Momentum and Impulse

Momentum and Impulse 8.01 W06D2 Associated Reading Assignment: Young and Freedman: 8.1-8.5

Announcements: No Math Review Night this Week Next Reading Assignment W06D3: Young and Freedman: 8.1-8.5

Momentum and Impulse Obeys a conservation law Simplifies complicated motions Describes collisions Basis of rocket propulsion & space travel

Quantity of Motion DEFINITION II Newton’s Principia The quantity of motion is the measure of the same, arising from the velocity and quantity of matter conjointly. The motion of the whole is the sum of the motions of all the parts; and therefore in a body double in quantity, with equal velocity, the motion is double; with twice the velocity, it is quadruple.

Momentum and Impulse: Single Particle Momentum SI units Change in momentum Impulse SI units

Newton’s Second Law “The change of motion is proportional to the motive force impresses, and is made in the direction of the right line in which that force is impressed”, When then then

Concept Question: Pushing Identical Carts Identical constant forces push two identical objects A and B continuously from a starting line to a finish line. If A is initially at rest and B is initially moving to the right, • Object A has the larger change in momentum. • Object B has the larger change in momentum. • Both objects have the same change in momentum • Not enough information is given to decide.

Concept Question: Pushing Non-identical Carts Kinetic Energy Consider two carts, of masses m and 2m, at rest on an air track. If you push one cart for 3 s and then the other for the same length of time, exerting equal force on each, the kinetic energy of the light cart is • larger than • equal to 3) smaller than the kinetic energy of the heavy car.

Concept Question: Same Momentum, Different Masses Suppose a ping-pong ball and a bowling ball are rolling toward you. Both have the same momentum, and you exert the same force to stop each. How do the distances needed to stop them compare? It takes a shorter distance to stop the ping-pong ball. Both take the same distance. It takes a longer distance to stop the ping-pong ball.

Demo: Jumping Off the Floorwith a Non-Constant Force

Demo Jumping: Non-Constant Force • Plot of total external force vs. time for Andy jumping off the floor. Weight of Andy is 911 N.

Demo Jumping: Impulse • Shaded area represents impulse of total force acting on Andy as he jumps off the floor

Demo Jumping: Height • When Andy leaves the ground, the impulse is • So the y-component of the velocity is

System of Particles: Center of Mass

Position and Velocity of Center of Mass Total mass for discrete or continuous body (mass densityρ) Position of center of mass Velocity of center of mass

Table Problem: Center of Mass of Rod and Particle A slender uniform rod of length d and mass m rests along the x-axis on a frictionless, horizontal table. A particle of equal mass m is moving along the x-axis at a speed v0. At t = 0, the particle strikes the end of the rod and sticks to it. Find a vector expression for the position of the center of mass of the system at t = 0.

System of Particles: Internal and External Forces, Center of Mass Motion

Internal Force on a System of N Particles The total internal force on the ith particle is sum of the interaction forces with all the other particles The total internal force is the sum of the total internal force on each particle Newton’s Third Law: internal forces cancel in pairs So the total internal force is zero

Total Force on a System of N Particles is the External Force The total force on a system of particles is the sum of the total external and total internal forces. Since the total internal force is zero

External Force and Momentum Change The total momentum of a system of N particles is defined as the sum of the individual momenta of the particles Total force changes the momentum of the system Total force equals total external force Newton’s Second and Third Laws for a system of particles: The total external force is equal to the change in momentum of the system

System of Particles: Newton’s Second and Third Laws The total momentum of a system remains constant unless the system is acted on by an external force

System of Particles: Translational Motion of the Center of Mass

Translational Motion of the Center of Mass Momentum of system System can be treated as point mass located at center of mass. External force accelerates center of mass Impulse changes center of mass momentum

Demo : Center of Mass trajectory B78 http://tsgphysics.mit.edu/front/index.php?page=demo.php?letnum=B%2078&show=0 Odd-shaped objects with their centers of mass marked are thrown. The centers of mass travel in a smooth parabola. The objects consist of: a squash racket, a 16” diameter disk weighted at one point on its outer rim, and two balls connected with a rod. This demonstration is shown with UV light.

CM moves as though all external forces on the system act on the CM so the jumper’s cm follows a parabolic trajectory of a point moving in a uniform gravitational field

Concept Question: Pushing a Baseball Bat Recall Issue with Phrasing 1 3 2 The greatest acceleration of the center of mass will be produced by pushing with a force F at Position 1 2. Position 2 3. Position 3 4. All the same

Table Problem: Exploding Projectile Center of Mass Motion An instrument-carrying projectile of mass m1 accidentally explodes at the top of its trajectory. The horizontal distance between launch point and the explosion is x0. The projectile breaks into two pieces which fly apart horizontally. The larger piece, m3, has three times the mass of the smaller piece, m2. To the surprise of the scientist in charge, the smaller piece returns to earth at the launching station. • How far has the center of mass of the system traveled from the launch when the pieces hit the ground? • How far from the launch point has the larger piece graveled when it first hits the ground?

System of Particles:Conservation of Momentum

External Forces and Constancy of Momentum Vector The external force may be zero in one direction but not others The component of the system momentum is constant in the direction that the external force is zero The component of system momentum is not constant in a direction in which external force is not zero

Table Problem: Center of Mass of Rod and Particle Post- Collision A slender uniform rod of length d and mass m rests along the x-axis on a frictionless, horizontal table. A particle of equal mass m is moving along the x-axis at a speed v0. At t = 0, the particle strikes the end of the rod and sticks to it. Find a vector expression for the position of the center of mass of the system for t > 0.

Strategy: Momentum of a System 1. Choose system 2. Identify initial and final states 3. Identify any external forces in order to determine whether any component of the momentum of the system is constant or not i) If there is a non-zero total external force: ii) If the total external force is zero then momentum is constant

Modeling: Instantaneous Interactions Decide whether or not an interaction is instantaneous. External impulse changes the momentum of the system. If the collision time is approximately zero, then the change in momentum is approximately zero.

Table Problem: Landing Plane and Sandbag A light plane of mass 1000 kg makes an emergency landing on a short runway. With its engine off, it lands on the runway at a speed of 40 ms-1. A hook on the plane snags a cable attached to a sandbag of mass 120 kg and drags the sandbag along. If the coefficient of friction between the sandbag and the runway is μ = 0.4, and if the plane’s brakes give an additional retarding force of magnitude 1400 N, how far does the plane go before it comes to a stop?

Momentum and Impulse