Chapter 6 - Momentum and Collisions6.1 - Momentum and Impulse

ReviewEnergy cannot be created or

destroyed, but only changed from one form to another

Newton’s 3 lawsKinetic energy is the energy of

motion: KE=1/2 mv2

Momentum and ImpulseMomentum is a vector quantity

defined as the product of an object’s mass and velocity

SI units are kilogram-meters per second

p =mv

kg ⋅m/s

Momentum and ImpulseA change in momentum takes force

and time

Impulse-Momentum TheoremFΔt=Δp

or

FΔt=Δp=mvf −mvi

Momentum and ImpulseExplains why follow-through is

important in many sportsImpulse is the product of the force

and the time over which it acts on an object

Determines stopping times and distances

Momentum and ImpulseA change in momentum over a longer

time requires less forceExample

The egg fall

HW AssignmentPage 209: Practice 6APage 211: Practice 6BPage 213: Practice 6C

Chapter 6 - Momentum and Collisions6.2 - Conservation of momentum

Conservation of momentumWe have looked at the momentum of

one objectIf two or more objects are interacting

with each other then the total momentum of all objects remains constant regardless of the nature of the forces between the objects

Conservation of momentumMomentum is conserved in collisions

Momentum is also conserved for objects pushing away from each other

m1v1, i +m2v2, i =m1v1, f +m2v2, f

total initial momentum = total final momentum

Chapter 6 - Momentum and Impulse6.3 - Elastic and inelastic collisions

Perfectly inelastic collisionsA collision in which two objects stick

together and move with a common velocity after colliding

After the collision, the two objects become essentially one object

Perfectly Inelastic CollisionsIn this case, we get a simplified

version of the equation for conservation of momentum

using this equation, pay attention to the signs indicating direction

What happens in terms of Kinetic Energy?

m1v1, i +m2v2, i =(m1 +m2)vf

Perfectly Inelastic CollisionsConsider the following Situation:m1 = 1kg v1 = 5 m/sm2 = 2kg v2 = 3 m/s

Conservation of MomentumVideo Demonstration

Practice ProblemA clay ball with a mass of 0.35 kg hits

another 0.35 kg ball at rest, and the two stick together. The first ball has an initial speed of 4.2 m/s.

• What is the final speed of the balls?• Calculate the decrease in kinetic

energy that occurs during the collision

• What percentage of the kinetic energy is converted to other forms of energy?

Elastic CollisionWhen two objects collide and return to

their original shapes with no change in momentum and no change in total kinetic energy

m1v1, i +m2v2, i =m1v1, f +m2v2, f

1

2m1v1, i2 +

12m2v2, i2 =

12m1v1, f2 +

12m2v2, f2

CollisionsMost collisions are neither elastic nor

perfectly inelasticEven in nearly elastic collisions, there

is some deformation and loss of kinetic energy as a result

In most collisions, some kinetic energy is converted into sound

HW AssignmentPage 224, Practice 6E: 1, 3, 5Page 226, Practice 6FPage 229, Practice 6G: 1, 3, 4