Physics 1 Academic Chapter 4 – Newton’s First Law of Motion – Inertia Sections 4.5 to 4.7

Mass – A Measure of Inertia  The amount of inertia an object has is related to its mass.  Think of kicking a soccer ball vs. kicking “The Rock.”  The greater the inertia, the greater the force that is required to change the object’s state of motion. 2

Mass is Not Volume  Mass – the amount of matter an object contains  Volume – the amount of space an object occupies  Nuclear material i.e. protons and neutrons are very dense.  If the volume is 1 cm 3  The mass is several thousand metric tons (1 metric ton = 1,000 kg) 3

Mass is Not Volume  If you have two identical boxes and one is filled with lead shot, the other with feathers, which has the greatest mass?  Which has the greatest volume?  Which has the greatest inertia? 4

Mass is Not Weight  Mass – the amount of matter an object contains  Measured by a balance  Would this measurement change on the moon? 5

Mass is Not Weight  Weight – the objects attraction to a gravitational field  Measured by a spring scale  Would this measurement change on the moon? 6

Mass is Not Weight 7  Remember that in a single given location: Mass α Weight

Mass and Force 8  The SI unit of mass is the kilogram  1 kg = 2.2 lbs.  The SI unit of force is the newton  1 newton is ≈ 0.25 lbs.  1 kg = 9.8 newtons

Weight and Mass 9 Weight = mass x acceleration due to gravity Or W (newtons) = m (kg) x g (m/s 2 )

Problem 10  What is the weight in Newtons of an object with a mass of 25 kg? W = m x g = (25 kg) x (10 m/s 2 ) NOTE – 1 kg x m/s 2 = 1 Newton = 250 kg x m/s 2 = 250 Newtons

Net Force 11  In the absence of force:  Objects at rest stay at rest  Objects in motion stay in motion  Really, the correct term is net force  If Sean and Kyle both push with equal force on opposite sides of a desk, with the same force, will the desk move?  Justify your answer.  What is the net force on the desk?

Net Force 12 Net force is the combination of all forces that act on an object. You must have a net force to change the motion of an object!!!!!!

Net Force 13  In this situation, the forces are equal and opposite.  The net force is zero

Net Force 14  In this situation, the net forces are additive.  The net force on the object is 10N.

Net Force 15  Here, the net forces are against each other, but not equal.  The forces subtract.  The net force is 2 N

Equilibrium – When Net Force Equals Zero 16 When an object is on a table, what forces are acting on it?

Equilibrium – When Net Force Equals Zero 17  Gravity pulls the book down  The table pushes the book up, balancing the force of gravity  Net force is zero  The book is not moving  The book is in equilibrium  Support force is also called normal force