study for momentum

• Momentum


  Inertia in motion - the mass of an object multiplied by its velocity
• Momentum is conserved for all collisions as long as external forces don't interfere
• How a Collision Affects the Motion of Marbles


  1. Place five marbles in the center groove of a ruler

  2. Launch a sixth marble toward the five stationary marbles

  3. Note any changes in the marbles' motion

  4. Launch two marbles at four stationary marbles

  5. Launch three marbles at three stationary marbles

  6. Remove all but two marbles from the groove

  7. Roll these two marbles at each other with equal speeds

  8. Note any changes in the marbles' motion
• Impulse


  The force acting on an object multiplied by the time over which it acts
• Impulse


  Changes momentum
• The greater the impulse exerted on something, the greater will be the change in momentum</b>
• Impulse


  Equals change in momentum (Ft = Δ(mv))
• Increasing Momentum


  • Apply the greatest force possible for as long as possible (e.g. golf swing, baseball swing)
• Decreasing Momentum


  • Extend the time of contact to reduce the force of impact (e.g. hitting a haystack vs. concrete wall, catching a ball)
• When jumping from an elevated position, bending knees on landing extends the time of momentum decrease and reduces the force
• Momentum transfer in a train

  1. Enough friction to set one car in motion

  2. Slack allows locomotive to get one car going

  3. When coupling is tight, next car is set in motion

  4. When coupling for two cars is tight, third car is set in motion

  5. Whole train is given momentum
• Falling on a floor with more give

  Less dangerous than falling on a floor with less give
• Reason why falling on a floor with more give is less dangerous

  • The floor with more give allows a greater time for the impulse that reduces the momentum of fall to zero
  • A greater time for a change in momentum results in less force
• Reducing force when jumping from an elevated position

  1. Bend knees when feet make contact with ground

  2. Extends time during which momentum decreases

  3. Reduces force on bones by 10 to 20 times
• Reducing force when a wrestler is thrown to the floor

  1. Relaxes muscles

  2. Spreads the impulse into a series of smaller ones as different body parts hit the mat
• Boxer moves away from punch

  Increases time of contact and reduces force
• Boxer moves toward punch

  Reduces time of contact and increases force
• If the boxer can make the contact time five times longer by "riding" with the punch, the force of the punch impact will be reduced
• Whether body A acts on body B, or body B acts on body A, in accordance with Newton's third law, both have the same amount of impulse Ft
• Bungee jumping


  1. Rubber cord stretches when jumper's fall is brought to a halt

  2. Cord has to apply an impulse equal to the jumper's momentum in order to stop the jumper

  3. Momentum, mv, is the amount gained before the cord begins stretching

  4. Ft is the impulse the cord supplies to reduce the momentum to zero

  5. Because the rubber cord stretches for a long time, a large time interval t ensures that a small average force F acts on the jumper

  6. Elastic cords typically stretch to twice their original length during the fall
• The change in momentum depends on the force that acts and the length of time it acts
• Flower pot bounces quickly from head

  If bouncing took a longer time, the force of the bounce would be much smaller
• The impulse required to bring an object to a stop and then to "throw it back again" is greater than the impulse required merely to bring the object to a stop
• Pelton Wheel design


  1. Curve-shaped paddle caused incoming water to make a U-turn upon impact

  2. Because the water "bounced", the impulse exerted on the waterwheel was increased
• • To change the momentum of an object, an outside push or pull is required
  • If no outside force is present, no change in momentum is possible
• • The force on the cannonball inside the cannon barrel is equal and opposite to the force causing the cannon to recoil
  • These forces are internal to the system comprising the cannon and the cannonball, so they don't change the momentum of the cannon–cannonball system
  • Before the firing, the system is at rest and the momentum is zero
  • After the firing the net momentum, or total momentum, is still zero
• Momentum, like the quantities velocity and force, has both direction and magnitude. It is a vector quantity.
• Momentum can be canceled. The momenta (plural form of momentum) of the cannonball and the cannon are equal in magnitude and opposite in direction. Therefore, these momenta cancel each other out for the system as a whole.
• Law of conservation of momentum

  In the absence of an external force, the momentum of a system remains unchanged.
• If a system undergoes changes wherein all forces are internal as for example in atomic nuclei undergoing radioactive decay, cars colliding, or stars exploding, the net momentum of the system before and after the event is the same.
• Whenever objects collide in the absence of external forces, the net momentum of both objects before the collision equals the net momentum of both objects after the collision.
• The vector sum of the momenta is the same before and after a collision.
• Momentum conservation holds true even in inelastic collisions. Whenever colliding objects become tangled or couple together, a totally inelastic collision occurs.
• Inelastic collision


  Colliding objects become distorted and generate heat during the collision
• One of the gliders is loaded so it has three times the mass of the other glider. The loaded glider is initially at rest. The unloaded glider collides with the loaded glider and the two gliders stick together.


  The motion of the gliders after the collision is that the combined gliders move with a velocity that is one-third the velocity of the unloaded glider before the collision.
• Perfectly elastic collisions are not common in the everyday world. We find in practice that some heat is generated during collisions.
• At the microscopic level, perfectly elastic collisions are commonplace. For example, electrically charged particles bounce off one another without generating heat; they don't even touch in the classic sense of the word.
• Momentum


  A moving object can have a large momentum if it has a large mass, a high speed, or both
• Impulse


  The change in momentum depends on the force that acts and the length of time it acts
• Law of conservation of momentum

  In the absence of an external force, the momentum of a system remains unchanged