Is it possible to have a negative impulse

You have also experienced this a multitude of times while driving. As you bring your car to a halt when approaching a stop sign or stoplight, the brakes serve to apply a force to the car for a given amount of time to change the car's momentum. An object with momentum can be stopped if a force is applied against it for a given amount of time.

A force acting for a given amount of time will change an object's momentum. Put another way, an unbalanced force always accelerates an object - either speeding it up or slowing it down.

If the force acts opposite the object's motion, it slows the object down. If a force acts in the same direction as the object's motion, then the force speeds the object up. Either way, a force will change the velocity of an object. And if the velocity of the object is changed, then the momentum of the object is changed.

These concepts are merely an outgrowth of Newton's second law as discussed in an earlier unit. This equation represents one of two primary principles to be used in the analysis of collisions during this unit. To truly understand the equation, it is important to understand its meaning in words. In words, it could be said that the force times the time equals the mass times the change in velocity. The equation really says that the. In a collision, objects experience an impulse; the impulse causes and is equal to the change in momentum.

Consider a football halfback running down the football field and encountering a collision with a defensive back. The collision would change the halfback's speed and thus his momentum. If the motion was represented by a ticker tape diagram , it might appear as follows:. At approximately the tenth dot on the diagram, the collision occurs and lasts for a certain amount of time; in terms of dots, the collision lasts for a time equivalent to approximately nine dots.

In the halfback-defensive back collision, the halfback experiences a force that lasts for a certain amount of time to change his momentum. Since the collision causes the rightward-moving halfback to slow down, the force on the halfback must have been directed leftward. If the halfback experienced a force of N for 0. In a collision, the impulse experienced by an object is always equal to the momentum change.

Now consider a collision of a tennis ball with a wall. Depending on the physical properties of the ball and wall, the speed at which the ball rebounds from the wall upon colliding with it will vary. The diagrams below depict the changes in velocity of the same ball. For each representation vector diagram, velocity-time graph, and ticker tape pattern , indicate which case A or B has the greatest change in velocity, greatest acceleration , greatest momentum change, and greatest impulse.

Support each answer. Click the button to check your answer. See Answer a. The velocity change is greatest in case B. The acceleration is greatest in case B.

Acceleration depends on velocity change and the velocity change is greatest in case B as stated above. The momentum change is greatest in case B. Momentum change depends on velocity change and the velocity change is greatest in case B as stated above. The impulse is greatest in case B. Impulse equals momentum change and the momentum change is greatest in case B as stated above. The velocity change is greatest in case A. The acceleration is greatest in case A. They recognize that the sad ball goes from 1 Ns to 0 Ns, so changes its momentum by 1 Ns.

That's easy. The happy ball also starts the collision by coming to rest, losing 1 Ns of momentum. But then, the happy ball has to rebound, too. It has to not only lose its original 1 Ns to stop, but it also has to gain 1 Ns in order to get back up to speed. So its total momentum change is 2 Ns : 1 Ns to stop, 1 Ns to rebound.

Explained this way and then practiced a few times , the students are entirely comfortable with this conceptual approach. The follow up question is to make sure they understand why the happy ball doesn't have zero momentum change -- after all, the happy ball had the same amount of momentum before and after collision! But they quickly recognize that "zero momentum change" means the object never collided with anything.

Rebounding takes more momentum change than just stopping. A professional boxer hits his opponent with a N horizontal blow that lasts for 0. Suppose a child drives a bumper car head on into the side rail, which exerts a force of N on the car for 0. You may neglect friction between the car and floor.

One hazard of space travel is debris left by previous missions. There are several thousand objects orbiting Earth that are large enough to be detected by radar, but there are far greater numbers of very small objects, such as flakes of paint. Calculate the force exerted by a 0. Military rifles have a mechanism for reducing the recoil forces of the gun on the person firing it.

An internal part recoils over a relatively large distance and is stopped by damping mechanisms in the gun. The larger distance reduces the average force needed to stop the internal part. A cruise ship with a mass of 1. It comes to rest 6. Calculate the average force exerted on the pier using the concept of impulse.

Hint: First calculate the time it took to bring the ship to rest. Calculate the final speed of a kg rugby player who is initially running at 8. Water from a fire hose is directed horizontally against a wall at a rate of Starting with the definitions of momentum and kinetic energy, derive an equation for the kinetic energy of a particle expressed as a function of its momentum.

What is the impulse delivered by the wall? When serving a tennis ball, a player hits the ball when its velocity is zero at the highest point of a vertical toss. The racquet exerts a force of N on the ball for 5. Using these data, find the mass of the ball.

A punter drops a ball from rest vertically 1 meter down onto his foot. What is the impulse delivered by the foot magnitude and direction? Licenses and Attributions. CC licensed content, Shared previously. Similarly, a padded dashboard increases the time over which the force of impact acts, thereby reducing the force of impact.

Cars today have many plastic components. One advantage of plastics is their lighter weight, which results in better gas mileage. Another advantage is that a car will crumple in a collision , especially in the event of a head-on collision. A longer collision time means the force on the occupants of the car will be less.

Deaths during car races decreased dramatically when the rigid frames of racing cars were replaced with parts that could crumple or collapse in the event of an accident. You may have heard the advice to bend your knees when jumping.

In this example, a friend dares you to jump off of a park bench onto the ground without bending your knees. You, of course, refuse.

Explain to your friend why this would be a foolish thing. Show it using the impulse-momentum theorem. No information is given about the direction of the football player or the football, so we can calculate only the magnitude of the momentum, p. A symbol in italics represents magnitude. In both parts of this example, the magnitude of momentum can be calculated directly from the definition of momentum:.

Although the ball has greater velocity, the player has a much greater mass. Therefore, the momentum of the player is about 86 times greater than the momentum of the football.