力学英语求解释.
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.
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
you usually resolve in the direction of the acceleration and perpendicular to the direction of the acceleration.
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.