Impulse, a fundamental concept in Physics, refers to the change in momentum of an object when an external force is applied to it. The impulse experienced by an object is a direct result of the force applied and the duration of its application. Formally, impulse (denoted as ‘J’) is given by the equation: Impulse (J) = Force (F) x Time (Δt). This quantifiable measure provides an essential tool for understanding and predicting an object’s behavior under varying forces.

The Interplay between Impulse and Momentum

One cannot delve into the depths of impulse without understanding its close relationship with momentum. Momentum, in simple terms, is the measure of the motion of an object. It’s a vector quantity given by the product of an object’s mass and its velocity.

The connection between impulse and momentum is explicit in Newton’s second law:

  • Often expressed as Force = mass x acceleration.
  • Can be rearranged to Force = change in momentum / time, showing that the applied force is directly proportional to the rate of change in momentum.
  • Consequently, if we multiply both sides by time, we get the impulse-momentum theorem: Impulse = change in momentum.

Newton’s Second Law and Impulse

Newton’s second law of motion is the bedrock on which the concept of impulse is built. This law asserts that the force exerted on an object is equal to the change in its momentum per unit time. Mathematically, it translates into the equation: Force (F) = mass (m) x acceleration (a), where acceleration is the rate of change of velocity.

When you multiply both sides of Newton’s second law by time, you get the impulse equation: Impulse = Force x Time. This equation is significant because it underscores that impulse is not just about the magnitude of the force but also the duration over which it’s applied.

The Role of Impulse in Collisions

Impulse is of paramount importance when studying and analyzing collisions, a phenomenon frequently observed in our everyday lives and numerous scientific contexts. This might involve common scenarios like billiard balls colliding on a pool table, cars crashing on a roadway, or more intricate situations like particle interactions in a particle accelerator.

In each collision event, an impulse is applied, and it’s this impulse that is responsible for altering the momentum of the colliding objects. As stated earlier, impulse is the product of the force exerted on an object and the time duration of this force. During a collision, a force acts upon the objects involved for a certain amount of time, resulting in a change in their momentum.

This change in momentum is especially pertinent when we consider a ‘closed’ or ‘isolated’ system, where the only forces at work are the internal forces of the system, and external forces like friction, air resistance, or gravitational pull are negligible or non-existent. In such scenarios, the principle of conservation of momentum comes into play.

The conservation of momentum principle posits that the total momentum of an isolated system remains constant if no external forces act upon it. Hence, in the context of collisions, it implies that the total momentum of all objects involved before the collision equals the total momentum after the collision.

Consider a simple two-object collision. The combined momentum of the two objects before they collide is equal to their combined momentum after they collide. This holds true regardless of whether the collision is elastic (where kinetic energy is also conserved) or inelastic (where kinetic energy is not conserved).

The role of impulse in collisions becomes even more fascinating when we observe that it can significantly change the direction and speed of the colliding objects, leading to a wide array of outcomes. For instance, in an elastic collision of two identical billiard balls, one initially at rest, the moving ball stops after the collision while the initially stationary ball moves with the initial speed of the first ball – all due to the interplay of impulse and the conservation of momentum.

Impulse: A Vector Quantity

Impulse, similar to other vector quantities in physics such as force, velocity, and momentum, has both a magnitude and a direction. This dual attribute of an impulse not only quantifies the “amount” of the impulse but also the “way” it acts.

The magnitude of impulse, given by the product of the force and the time for which it is applied, essentially measures the strength or impact of the force. It tells us how ‘forceful’ or ‘influential’ the impulse is. For example, a more considerable force or a longer duration results in a larger impulse and consequently a more significant change in momentum.

The direction of impulse, on the other hand, is inherently tied to the direction of the force applied to the object. The impulse follows the path of the force. If you push an object to the right, the impulse is directed to the right. Conversely, if you pull an object toward yourself, the impulse is oriented in your direction.

It’s noteworthy that the direction of impulse plays a pivotal role in determining the final motion of the object. When the force is applied along the line of the initial motion of an object, the impulse only alters the speed of the object, not its direction. For instance, if a car moving eastward experiences a force in the eastward direction, it will only speed up but keep moving east.

However, if the force is applied in a direction not along the line of the initial motion, the impulse can indeed change the object’s direction of motion. Consider a moving billiard ball hit at an angle. The applied force generates an impulse that changes the ball’s direction of motion.

Real-life Implications of Impulse

Impulse has significant real-world implications:

  1. For instance, the effectiveness of airbags in cars is a direct application of the concept of impulse. An airbag increases the time over which the driver’s momentum is brought to zero, thereby reducing the force experienced and minimizing injury.
  2. Similarly, athletes use the concept of impulse when they “follow through” while hitting a ball, thus maximizing the time of contact and increasing the ball’s final momentum.


Impulse is a fundamental concept in physics that encapsulates the interplay between force, time, and change in momentum. It’s an integral part of understanding how objects move and interact in the world, from the macroscopic collisions of vehicles to the microscopic interactions of particles. The concept of impulse helps us predict outcomes, protect lives, and even enjoy our favorite sports.


What are the units of impulse?

The units of impulse are Newton-seconds (N.s) in the International System of Units (SI), reflecting its definition as force multiplied by time.

What is the relationship between impulse and momentum?

Impulse is equal to the change in momentum of an object. If an external force acts upon an object, it results in an impulse which changes the object’s momentum.

Can impulse change the direction of motion?

Yes, since impulse is a vector quantity, it can change an object’s direction of motion if the force applied is not along the line of the initial motion.

What is the principle of conservation of impulse?

Impulse doesn’t have a conservation law. However, momentum, which is closely related to impulse, is conserved in a closed system. This means the total momentum before a collision is equal to the total momentum after the collision.

How does impulse affect collisions?

Impulse determines the change in momentum of an object in a collision. The greater the impulse, the greater the change in momentum.

Is impulse a scalar or vector quantity?

Impulse is a vector quantity. It has both a magnitude (size) and direction. The direction of the impulse is the same as the direction of the force that causes it.

What are some real-life examples of impulse?

Real-life examples of impulse include a bat hitting a ball, a car crash (and the role of airbags in reducing injury), and an athlete jumping off a starting block.

What are Impulse and change in momentum?

Impulse is the change in an object’s momentum when a force is applied to it over a period of time. Therefore, impulse and change in momentum are equal.

What is the Importance of impulse in Physics?

Impulse is important in physics as it allows us to calculate the change in an object’s momentum. This is particularly useful in analyzing collisions and other situations where forces cause changes in motion.

Impulse is the product of the force applied to an object and the time period over which it is applied. Hence, impulse and force are directly related.

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