INERTIA AND EQUILIBRIUM OF FORCES

Inertia Taken from Hewitt's "Conceptual Physical Science"  Galileo, an italian scientist, is considered the father of modern science since he started doing scientific experimentation to prove or disprove his ideas.  He showed that moving things, once moving, continued in motion without the application of forces.  A force is a push or a pull. A force is needed to start an object moving, but once moving, no force is needed to keep it moving (except to overcome friction).  A ball moving horizontally would move forever if friction were absent as a result of inertia.  Inertia is the property by which objects resist changes in motion.

Mass Taken from Hewitt's "Conceptual Physical Science"  Every material object has inertia. How much depends on amount of matter/mass. The more matter/mass, the more inertia.  The greater the mass, the greater the amount of matter, the greater its inertia.  Mass: The quantity of matter in an object, and also the measure of its inertia.  Weight: The force upon an object due to gravity.

Mass and Weight Taken from Hewitt's "Conceptual Physical Science"  Mass and Weight are directly proportional to each other, or directly related. If you change one, the other changes proportionally.  If the mass of an object is doubled, its weight is also doubled. This is why mass and weight are often interchanged. w = m x g  Mass is measured in kilograms, weight is measured in units of Force (newtons).

Review Questions Taken from Hewitt's "Conceptual Physical Science"  Does a 2 kg iron block have twice as much inertia as a 1 kg iron block? …yes  Does a 2 kg iron block have twice as much inertia as 1 kg bunch of bananas?...yes Twice as much mass?...yes Twice as much volume?...no Twice as much weight when weighed in same location?...yes  How does the mass of a bar of gold vary with location? It doesn’t vary.

Weight Units are not kg but N Taken from Hewitt's "Conceptual Physical Science"  1 kg of anything on Earth has a weight of 9.8N. If gravity is less (like on the Moon) the material would weigh less.  Weight can change with location (different planets), but Mass doesn’t change.

Net Force Taken from Hewitt's "Conceptual Physical Science"  A force is a push or a pull. Objects don’t speed up, slow down or change direction unless a force acts.  By force we mean the total force, or net force, acting on an object.  When you kick a soccer ball, the force of gravity, air friction, and the pushing force you apply with your muscles all act on the ball.  Net force is the combination of all these forces.

Net Force - continuation Taken from Hewitt's "Conceptual Physical Science"  You pull on a box with a force of 5N. If your friend pulls 5N in same direction, net force is 10 N.  If friend pulls on box with same force but opposite direction, net force is zero.  If you increase pull to 10N and friend pulls opposite 5N, net force is 5N in your direction.  Force is a vector quantity, meaning it has magnitude (how much) and direction (which way). Arrow length represents magnitude.

The Equilibrium Rule Taken from Hewitt's "Conceptual Physical Science"  When the net force on something is zero, we say that the object is in equilibrium.  Exercise: If you hang from a trapeze at rest, what is the tension in each of the two supporting vertical ropes?  Answer: Assuming that the weight of the trapeze’s seat is negligible, the tension on each rope would be half of your weight.

Support Force Taken from Hewitt's "Conceptual Physical Science"  A book lying at rest on a table is in equilibrium. One force acting on book is a consequence of gravity, the book’s weight. As book is in equilibrium there must be opposite force, of same magnitude.  This upward support force works as a tension force. Imagine you pushing a spring. The balancing force is the support force, or tension in the spring.  The weight of book pushing on the table actually compresses atoms in the table, like microscopic springs, and the atoms squeeze upward on the book.

Review Exercise Taken from Hewitt's "Conceptual Physical Science"  Suppose you stand on two bathroom scales with your weight evenly divided between the two scales. What will each scale read? How about if you stand with more of your weight on one foot than the other?  Answer: Each scale would indicate half of your weight. If more of your weight is on one side than the other, that side would show more weight.

Dynamic Equilibrium Taken from Hewitt's "Conceptual Physical Science"  When an object isn’t moving, the forces on it add up to zero- it’s in equilibrium, static equilibrium.  An object moving at constant speed in a straight-line path is also in equilibrium, dynamic equilibrium.  Once in motion, it there is no net force to change the state of motion, object moves at same speed (inertia).  In static or dynamic equilibrium, sum of forces = 0.  If you push desk and it moves steadily at constant speed, the force of friction between desk and force must be equal and opposite to our pushing force.

The Force of Friction Taken from Hewitt's "Conceptual Physical Science"  Friction is a force. A force is a push or a pull. All forces have two properties: magnitude and direction.  Friction is the resistive force that opposes the motion or attempted motion of an object when it rubs against something else.  Friction occurs for solids, liquids, and gases.  Friction always acts in a direction to oppose motion.  When an object falls downward through the air, the force of friction, air resistance, acts upward.  Frictions always acts in a direction to oppose motion.

The Force of Friction cont. Taken from Hewitt's "Conceptual Physical Science"  Amount of friction between two surfaces depends on kinds of material and how much they are pressed together.  Friction is due to tiny surface bumps and also to the “stickiness” of the atoms on the surfaces.  Friction between a sliding desk you’re pushing and a smooth linoleum floor is less than between desk and a rough concrete uneven floor. If surface is inclined friction is less because it doesn’t press as much on the inclined surface.

Types of Friction Taken from Holt's “Physical Science"  If you push horizontally hard enough on piece of furniture to match friction of floor, the net force on it is zero, and it slides across floor at constant speed.  What affects the amount of friction?  Imagine that a ball is rolled over a carpeted floor and another ball is rolled over a wood floor. In which case would the ball move faster? Why?  There are two types of friction. Kinetic friction occurs when object moves (kinetic means moving). Static friction occurs when force is applied but object does not move.

Changing Friction Taken from Holt's “Physical Science"  How can Friction be decreased?  To reduce amount of friction you can apply a lubricant between two surfaces (like oil, grease, and wax).  You can also roll rather than slide an object.  You can smooth the surfaces that rub against each other (skiers have their skis sanded to make them smoother).  How can Friction be increased?  Increasing friction can be necessary, for example when talking about the tires of a car so that it can stop easier.  On icy roads sand can be used to make them less slippery.  By increasing force (like cleaning a pan or pot).