# Physics in Everyday Life

## Physics and Me

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After viewing our curriculum units, please take a few minutes to help us understand how the units, which were created by public school teachers, may be useful to others.

## Motion

The terms up, down, backward, forward, left and right all describe directions in which an object can move. To determine how fast an object is moving in any direction you need a unit of measure. Today the most popular unit of measurement in the developed world (excluding the U.S.) is the Metric System. This unit of measurement is a base 10 unit, which makes it easy to go from one unit to another. Throughout history, many different systems of measurement have been developed. Until the late 18th century systems of measurement were very confusing. Every nation and sometimes every town or village had its own unique system of measurement. If you moved from one country to another you had to learn a whole new system of measurement. Also, a unit of measurement did not always mean the same thing to everyone within the same country.

- To solve this problem, in the 1790's the French National Assembly ordered the development of a new system of measurement, which has come to be known as the Metric System. As previously stated, this system is based on the number 10 and multiples 0f 10. The units of measure in the metric system includes the meter, kilometer, centimeter, liter, gram and kilogram. These units are based on standards that do not vary. So regardless of whether you are in France, Mexico, or Japan, if you are given directions to go south for 10 km, you can be sure that you know exactly how far to move.
^{ 1 }- Sir Isaac Newton's First Law of motion deals with the nature of motion. Prior to his first Law of Motion- which states that objects at rest tend to stay at rest; objects in motion tend to stay in motion- scientists thought that a constant force was needed to keep an object in motion It seemed obvious to them that if a force stopped acting on an object, the object would slow down and eventually stop.
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Inertia

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The tendency of an object to remain at rest or to remain in motion is called inertia. We expect a rock to remain sitting at the top of a hill. Similarly, a ball rolling across a table top might be expected to continue rolling in the same direction; no force is needed to keep it in motion. A change in conditions could influence an object's motion. For example, suppose that the rock starts falling down the hill. What change in condition caused the rock to start falling down the hill? Further suppose that a rolling ball speeds up, slows down or changes directions. Some type of force was applied in each instance to cause a change of the motion of the objects.

- Many examples of inertia can be found in everyday life. Finding examples of inertia would be a good activity for students. One example: Suppose you are standing in the aisle of a bus that is traveling at a constant speed of 40km/h(25 mph). What would happen if the driver suddenly slammed on the brakes? Chances are you'd fall forward as the bus comes to a stop. This would happen because your body has inertia while the bus is traveling forward. Your body is moving in the same direction and at the same speed as the bus. When the driver hits the brakes, the bus comes to a stop, but inertia keeps your body moving forward.

Force, Mass and Acceleration

Newton's Second Law of Motion states that an object begins to move, slows down, speeds up, comes to a stop, or changes direction only when some force acts on the object. Newton's Second Law of Motion states that an object begins to move, speed up, slows down, comes to a stop, or changes direction only when some force acts on the object. For example, a rock on top of a hill might begin rolling down the hill if someone exerted a force on it. Once started down the hill, the rock would continue to gain speed because of the force of gravity acting on it. The rock would continue moving along a straight course down the hill until some new force acted on it. This new force could change its direction, slow it down, speed it up or stop it.

- Consider a ball rolling across a billiard table. The ball might speed up, slow down or change direction. Why? The second law says that such a change occurs when a force acts on the ball. What might provide the necessary force on a billiard ball to change its motion?
^{ 3 }- Perhaps the billiard ball is hit by another ball from behind, from the front, or from the side. If contact with the ball is from behind, a pushing force changes the direction of the ball. If contact with the other ball is from the side or the front, a pushing force changes not only the speed of the ball but also the direction in which the ball is moving, Newton discovered a mathematical formula that shows how force causes a change in the speed or direction of an object.

The formula is Force = mass x acceleration. F = m x a. The units used in this formula are newtons (N) for force, kilograms (kg) for mass, and meters per second per second (m/s squared) for acceleration.

- The Newton is a unit of force in the metric system. A Newton is defined as the force needed to accelerate a 1-kg object 1 meter per second per second. N = kg x m/s squared. The formula for force tells us many things about the way in which a force acts on an object. For example, suppose an object with a mass of 2 kg accelerates at 5 m/s squared. What force was needed to achieve this acceleration? To answer that question, first write the formula for Newton's second law. F = m x a. Then substitute the values you know for this question. m = 2 kg; a = 5 m/s squared. Finally, use the formula to find the unknown quantity-force.
- F = m x a
- F = 2 kg x 5 m/s squared
- F = 10 kg x m/s squared
- F = 10 N
^{ 4 }- It would require a force of 10 N to make a 2 kg object accelerate at the rate 5 m/s squared.

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Friction
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Friction is a force that occurs between surfaces that are in contact with each other. Friction resists the motion of one surface over another. When a racecar is at rest on the track, it has no motion. There is no friction between the car's tires and the track. But when the driver steps on the accelerator, the car's tires begin to rotate. Friction begins to develop between the tires and the track beneath them.

- The amount of reaction between two bodies depends on many factors but especially on the properties of each surface. Rough surfaces generally result in more friction than do smooth surfaces. Imagine sliding an ice cube across the frozen surface of a lake. Ice is usually very smooth, so there is little friction between the ice cube and the ice on the lake. The ice cube will slide a long way before coming to a rest. What would happen if you slid the ice cube across a rough surface?
- Friction also varies with the kind of motion taking place. Objects that roll over a surface produce less friction than objects that slide. Ball bearings are small metal balls inserted between surfaces that rub against each other. There is much less friction with the ball bearings rolling between the surfaces than with the two surfaces rubbing directly against each other.

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Friction in Sports
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In many winter sports, participants want to reduce friction as much as possible. Downhill skiers often put wax on their skis. The wax reduces the friction between the skis and the snow, causing the skier's speed to increase.

- Although friction slows speed, it can be helpful. Walking is possible because friction prevents your feet from sliding over the ground. In some sports, players want to increase friction. A person who runs the 100 m dash wants the maximum friction between his or her feet and the running track.
- Sir Isaac Newton's Third Law of Motion states that for every action force there is an equal and opposite reaction force. For example, when you shoot a basketball, the ball pushes on you just as hard as you push on it. There are two important things to remember about Newton's third law:
- - forces always occur in pairs made up of an action force and a reaction force
- - the action force and the reaction force always act on different bodies
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Mass Matters

Imagine that you are attempting to shoot a basket and your friend is attempting to block your shot. Let's further suppose that she had pushed off the floor with a force exactly equal to the force you used to push off, but she has less mass (the amount of "stuff" in an object) than you. The third law explains that the reaction force of the floor would be equal to your friends action force. The reaction force however, would be acting on a smaller mass; according to Newton's second law, your friend would have a greater acceleration and go higher than you! According to Newton's third law your momentum is equal to your mass multiplied by your velocity. p = m x v.