When it becomes difficult for an object to freely move across a surface, friction may be the preventing force. Friction is the resistance of motion when an object rubs against and acts in the opposite direction of another object. When any two objects rub against each other they cause friction. Friction is a force that holds back another object from freely moving. A simple example are the brakes on an automobile.
当物体难以在接触面自由移动时，摩擦力可能是其阻力。摩擦力是物体运动表面相互摩擦产生的，其方向与物体相对运动方向相反，阻力是物体磨擦时，产生的的反方向作用力。任何两个物体相互摩擦，都会产生摩擦力。摩擦力是阻碍相互接触的物体阻碍另一物体自由移动的力。一个简单的例子是在汽车刹车。
Friction causes a second object to lose energy by slowing its motion. The energy does not disappear, but it changes from moving energy, which is called kinetic energy to heat energy or thermal energy. When a person rubs their hands together friction is generated and then it turns into heat. This is why cold hands become warm after rubbing them together. This is also called kinetic friction.
摩擦导致第二对象通过能量消耗从而减速。能量不会消失，但它从移动的能量，也就是所谓的动能转热能。当一个人搓自己的双手会产生摩擦力，然后将其转变成热量。这就是为什么冰冷的手相护揉搓后会变得温暖。这也被称为运动摩擦。
Friction can be found anywhere objects come into contact with each other. The brakes on a car causes friction between the brake pads and the wheels of the car, allowing the car to come to a stop. A person running on a sidewalk may stop quickly because of friction caused between the bottom of the shoes and the asphalt or concrete.
物体与其接触面的摩擦力随处可见。汽车刹车是因为刹车踏板与汽车车轮之间的摩擦力让车停下来。在人行道上跑的人能快速停止是源于鞋底和沥青或混凝土之间的摩擦力。
However, other variables can lessen the effects of friction. For instance, if the sam e person running tried to stop on a water-covered sidewalk, friction would be less and the runner may not be able to stop as quickly, and in some cases may fall. This is similar to a car trying to stop on an ice-covered roadway. The friction is still there, but it is much less and may lead to accidents. Also, during rain, there is still friction between the brakes and the wheels, yet if the brakes are wet, the wheels would not be as much in contact with the ground. As a result, cars hydroplane when they go too fast on puddles of water.
然而，其他变量可以减少摩擦力的作用。例如，跑步者试图在积水的人行道上停下来，但是因为摩擦力减小他可能很难立刻停下来，而且有时会摔倒。同样，汽车试图在冰雪覆盖的道路上刹车。摩擦仍然存在，但要小得多，而且可能引起事故。又例如，下雨时，刹车踏板和车轮之间的摩擦力仍然存在，但如果离合器是湿的，车轮不会与地面接触一样多。其结果是，车子如果在水洼行驶速度过快如同水上飞机。
There are times when friction needs to be prevented so things will move more easily. Lubricants like grease and oil can help reduce friction between two objects. Engines and machines use grease and oil to reduce friction and wear so they can last much longer. Friction can also be reduced by using a ball or wheel on certain objects, which is called rolling friction. Changing the types of materials that come in contact with each other is another way of preventing friction. A good example is the use of roller skates on a concrete surface, ice skates on a lake covered with ice, or wearing rubber shoes on a wet sidewalk.
有些时候，需要减少摩擦使物体更容易移动。像润滑油之类的油脂可以减少两个物体之间的摩擦力。发动机和机器使用润滑油以减少摩擦和磨损，使其能够持续更长的时间的工作。还可以在某些物体上使用球体或滑轮来减少摩擦，这被称为滚动摩擦。改变接触材料是防止摩擦的另一种方式。一个很好的例子是在混凝土表面使用旱冰鞋，在结冰的湖面使用溜冰鞋，或者在湿滑的人行道使用胶鞋。
Besides dry friction as in some of the examples above, there is also static friction when objects are not moving such as the touching a metal surface and feeling a shock. When friction involves a fluid or air it is called fluid friction. The air resistance on an airplane, water resistance on a boat, and the slides at water parks are examples of fluid friction.
除了上文中提及的动摩擦外，也有非运动物体的静摩擦力，如触摸金属表面和撞击。当摩擦涉及流体或空气，被称为流体摩擦。飞机的空气阻力，船的水的阻力，包括水上公园的滑梯都是流体摩擦的例子。
Finally, the two main factors that influence the total amount of friction include the roughness of the objects' surfaces and the force applied between the two objects. The measure of friction, its coefficient, is based on the roughness of the materials that come in contact with each other. For example, concrete on concrete has a very high coefficient of friction, and the Teflon surfaces of pots and pans have a low coefficient because it is a surface where most things do not stick.
最后，影响摩擦大小的两个主要因素是接触表面的粗糙度和作用在两个对象之间的力。摩擦力，其系数是基于相护接触的材料的粗糙度。例如，混凝土之间的摩擦系数非常高的系数，而锅具的特氟隆表面的摩擦系数很低，因为大部分东西都不沾附其表面。
In summary, friction is the resistance of motion when an object rubs against and acts in the opposite direction of another object. There are four types of friction which includes kinetic friction, dry friction, rolling friction, static friction, and fluid friction. Common examples of friction are brakes on a car, ice skates, and wearing rubber shoes on a wet surface. Roughness and force of objects are factors of friction, and the coefficient of friction is a measure of how easily one object moves in relationship to another.
总之，摩擦力是当物体摩擦另一对象而产生的相反方向的作用力。有四种类型的摩擦力，其包括动摩擦，干摩擦，滚动摩擦，静摩擦和流体摩擦。摩擦常见的例子是在一个汽车刹车，溜冰鞋，在湿地穿胶鞋。摩擦因素是粗糙度和作用在对象上的力，摩擦系数用来评断在一物体在另一物体上移动的难易程度。