Newton's second law of motion states that the force needed to accelerate a particle is equal to the product of the mass of the particle and the acceleration produced.
牛顿第二定律指：使质点加速的力等于质点的质量乘以力所产生的加速度.也就是F=ma
The unit of force is the newton(N).It is defined as the force that will cause a mass of 1 kg to accelerate at a rate of 1 m/s^2
力的单位是牛顿N,1N等同于使1kg的物质产生1m/s^2的加速度时所用的力.
you usually resolve in the direction of the acceleration and perpendicular to the direction of the acceleration
我们通常（将力）分解为一个加速度方向的力和一个垂直于加速度方向的力.
the normal reaction is the force which acts perpendicular to a surface when an object is in contact with the surface.In the example on the previous page,the normal reaction is equal in magnitude to the weight of the book
反作用力垂直于物体的接触面,在前页的例子中,反作用力的大小等于书地重力.