How much impact force?Īirplane & Duck Force Estimate For the airplane and duck force estimate, the mass of the duck is needed, but the weight in the U. This is an example of the use of impulse of force. Using the work-energy principle to evaluate the distance average gives 15.8 Newtons.Įstimate the average impact force between an airliner traveling at 600 mi/hr and a 1 pound duck whose length is 1 foot. So the time to stop the mass would be about a half second and the time average of force would be 20.1 Newtons. This system would have a natural oscillation period of 1.99 seconds. To provide a numerical example, consider a 1 kg mass traveling at 10 m/s that strikes a spring with spring constant k=10 N/m. It is clear that the two approaches to an average force are not the same, although in this physical system they are proportional. The work-energy principle may also be applied, using the fact that in stopping the ball, the kinetic energy would be converted to spring potential energy. The elastic motion would follow Hooke's law with a force constant k. To make the comparison, the nature of simple harmonic motion is used, and the time to bring the ball to rest is one-fourth of a period of the periodic motion. This provides an opportunity to evaluate the force average over time and the force average over distance. In the example illustrated above, a ball is brought to rest by impact with a spring. Such an average over distance is not the same as the time average of force, but it is nevertheless useful in a case like evaluating the wisdom of wearing your seatbelt at all times in your automobile because without it, your stopping distance in a collision will be much shorter, and the average force on you much greater. In a case like a car crash with a tree, one can measure the distance of the collision and use the work-energy principle to evaluate the average of the force over the distance. There are, however, situations in which the distance traveled in a collision is readily measured while the time of the collision is not. This average force is also an average over time. When you strike a golf ball with a club, if you can measure the momentum of the golf ball and also measure the time of impact, you can divide the momentum change by the time to get the average force of impact. Another approach that is valuable for assessing the impact force in high speed collisions is to use the impulse of force. The most straightforward way to approach the concept of average force is to multiply the constant mass times the average acceleration, and in that approach the average force is an average over time. The net external force on a constant mass object obeys Newton's second law, F net external=ma. The profile of force during a collision such as that of a car with a tree can be complex, whether expressed in terms of time or distance. Note: this approach to minimizing force during a collision uses the average of force over distance rather than the average over time. Automobiles are made to collapse upon impact, extending the time of collision and lessening the impact force.Īlternatively, the same scenario can be examined with the aid of the work-energy principle.Īn impact which stops a moving object must do enough work to take away its kinetic energy, so extending the distance moved during the collision reduces the average impact force. A boxer moves away from a punch, extending the time of impact and lessening the force.If you jump to the ground from any height, you bend your knees upon impact, extending the time of collision and lessening the impact force.This principle is applied in many common-sense situations: If an impact stops a moving object, then the change in momentum is a fixed quantity, and extending the time of the collision will decrease the time average of the impact force by the same factor. The process of minimizing an impact force can be approached from the definition of the impulse of force: HyperPhysics***** Mechanics ***** Newton's laws If the time of collision can be measured, then the average force of impact can be calculated.Īpplication in Rutherford scattering experiment For collisions, the mass and change in velocity are often readily measured, but the force during the collision is not. The main utility of the concept is in the study of the average impact force during collisions. The impulse of force can be extracted and found to be equal to the change in momentum of an object provided the mass is constant: The product of average force and the time it is exerted is called the impulse of force.
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