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Momentum and Impulse

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Momentum and Impulse

Rachel Flatt performs a layback spin at the 2 011 Rostelecom Cup in Moscow, Russia.

When an ice skater spins, angular momentum must be conserved. When her arms or feet are far away from her body, her spin slows; when she brings her arms and feet close in to her body, she spins faster.

Momentum and Impulse

If a bowling ball and a ping-pong ball are each moving with a velocity of 5 mph, you intuitively understand that it will require more effort to stop the bowling ball than the ping pong ball because of the greater mass of the bowling ball.  Similarly, if you have two bowling balls, one moving at 5 mph and the other moving at 10 mph, you know it will take more effort to stop the ball with the greater speed. It is clear that both the mass and the velocity of a moving object contribute to what is necessary to change the motion of the moving object. The product of the mass and velocity of an object is called its  momentum Momentum is a vector quantity that has the same direction as the velocity of the object and is represented by a lowercase letter p .

p = mv

The momentum of a 0.500 kg ball moving with a velocity of 15.0 m/s will be

p=mv=(0.500 \ \text{kg})(15.0 \ \text{m/s})=7.50 \ \text{kg} \cdot \text{m/s}

You should note that the units for momentum are kg·m/s.

According to Newton’s first law, the velocity of an object cannot change unless a force is applied. If we wish to change the momentum of a body, we must apply a force. The longer the force is applied, the greater the change in momentum. The impulse  is the quantity defined as the force multiplied by the time it is applied. It is a vector quantity that has the same direction as the force.  The units for impulse are N·s but we know that Newtons are also kg·m/s 2 and so N·s = (kg·m/s 2 )(s) = kg·m/s. Impulse and momentum have the same units; when an impulse is applied to an object, the momentum of the object changes and the change of momentum is equal to the impulse.

Ft = \Delta mv

Example Problem: Calculating Momentum

A 0.15 kg ball is moving with a velocity of 35 m/s.  Find the momentum of the ball.

Solution: p = mv = (0.15 \ \text{kg})(35 \ \text{m/s}) = 5.25 \ \text{kg} \cdot \text{m/s}  

Example Problem: If a ball with mass 5.00 kg has a momentum of 5.25 \ \text{kg} \cdot \text{m/s} , what is its velocity?

Solution: v=\frac{p}{m}=\frac{5.25 \ \text{kg} \cdot \text{m/s}}{5.00 \ \text{kg}}=1.05 \ \text{m/s}

It should be clear from the equation relating impulse to change in momentum, Ft = \Delta mv , that any amount of force would (eventually) bring a moving object to rest. If the force is very small, it must be applied for a long time, but a greater force can bring the object to rest in a shorter period of time. 

If you jump off a porch and land on your feet with your knees locked in the straight position, your motion would be brought to rest in a very short period of time and thus the force would need to be very large – large enough, perhaps, to damage your joints or bones.

Suppose that when you hit the ground, your velocity was 7.0 m/s and that velocity was brought to rest in 0.05 seconds.  If your mass is 100. kg, what force was required to bring you to rest?

F=\frac{\Delta mv}{t}=\frac{(100. \ \text{kg})(7.0 \ \text{m/s})}{0.050 \ \text{s}}=14,000 \ \text{N}

If, on the other hand, when your feet first touched the ground, you allowed your knees to flex so that the period of time over which your body was brought to rest is increased, then the force on your body would be smaller and it would be less likely that you would damage your legs.

Suppose that when you first touch the ground, you allow your knees to bend and extend the stopping time to 0.50 seconds. What force would be required to bring you to rest this time?

F=\frac{\Delta mv}{t}=\frac{(100. \ \text{kg})(7.0 \ \text{m/s})}{0.50 \ \text{s}}=1400 \ \text{N}

With the longer period of time for the force to act, the necessary force is reduced to one-tenth of what was needed before.

Extending the period of time over which a force acts in order to lessen the force is a common practice in design. Padding in shoes and seats allows the time to increase. The front of automobiles are designed to crumple in an accident; this increases the time the car takes to stop. Similarly, barrels of water or sand in front of abutments on the highway and airbags serve to slow down the stoppage time. These changes all serve to decrease the amount of force it takes to stop the momentum in a car crash, which consequently saves lives.

Example Problem: An 0.15 kg baseball is thrown horizontally at 40. m/s and after it is struck by a bat, it is traveling at -40. m/s.

(a) What impulse did the bat deliver to the ball?

(b) If the contact time of the bat and bat was 0.00080 seconds, what was the average force the bat exerted on the ball?

(c) Calculate the average acceleration of the ball during the time it was in contact with the bat.

Solution:  We can calculate the change in momentum and give the answer as impulse because we know that the impulse is equal to the change in momentum.

(a)

  p=m \Delta v&=(0.15 \ \text{kg})(-40. \ \text{m/s}-40. \ \text{m/s})\\&=(0.15 \ \text{kg})(-80. \ \text{m/s})=-12 \ \text{kg} \cdot \text{m/s}

The minus sign indicates that the impulse was in the opposite direction of the original throw.

(b) F=\frac{\Delta mv}{t}=\frac{-12 \ \text{kg} \cdot \text{m/s}}{0.00080 \ \text{s}}=-15000 \ \text{N}

Again, the negative sign indicates the force was in the opposite direction of the original throw.

(c) a=\frac{F}{m}=\frac{-15000 \ \text{N}}{0.15 \ \text{kg}}=-100,000 \ \text{m/s}^2

Summary

  • The product of the mass and velocity of an object is called momentum, given by the equation  \rho = mv
  • Momentum is a vector quantity that has the same direction as the velocity of the object.
  • The quantity of force multiplied by the time it is applied is called impulse. 
  • Impulse is a vector quantity that has the same direction as the force. 
  • Momentum and impulse have the same units: kg·m/s.
  • The change of momentum of an object is equal to the impulse. Ft = \Delta mv

Practice

Use this resource to answer the question that follows.

https://www.youtube.com/watch?v=3g4v8x7xggU

  1. Why don't the glasses of water spill when the tablecloth is pulled out from under them?
  2. How does the video get from momentum to impulse?

Review

  1. A small car with a mass of 800. kg is moving with a velocity of 27.8 m/s.
    1. What is the momentum of the car?
    2. What velocity is needed for a 2400. kg car in order to have the same momentum?
  2. A scooter has a mass of 250. kg.  A constant force is exerted on it for 60.0 s.  During the time the force is exerted, the scooter increases its speed from 6.00 m/s to 28.0 m/s.
    1. What is the change in momentum?
    2. What is the magnitude of the force exerted on the scooter?
  3. The brakes on a 15,680 N car exert a stopping force of 640. N.  The car’s velocity changes from 20.0 m/s to 0 m/s.
    1. What is the car’s mass?
    2. What was its initial momentum?
    3. What was the change in momentum for the car?
    4. How long does it take the braking force to bring the car to rest?

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