CH+12+Vocabulary

Chapter 12:


 * A ** force ** is a push or a pull that acts on an object. A force can cause a resting object to move, or it can accelerate a moving object by changing the object's speed or direction.
 * Units of Force Force is measured in newtons, abbreviated as N. One ** newton ** is the force that causes a 1-kilogram mass to accelerate at a rate of 1 meter per second each second (1 m/s2). In fact, 1 newton is equal to 1 kilogram-meter per second squared (1 N = 1 kg·m/s2).
 * The ** net force ** is the overall force acting on an object after all the forces are combined.
 * Balanced forces are forces that combine to produce a net force of zero. When the forces on an object are balanced, the net force is zero and there is no change in the object's motion.
 * An **unbalanced force** is a force that results when the net force acting on an object is not equal to zero. When an unbalanced force acts on an object, the object accelerates.
 * How Forces Combine: [[image:file:hsp00443c04.gif]]
 * [[image:hsp00443c04.gif]]


 * All moving objects are subject to ** friction **, a force that opposes the motion of objects that touch as they move past each other.


 * There are four main types of friction: static friction, sliding friction, rolling friction, and fluid friction.
 * ** Static friction ** is the friction force that acts on objects that are not moving. Static friction always acts in the direction opposite to that of the applied force.
 * ** Sliding friction ** is a force that opposes the direction of motion of an object as it slides over a surface. Because sliding friction is less than static friction, less force is needed to keep an object moving than to start it moving.
 * **Rolling Friction** is when a round object rolls across a flat floor, both the object and the floor are bent slightly out of shape. This change in shape at the point of rolling contact is the cause of ** rolling friction **, the friction force that acts on rolling objects. For a given set of materials, the force of rolling friction is about 100 to 1000 times less than the force of static or sliding friction. Because of this, professional movers often use wheeled dollies to move heavy objects.
 * **Fluid Friction** also acts on a submarine moving through water and on an airplane flying through air. Water and a mixture of gases such as air are known as fluids. The force of ** fluid friction ** opposes the motion of an object through a fluid. You feel fluid friction when stirring thick cake batter. The motion of the spoon through the batter is slowed by fluid friction. Fluid friction increases as the speed of the object moving through the fluid increases. Thus the faster you stir, the greater the friction is.

media type="custom" key="564571" **Mass** is a measure of the inertia of an object; weight is a measure of the force of gravity acting on an object. media type="custom" key="563917"
 * Fluid friction acting on an object moving through the air is known as ** air resistance **. At higher speeds, air resistance can become a significant force. For this reason, bicyclists and speed skaters often wear slick racing suits to reduce air resistance.
 * ** Gravity ** is a force that acts between any two masses. Gravity is an attractive force, that is, it pulls objects together. Earth's gravitational force exerts a force of attraction on every other object that is near Earth.
 * Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion and reduces acceleration.
 * ** Terminal velocity ** is the constant velocity of a falling object when the force of air resistance equals the force of gravity.
 * When you throw a ball forward, you'll notice that it actually follows a curved path. This curved path is an example of ** projectile motion **, the motion of a falling object (projectile) after it is given an initial forward velocity. Air resistance and gravity are the only forces acting on a projectile.
 * Projectile Motion**
 * Newton's first law of motion is sometimes called the law of inertia (in ur shuh). ** Inertia ** is the tendency of an object to resist a change in its motion. In other words, an object at rest tends to remain at rest, and an object in motion tends to remain in motion with the same direction and speed.
 * ** Mass ** is a measure of the inertia of an object and depends on the amount of matter the object contains.
 * ** Weight ** is the force of gravity acting on an object. An object's weight is the product of the object's mass and acceleration due to gravity acting on it.
 * According to **Newton's second law of motion**, the acceleration of an object is equal to the net force acting on it divided by the object's mass. Thus, doubling the mass of an object cuts its acceleration in half. Newton was able to put these ideas into a single formula.
 * Momentum[[image:file:hsp00443c04.gif]]
 * ** Momentum ** is the product of an object's mass and its velocity. An object with large momentum is hard to stop.
 * An object has a large momentum if the product of its mass and velocity is large. The momentum for any object at rest is zero. A huge rocket such as the space shuttle has zero momentum as it sits on the launch pad. A small 1-kilogram meteor traveling at the very high speed of 20 kilometers per second has a very large momentum.
 * Momentum is measured in units of kilogram-meters per second.

Figure 19
 * According to the ** law of conservation of momentum **, if no net force acts on a system, then the total momentum of the system does not change.
 * In a closed system, the loss of momentum of one object equals the gain in momentum of another object—momentum is conserved.
 * ** Electromagnetic force ** is associated with charged particles.
 * Electric force and magnetic force are the only forces that can both attract and repel. To understand electric and magnetic forces, recall what you learned about charged particles in Chapter 4.
 * **Electric Forces**: act between charged objects or particles such as electrons and protons. Objects with opposite charges—positive and negative—attract one another. Objects with like charges repel one another.
 * Figure 19 shows that clothes often cling together when they are removed from a dryer. Some clothes, such as cotton socks, lose electrons easily and become positively charged. Other clothes, such as polyester shirts, gain electrons easily and become negatively charged. Because the oppositely charged particles attract one another, the clothes cling together.

**Nuclear Forces** Two forces, the strong nuclear force and the weak nuclear force, act within the nucleus to hold it together.
 * The ** strong nuclear force ** is a powerful force of attraction that acts only on the neutrons and protons in the nucleus, holding them together.
 * The ** weak nuclear force ** is an attractive force that acts only over a short range.
 * The weak nuclear force is involved in certain types of radioactive processes.


 * Gravitational force ** is an attractive force that acts between any two masses.
 * Newton's law of universal gravitation states that every object in the universe attracts every other object.
 * Gravity is the weakest universal force, but it is the most effective force over long distances. Gravity holds you on Earth. It keeps the moon in orbit around Earth, the planets in orbit around the sun, and the stars in orbit in their galaxies.

A ** centripetal force ** is a center-directed force that continuously changes the direction of an object to make it move in a circle.
 * Much like the moon, the satellite needs only the centripetal force provided by gravity and its inertia to maintain its orbit.