12 Collisions: Impulse and Momentum

12 Collisions: Impulse and Momentum

• The techniques of impulse and momentum can be used to describe or predict the result of a collision.
• The total momentum of all objects is the same before and after a collision.

• There is movement in the direction of it.

• The net impulse on an object is the same as the change in the object's momentum.

• Before you treat a set of objects as a system, you need to define the system you are considering.

• The techniques of impulse and momentum are useful when you see a col lision.
• If you want to use force or energy, try impulse and momentum first.

• A teacher with a weight of 900 N jumps from a platform scale.
• The preceding figure shows the scale reading as a function of time.

• A force versus time graph is an invitation to calculate impulse.

• The scale reading on the vertical axis is not the net force, it is the scale reading in excess of the person's weight.

• The areas above and below the zero net force line are canceled out because they are close to the same area.

• You need to be comfortable with this rough approximation.
• The reasoning behind your calculation will be more important than the answer itself on a free-response item.

• The teacher changed his momentum by 220 N[?
• ]s since impulse is the change in an object's momentum.
• After leaving the scale, his momentum is 220 N[?]s.
• Mass times speed is what momentum is.
• His speed is close to 2.5 m/s.

• Cart B is at rest.

• His weight is 900 N and on Earth it is 10 N.

• Calculating the speed of one or both objects after a collision is a common task.
• It's a good idea to calculate speeds after a collision, even if the question is qualitative or conceptual.

• Then write the equation.

• The definition of "conservation" means an unchanging quantity.
• The system of the two carts has to have a zero change in momentum.
• Since the table above has values and units, it's clear what units are intended, so I'm going to leave off the units.

• It's only one equation with two variables.
• The information about this collision is incomplete.

• There is a chance that the carts stick together.

• The problem statement tells us the speed of one of the carts after the collision.

• The track wasn't considered part of the system and it's applying a force to the carts if the track is significant.

• If Cart A rebounded after the collision, the only tricky part would be.

• A negative value would be A'.
• The solution would be the same.

• Two identical balls are dropped from the same height above the ground, so that they are traveling 50 cm/s just before they hit the ground.

• Ball A has a speed of 50 cm/s and Ball B has a speed of 10 cm/s.

• Each is in contact with the ground for the same amount of time.

• The system should be defined here.
• The problem says that Ball A rebounded, which means it changed direction.

• This is not correct.
• An object that changes direction gains some more.

• If we take the system of the Earth and Ball A into account, then momentum is conserved.
• The change in Ball A's momentum is the same as the change in the Earth's.
• The Earth is so large that it won't change its speed.

• The system of objects has the same total momentum.
• The total momentum can't change in a single collision.
• Balls A and B are involved in two separate accidents.
• When Balls A and B are involved in separate crashes, don't use "conservation of momentum" as a reason for anything to be equal.

• The impulse-momentum theorem can be used to find out what you can do when a ball collides with the Earth.

• That would be Ball A.
• Since Ball A rebounded faster and the balls have the same mass, this is not an effective approach to consider.

• If you want to calculate the total momentum change for each ball, you can use a mass of 2 kg for each ball and a plus and minus sign for the direction of velocity.
• If the words are confusing you, that's not a bad approach.

• The bigger force is exerted by the ball with the greater momentum change, that's Ball A.

• Similar reasoning can help explain why a car is safer.
• You lose all your momen tum in a crash regardless of how you rest.
• The time of the collision between you and the car can be extended by the use of the Airbags.

• Before and after a collision, the total energy of both objects is the same.
• Add up the energy of the two objects before and after the collision.
• The collision is elastic if the energy is essentially the same.
• After colliding, the carts bounce off each other, each regaining 30 cm/s of speed, but now moving in the opposite direction.

• Don't write anything about energy.
• The first thing to do is conserve of momentum.

• You don't have to do calculations to show it.
• In a collision, the only forces acting on the carts are the carts themselves.

• To check, you have to do the calculation.

• The carts flew off each other.

• The collision can't be elastic when carts stick together.
• The collision might be elastic or not.

• In this case, make up a mass for each cart and call them 1 kilo each.
• The speed of each cart is 0.30 m/s, so the energy of each cart before the collision is 1/2.
• The combined energy before the collision is 0.090 J.

• When the objects are initially at rest and then blown apart, you'll most often see this type of collision.

• One cart was moving right and the other was moving left.

• The energy is not directionless.
• Positive energy can never take on a negative value.
• The total energy of each object should be added to the system.
• The calculation is the same after the collision.
• The collision is elastic.

• She collides with a penguin.
• The following figure shows the directions of the penguin's andMaggie's motion after the collision.

• Quantitative analysis of a two-dimensional collision is not likely to be asked of you.

• To answer simple qualitative questions, you need to be able to explain how you would carry out the analysis.

• Momentum is also zero.
• To subtract to zero, their momentums must be equal and opposite.

• Of course it is.
• Momentum is also 125 N. The cosine of 30deg is hermomentum.
• The only way to get values for these components is to do a complicated algebra, which is beyond the scope of AP physics 1.
• You should be able to explain everything about this collision in words.

• The center of mass obeys the second law.

• Imagine if an astronauts on a spacewalk throws a rope around a small asteroid and pulls it towards him.

• The center of mass has no acceleration since no forces acted except for the astronauts and asteroid.
• All the way until the objects collide, the center of mass started at rest.

• A toy rocket is on its way to land 30 m from its launch point.
• The rocket explodes into two pieces, one of which lands 35 m from the launch point.

• The center of mass on the rocket must stay in motion and land 30 m from the launch point.
• If one piece is 5 m beyond the center of mass's landing point, the other must be 5 m short.

• The location of the center of mass is obvious.
• If one mass is heavier than the others, the cm is closer to the heavier mass.

• The coconut hit 10,000 N. The person rests on their head.

• The change in momentum leads to a smaller force on the person's head.

• The only forces in this place are the car and time on the mosquito and mosquito on the car.

• The mosquito went from rest to moving free after it hit the car.
• The mass of all objects is the same.
• The mosquito's speed increased.

• The car had 300 kJ of energy, and the car-mosquito B had 375 kJ, for a total of 675 kJ before the system doesn't change.

• The car's speed will serve in a collision regardless of whether the hardly change or not.
• You were aware that a collision is elastic.

• This is because the total momentum is not changing.

• Car B must be lost if it moves 1 m/s.

• The force of the mos Car A is found to be moving 25 m/s after the quito on the car is equal to the force of the car collision.

• The B + 1,500 kilogram)(5 m/s) to the left is lost.

• Let's go in a positive direction.
• The collision is elastic.

• The B needs to be conserved to get a large amount of food.
• Car B was going in the opposite direction.
• Car A is the only moving car and it has more momentum than Car B.
• Car A was moving 30 m/s because it was going mass.

• The collision is elastic.

• The total is not -75 kJ.
• The energy can't have a direction and can't be negative.
• The total energy of a system is the sum of all the energy of the objects, regardless of how they are moving.

• The only forces acting between objects in a system are called momen tum.

• The center of mass obeys the second law.

• When objects collide, the impulse-momentum theorem is most useful.