# Tuesday, November 6, 2007 Momentum and Momentum Change

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Tuesday, November 6, 2007 Momentum and Momentum Change

Which do you think has more momentum?

Momentum Momentum is a measure of how hard it is to stop or turn a moving object. What characteristics of an object would make it hard to stop or turn?

Sample Problem Calculate the momentum of a 65-kg sprinter running east at 10 m/s.

Sample Problem Calculate the momentum of a system composed of a 65-kg sprinter running east at 10 m/s and a 75-kg sprinter running north at 9.5 m/s.

Change in momentum Like any change, change in momentum is calculated by looking at final and initial momentums. p pf – pi – p: change in momentum – pf: final momentum – pi: initial momentum

Momentum change demonstration Using only a meter stick, find the momentum change of each ball when it strikes the desk from a height of exactly one meter. Which ball, Bouncy or Lazy, has the greatest change in momentum?

Wording dilemma In which case is the magnitude of the momentum change greatest? In which case is the change in the magnitude of the momentum greatest?

Wednesday, November 7, 2007 Impulse

Impulse (J) Impulse is the product of an external force and time, which results in a change in momentum of a particle or system. J F t and J P Therefore Ft P Units: N s or kg m/s (same as momentum)

Impulsive Forces Usually high magnitude, short duration. Suppose the ball hits the bat at 90 mph and leaves the bat at 90 mph, what is the magnitude of the momentum change? What is the change in the magnitude of the momentum?

Impulse (J) on a graph F(N) 3000 2000 area under curve 1000 0 0 1 2 3 4 t (ms)

Sample Problem Suppose a 1.5-kg brick is dropped on a glass table top from a height of 20 cm. a) What is the magnitude and direction of the impulse necessary to stop the brick? b) If the table top doesn’t shatter, and stops the brick in 0.01 s, what is the average force it exerts on the brick? c) What is the average force that the brick exerts on the table top during this period?

Sample Problem F(N) 2,000 1,000 0.20 0.40 0.60 0.80 t(s) This force acts on a 1.2 kg object moving at 120.0 m/s. The direction of the force is aligned with the velocity. What is the new velocity of the object?

Wednesday, November 22, 2006 Law of Conservation of Momentum

Law of Conservation of Momentum If the resultant external force on a system is zero, then the vector sum of the momentums of the objects will remain constant. Pbefore Pafter

Sample problem A 75-kg man sits in the back of a 120-kg canoe that is at rest in a still pond. If the man begins to move forward in the canoe at 0.50 m/s relative to the shore, what happens to the canoe?

External versus internal forces External forces: forces coming from outside the system of particles whose momentum is being considered. – External forces change the momentum of the system. Internal forces: forces arising from interaction of particles within a system. – Internal forces cannot change momentum of the system.

An external force in golf The club head exerts an external impulsive force on the ball and changes its momentum. The acceleration of the ball is greater because its mass is smaller. The System

An internal force in pool The forces the balls exert on each other are internal and do not change the momentum of the system. Since the balls have equal masses, the magnitude of their accelerations is equal. The System

Explosions When an object separates suddenly, as in an explosion, all forces are internal. Momentum is therefore conserved in an explosion. There is also an increase in kinetic energy in an explosion. This comes from a potential energy decrease due to chemical combustion.

Recoil Guns and cannons “recoil” when fired. This means the gun or cannon must move backward as it propels the projectile forward. The recoil is the result of action-reaction force pairs, and is entirely due to internal forces. As the gases from the gunpowder explosion expand, they push the projectile forwards and the gun or cannon backwards.

Sample problem Suppose a 5.0-kg projectile launcher shoots a 209 gram projectile at 350 m/s. What is the recoil velocity of the projectile launcher?

Sample Problem An exploding object breaks into three fragments. A 2.0 kg fragment travels north at 200 m/s. A 4.0 kg fragment travels east at 100 m/s. The third fragment has mass 3.0 kg. What is the magnitude and direction of its velocity?

Friday, November 9, 2006 Inelastic Collisions

Collisions When two moving objects make contact with each other, they undergo a collision. Conservation of momentum is used to analyze all collisions. Newton’s Third Law is also useful. It tells us that the force exerted by body A on body B in a collision is equal and opposite to the force exerted on body B by body A.

Collisions During a collision, external forces are ignored. The time frame of the collision is very short. The forces are impulsive forces (high force, short duration).

Collision Types Elastic collisions – Also called “hard” collisions – No deformation occurs, no kinetic energy lost Inelastic collisions – Deformation occurs, kinetic energy is lost Perfectly Inelastic (stick together) – Objects stick together and become one object – Deformation occurs, kinetic energy is lost

(Perfectly) Inelastic Collisions Simplest type of collisions. After the collision, there is only one velocity, since there is only one object. Kinetic energy is lost. Explosions are the reverse of perfectly inelastic collisions in which kinetic energy is gained!

Sample Problem An 80-kg roller skating grandma collides inelastically with a 40-kg kid. What is their velocity after the collision? How much kinetic energy is lost?

Sample Problem A fish moving at 2 m/s swallows a stationary fish which is 1/3 its mass. What is the velocity of the big fish after dinner?

Sample problem A car with a mass of 950 kg and a speed of 16 m/s to the east approaches an intersection. A 1300-kg minivan traveling north at 21 m/s approaches the same intersection. The vehicles collide and stick together. What is the resulting velocity of the vehicles after the collision?

Monday, November 12, 2007 Elastic Collisions

Elastic Collision In elastic collisions, there is no deformation of colliding objects, and no change in kinetic energy of the system. Therefore, two basic equations must hold for all elastic collisions pb pa (momentum conservation) Kb Ka (kinetic energy conservation)

Sample Problem A 500-g cart moving at 2.0 m/s on an air track elastically strikes a 1,000-g cart at rest. What are the resulting velocities of the two carts?

Sample Problem Suppose three equally strong, equally massive astronauts decide to play a game as follows: The first astronaut throws the second astronaut towards the third astronaut and the game begins. Describe the motion of the astronauts as the game proceeds. Assume each toss results from the same-sized "push." How long will the game last?

2D-Collisions Momentum in the x-direction is conserved. – Px (before) Px (after) Momentum in the y-direction is conserved. – Py (before) Py (after) Treat x and y coordinates independently. – Ignore x when calculating y – Ignore y when calculating x Let’s look at a simulation: – http://surendranath.tripod.com/Applets.html

Sample problem Calculate velocity of 8-kg ball after the collision. 2 m/s y 2 kg y 3 m/s 50o x 2 kg x 8 kg 0 m/s 8 kg v Before After

Tuesday, November 13, 2007 Review of Momentum