Impulse

Impulse is the change in momentum of an object, calculated as the force applied multiplied by the time interval over which it acts. The direction of impulse is always the same as the direction of the force acting on the object.

How force and time relate to impulse

Momentum is the product of an object's mass and its velocity. Force equals the change in momentum divided by time, which means multiplying force by time gives the total change in momentum, or impulse. A given change in momentum can be achieved either by applying a large force over a very short time, or by applying a much smaller force over a longer time.

The egg and impact time

• A 0.2-kilogram egg traveling at 5 meters per second has a momentum of 1 kilogram meter per second.
• Bringing the egg to a stop requires an impulse of minus 1 kilogram meter per second, whether it hits a stiff board or a hanging cloth.
• Hitting the board stops the egg in only 0.04 seconds, producing a force of 25 newtons that breaks the shell.
• Hitting the cloth extends the impact time to 1.25 seconds, reducing the force to about 0.8 newtons so the egg does not break.

Catching a ball safely

When catching a fast-moving cricket ball, holding the hands completely stiff acts like a solid board, forcing the ball to stop in an instant and delivering a large, painful force. Pulling the hands backward along the path of the ball increases the impact time. This allows the ball's momentum to decrease gradually, significantly reducing the impact force on the hands.

Real-world applications

• Car safety designs increase impact time during accidents to prevent major injuries to passengers.
• Bike suspension systems extend the time over which bumps and potholes deliver force, making rides smoother.
• Trampolines, nets, and landing mats protect gymnasts and high jumpers by allowing their momentum to decrease slowly.