Chapter 12

 Force – push or a pull that acts on an object  Balanced forces – two forces that are equal in size and opposite in direction  Unbalanced forces – NOT equal and opposite  Net force – changes the velocity of the object 2

 Friction – a force that opposes motion 3

1. Static Friction – acts on objects that are not moving 2. Sliding friction – when two solid surfaces slide over each other 3. Rolling friction – acts on rolling objects 4. Fluid friction – when an object moves through liquid or gas 4

 Force that acts between any two masses  Gravity pulls down, air resistance pushes up  Terminal Velocity–  constant velocity of a falling object  Air resistance equals force of gravity 5

 An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force. 6

1 st Law Continued… Inertia is the tendency of an object to resist changes in its velocity: whether in motion or motionless. These pumpkins will not move unless acted on by an unbalanced force. 7

1 st Law Continued… Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever. 8

1 st Law Continued… Once airborne, unless acted on by an unbalanced force (gravity and air – fluid friction), it would never stop! 9

Newton’s 1 st Law and You Don’t let this be you. Wear seat belts. Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 m/hour. 10

Why then, do we observe every day objects in motion slowing down and becoming motionless seemingly without an outside force? It’s a force we sometimes cannot see – friction. 11

The net force of an object is equal to the product of its mass and acceleration, or F=ma. 12

Newton’s 2 nd Law proves that different masses accelerate to the earth at the same rate, (the rate of gravity) but land with different forces. So why is it that different objects appear to fall slower or faster? 13

 “For every action there is an equal and opposite reaction”  Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object 14

 Example: If you punch someone in the face, your hand applies a force to that person’s face and their face applies a force to your hand (don’t try this at home!) 15

 Momentum is the product of mass and velocity  Abbreviated with a little “p”  Law of Conservation of Momentum: In a closed system, the loss of momentum of one object equals the gain in momentum of another; momentum is conserved 16

 There are four universal forces that we currently know of. Scientists continue to study these and are searching for a unified theory that relates them better!  Universal forces act over a distance between particles of matter. The particles need not be in contact. Force is affected by the distance between particles. 1. Electromagnetic 2. Strong Nuclear 3. Weak Nuclear 4. Gravitational 17

 Associated with charged particles  Electric and magnetic force are two different aspects of the electromagnetic force.  Electric and magnetic force are the only forces that both attract and repel Objects with opposite charges attract one another. Objects with like charges repel one another. 18

1. Strong Nuclear Force: powerful attraction acting on neutrons and protons in the nucleus, holding them together. Overcomes the electric force of repulsion that acts among the protons in the nucleus. More than 100 times stronger than the electromagnetic force. Very short range. 1. Weak Nuclear Force: weaker than the strong force. Involved in certain types of radioactive processes. 19

 Gravity is the weakest universal force, but it is the most effective force over long distances.  Newton’s law of universal gravitation states that every object in the universe attracts every other object.  Gravitational force is an attractive force that acts between any two masses.  Gravitational force depends upon mass and distance. F=Gm 1 m 2 r 2 20

 The moon’s inertia acts to move it away from Earth.  Earth’s gravitational attraction keeps the moon in a nearly circular orbit around Earth.  A centripetal force is a center-directed force that continuously changes the direction of an object to make it move in a circle. 21