Short Answer:
Impact is defined as the sudden collision between two bodies for a very short time, during which large forces act on them and change their velocities or directions of motion. The effect of impact depends on the nature of the bodies and the type of collision.
There are mainly two types of impact based on the direction of motion — Direct Impact and Oblique Impact. Based on the elasticity of bodies, impacts are further classified as Elastic Impact, Inelastic Impact, and Perfectly Inelastic Impact. The study of impact helps engineers analyze collisions, design safe machines, and control energy transfer during motion.
Detailed Explanation :
Impact and Types of Impact
Definition of Impact:
Impact is a phenomenon in which two bodies collide with each other for a very short duration of time, resulting in a large force of interaction between them. During impact, the velocity of the bodies changes abruptly. The collision may be between moving bodies or between a moving and a stationary body.
The impact force acts only for a short duration, but it produces a noticeable change in the momentum of the colliding bodies. Hence, the study of impact is closely related to the principle of conservation of momentum and the coefficient of restitution (which measures elasticity of collision).
In mechanical engineering, the concept of impact is used to study collisions in machinery, vehicles, hammers, gears, and other moving systems, where forces act suddenly and significantly.
Basic Concepts of Impact
When two bodies collide, the forces developed between them are very large and act only for a small time interval. During this time, the external forces (like gravity or friction) can be neglected compared to the impact forces.
According to the law of conservation of momentum, the total momentum of the system before and after impact remains constant, provided no external forces act on the bodies.
Mathematically,
where,
= masses of the two bodies,
= initial velocities,
= final velocities after impact.
The coefficient of restitution (e) is defined as the ratio of relative velocity after impact to the relative velocity before impact:
Its value lies between 0 and 1, which determines how elastic or inelastic the collision is.
Types of Impact
The impact between two bodies can be classified based on direction of motion and elastic properties of the materials.
- Based on Direction of Motion
(a) Direct Impact:
When the line of motion of the two colliding bodies coincides (i.e., they move along the same straight line before and after collision), the impact is called direct impact.
In this case, both the velocity and the force act along the same line.
Example:
- A ball dropped vertically on the floor.
- Two vehicles moving along the same straight path and colliding head-on.
In direct impact, the bodies either move in the same or opposite directions after collision, depending on their masses and the coefficient of restitution.
(b) Oblique Impact:
When the direction of motion of colliding bodies is not along the same straight line, the impact is called oblique impact.
Here, the motion has components both along and perpendicular to the line of impact.
Example:
- A ball hitting the floor at an angle and bouncing off.
- A bat striking a ball at an angle in cricket or baseball.
In oblique impact, analysis is done by resolving velocities into components along and perpendicular to the line of impact. Only the velocity component along the line of impact changes after collision, while the perpendicular component remains the same.
- Based on Elasticity of Bodies
(a) Elastic Impact:
If the bodies regain their original shape and size after impact and there is no loss of kinetic energy, the impact is called perfectly elastic impact.
Here, the coefficient of restitution .
Example:
- Collision between steel or glass balls (used in physics experiments).
In perfectly elastic impact, both momentum and kinetic energy are conserved.
(b) Inelastic Impact:
If the bodies do not regain their original shape completely after impact and there is some loss of kinetic energy due to deformation or heat, the impact is called inelastic impact.
Here, .
Example:
- Collision between clay balls or a rubber ball on the ground.
Momentum remains conserved, but kinetic energy decreases due to internal deformation.
(c) Perfectly Inelastic Impact:
If the two bodies stick together and move as a single body after the collision, the impact is called a perfectly inelastic impact.
Here, .
Example:
- A lump of clay hitting another and sticking to it.
- A bullet embedding itself into a wooden block.
In a perfectly inelastic impact, maximum loss of kinetic energy occurs, although momentum remains conserved.
Mathematical Relation in Impact
For all types of impact, two fundamental equations are used:
- Law of Conservation of Momentum:
- Coefficient of Restitution:
By solving these two equations simultaneously, the final velocities and of both bodies after impact can be determined.
Applications in Engineering
The study of impact is essential in mechanical and civil engineering because many systems experience collision or sudden force during operation. Some key applications include:
- Design of shock absorbers and bumpers in vehicles to reduce damage during collisions.
- Impact analysis in gears and cams to avoid failure due to sudden forces.
- Ballistics and projectile motion in defense and aerospace engineering.
- Material testing machines that determine toughness and resistance to impact.
- Energy transfer analysis in machines such as hammers and presses.
Conclusion
Impact is the sudden collision between two bodies for a very short time, resulting in large forces and a change in their velocities. It is classified mainly as direct or oblique, and further as elastic, inelastic, and perfectly inelastic based on material behavior. The study of impact is fundamental in engineering mechanics for analyzing motion, energy loss, and force transmission during collisions, helping engineers design safer and more efficient machines and structures.