Chapter 5 Momentum

Momentum inertia and motion Newton’s “quantity of motion” helps us understand collisions and explosions

Momentum Momentum = mass ´ velocity p = mv Momentum is a vector quantity.

A large truck has more momentum than a car moving at the same speed because it has a greater mass. Which is more difficult to slow down? The car or the large truck?

P = mv P = mv Large Momentum Examples: Huge ship moving at a small velocity High velocity bullet P = mv P = mv

Impulse In order to change the momentum of an object you must apply a force over some time interval. Impulse = Force ´ time interval = F t

Impulse and Momentum Impulse = Change in Momentum = Final (mv) - Initial (mv) F t = D(mv)

Examples: Air bags in cars. Catching an egg. Boxing http://www.youtube.com/watch?v=nlWJayKWvw0 http://www.youtube.com/watch?v=_Av0WGrlTGY http://www.youtube.com/watch?v=D-ogrMr4lWc Catching an egg. Boxing

Conservation of Momentum This means that the momentum doesn’t change. Recall that F t = D(mv) In this equation, F is the "external force". Internal forces cannot cause a change in momentum.

Examples Conservation of Momentum: If there are no external forces, the total momentum for a system remains unchanged. Example 1: a person sitting inside a car pushing against the dashboard Example 2: a bullet fired from a rifle Example 3: a rocket is space

Question Which has more momentum? (A) A parked cruise ship (B) You in your vehicle driving home

Clicker Question 3. Which of the following is true about momentum? (a) it is a vector (b) it is a product of mass times velocity (c) impulses are required to change it (d) all of the above

Chapter 6 Energy

Energy Universe is made up of matter and energy. Energy is the mover of matter. Energy has several forms: Kinetic Potential Electrical Heat etc.

Work Now instead of a force for how long in time we consider a force for how long in distance. Work = Force ´ Distance W = F d The unit for work is the Newton-meter which is also called a Joule.

How much work? A weight lifter lifts a barbell weighing 1000 Newtons 1.5 meters A weight lifter pushes on a wall with a force of 1000 Newtons for 15 sec?

Power Power is equal to the amount of work done per unit time. The unit for power is the Joule/second which is also called a Watt.

Light Bulbs and Appliances How much energy does a 100 Watt light bulb use in one hour? How about a 40 Watt light bulb?

Mechanical Energy When work is done on an object, the object generally has acquired the ability to do work. This "ability to do work" is called energy and it has the same units as work….Joules. Two Types of Mechanical Energy Potential Energy and Kinetic Energy

Potential Energy The energy that is stored is called potential energy. Examples: Rubber bands Springs Bows Batteries Gravity

Gravitational Potential Energy PE = Weight ´ height PE = m g h Question: How much potential energy does a 10kg mass have relative to the ground if it is 5 meter above the ground?

Kinetic Energy Kinetic Energy is the energy of motion. Kinetic Energy = ½ mass ´ speed2 Question: How much kinetic energy does a 1kg mass have if it is moving at 10 meters/second?

Work/Energy Relationship If you want to move something, you have to do work.

Conservation of Energy Energy cannot be created or destroyed... ...it may be transformed from one form into another... ...but the total amount of energy never changes. Demos Galileo's incline Bowling ball pendulum

Pendulum Example Problem At what point is the kinetic energy the highest? (A) At the bottom of its path (B) At the ends of its path (C) Somewhere in between

Pendulum Example Problem At what point is the potential energy the highest? (A) At the bottom of its path (B) At the ends of its path (C) Somewhere in between

Example Problem A 100 kg mass is dropped from rest from a height of 1 meter. How much potential energy does it have when it is released? How much kinetic energy does it have just before it hits the ground? What is its speed just before impact? How much work could it do if it were to strike a nail before hitting the ground?

100 kg 1 meter 100 kg 100 kg nail

Comparison of Kinetic Energy and Momentum Kinetic energy is a scalar quantity. Momentum is a vector quantity.

A 1 kg mass is lifted 1 meter A 1 kg mass is lifted 1 meter. The change in potential energy of the weight is (a) 1 N (b) 9.8 N (c) 1 J (d) 9.8 J (e) 9.8 m/s2

End of Chapter 6