What forces oppose motion? Friction would be one force that opposed motion. Friction occurs when objects rub against one another, and this rubbing action tends to slow objects down. There are different types of three different types of friction: sliding friction, rolling friction, and fluid friction. Sliding friction occurs when two solid objects slide over each other. If the surface of the two objects are rough or if the objects are heavy, more friction occurs. Can sliding friction be reduced? Fluid friction is usually less than sliding friction. Substances called lubricants, which are "slippery" substances such as grease, change sliding friction to fluid friction, thus reducing friction. A fluid is anything that flows such as, water and air. Think of dropping a quarter in a swimming pool, why doesn't it drop straight down like it does in air? Because of fluid friction. Why doesn't a feather drop straight down? Because of fluid friction. The resistance air offers slows its path to the ground. If you removed the air, the feather would fall straight down without any interference. Another way to reduce sliding friction is by reducing the surface area between two objects. By that I mean, use wheels. The friction produced by objects such as wheels or ball bearings is called rolling friction. Rolling friction tends to be less than sliding friction, so wheels are often placed under objects that are being moved.

Legend has it that Galileo stood at the top of the Leaning Tower of Pisa in Italy and dropped two cannonballs of different mass (one was 10X the mass of the other) to see which one would strike the ground first. What do you predict happened? They both struck the ground at the same time. To Issac Newton, it meant that both objects were accelerating at the same velocity, regardless of their masses. This means that objects like a falling apple gain speed as it continues to drop. Newton calculated that all objects accelerated at 9.8 meters/second/second or 32 ft/sec/sec. Why then doesn't a piece of paper fall to the ground at 9.8 meters/second/second? Because air offers resistance (remember fluid friction?). Air opposes the downward fall of the paper. Air resistance will slow the acceleration of a falling object until it no longer accelerates as it falls. It will then drop at a constant speed, we say the object has reached terminal velocity. Sky divers cannot accelerate any further once they reach terminal velocity of about 118 mph.

The pull of gravity on an object determines the objects weight. Which means for us, the closer we are to the center of Earth's gravity the more we weigh, and the farther we are from the center of Earth's gravity the less we weigh. The center of Earth's gravity is at its core, so we weigh more at sea level than we would at the top of a mountain. Weight, unlike mass, is dependent upon location! Where would you weigh more, the top of Mount Everest or Death Valley?

Now think of this. If I were to fire a cannonball straight ahead (horizontally) and drop one vertically at the same time, which one would strike the ground first? Take a look at the diagram below and figure out what happens:

Horizontal velocity had no effect on the vertical pull of gravity. Both cannonballs would strike the ground at the same time. For a long time scientist thought forces on Earth were different from forces elsewhere in the universe. Newton proposed that a universal law applied to all objects in the universe. His Law of Universal Gravitation states that all objects in the universe attract each other by the force of gravity. The strength of gravity depends upon the mass of the objects and distance between them. Quite simply, the more mass an object has the more gravity it exerts. Gravity also increases as the objects get closer to one another. Gravity is what holds the Earth and all the other planets together and it's what gives us our weight. Weight is a measure of the pull of gravity. As we stand on the Earth, gravity holds us down and the amount of force that gravity exerts on us keep us on the earth is what gives us our weight. Weight is measured by you position in relation to the center of gravity. The closer you are to the center of gravity, the more you weigh. The center of gravity on Earth is at its core, so it is safe to say that you weigh more at Death Valley than you do on top of Mt. Everest. Remember that weight is relative to your position on Earth.

Newton's 3rd Law of Motion states that forces occur in pairs. Another way to state his 3rd Law is that for every action there is an equal and opposite reaction. Look at the animation to the right, as the rocket fuel burns and escapes out the bottom of the rocket, the space shuttle moves in the opposite direction at the same speed as the fuel escapes. The faster the gases escape the faster the rocket flies in the opposite direction. When you think of the 3rd Law of Motion think of a water sprinkler. As the water exits out of the sprinkler, the sprinkler spins in the opposite direction.

The ESPN sports announcer says "Going into the NCAA Tournament, the Crimson Tide of Alabama has momentum." The newspapter headlines declare, "Crimson Tide Gaining Momentum." Coach Gottfried pumps his team up at practice saying, "You have the momentum; what we need to do is use our momentum in the tournament." Momentum is a commonly used term in sports, and basically means that a team with a lot of momentum is going to be hard to stop. Momentum depends upon mass and velocity. The more mass and velocity an object has the more momentum it has and the harder it is to stop. Momentum can be defined as "mass in motion." All objects have mass; so if an object is moving, then it has momentum - it has it's mass in motion! Look at the animation below and notice how it was difficult for the big fish to stop the little fish.

Why did the big fish move in the direction of the little fish? The momentum of the little fish was not lost, it's speed and direction (velocity) was transferred into the big fish. We call this Conservation of Momentum, one object's momentum is transferred into another object. So what stops the object? Outside forces like friction and gravity alter the object's movement or momentum.