the swing to potential energy at the top of the swing. The energy transfers sustain the motion without the pendulum requiring any additional pushes. Similar energy transfers happen with many moving objects, such as roller coasters going up and down hills.

Friction and other dissipative forces cause some of the kinetic and potential energy to be lost as heat. So the transfers are never perfectly efficient. But as long as some kinetic energy remains or can be regenerated from potential energy, an object’s motion can continue.


Friction is a force that opposes the motion of objects. There are two main types of friction: static friction and kinetic friction.

Static friction occurs between two surfaces that are not moving relative to each other. For example, static friction exists between a book sitting on a table and the table itself. This friction force arises from the microscopic roughness between the two surfaces. Static friction prevents the surfaces from sliding past one another.

Kinetic friction occurs when two surfaces are already sliding past each other. Kinetic friction is usually a little less than static friction between the same two surfaces. Going back to the book example, if you push the book across the table, kinetic friction comes into play. This opposes the book’s motion across the table.

Friction always acts in the opposite direction of an object’s motion. Without friction, objects would keep moving forever. Friction dissipates kinetic energy, eventually bringing moving objects to rest. This is why friction is so important – it allows us to walk without slipping, drive cars, and more. Understanding the forces of friction helps scientists and engineers design various technologies to minimize or maximize frictional effects.


Gravity is one of the fundamental forces in nature that causes objects to accelerate. According to Newton’s law of universal gravitation, every particle 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.

On Earth, gravity gives weight to physical objects and causes them to fall toward the ground when dropped. Gravity is responsible for keeping the Moon in orbit around Earth and Earth in orbit around the Sun. The gravitational pull between two objects depends on their mass and the distance between them. More massive objects exert a greater gravitational force.

Gravity is always attractive and acts along a line joining the centers of two objects. It is a long-range force that acts between objects across space. Gravity keeps planets, stars, and other celestial bodies in orbit around each other. For example, the gravity of the Sun keeps Earth and the other planets orbiting around it.

Gravity is ultimately responsible for many of the large-scale structures and motions in the universe. It drives the formation and evolution of stars, galaxies, and galaxy clusters. Understanding gravity has been key to discovering the age, size, structure, and fate of the universe.


This article discussed several key scientific principles that explain why objects keep moving. The main reasons are inertia, momentum, energy transfers, and forces acting on the objects.

Inertia, as described by Newton’s First Law, means objects at rest tend to stay at rest while objects in motion tend to stay in motion unless acted on by an outside force. Momentum relates to an object’s mass and velocity – the greater the momentum, the more it resists changes in its motion. Energy transfers, like a push or kick, can initiate or sustain an object’s movement by imparting kinetic energy. Lastly, forces like gravity, friction, and propulsion determine if an object speeds up, slows down, or changes direction.

In summary, inertia keeps objects moving, momentum resists changes in motion, energy transfers sustain movement, and forces cause acceleration. Together, these scientific concepts explain why objects continue moving rather than coming to an immediate stop. Understanding motion provides insight into the underlying laws of physics that govern our universe.

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