Acceleration is the ratio (△V/△t) between the speed change and the time taken for this change. It is a physical quantity that describes the speed change of an object, usually expressed by a, and the unit is m/s 2. Acceleration is a vector, and its direction is the direction of the speed change (quantity) of the object, which is the same as the direction of the external force. Speed is an inertial quantity, which has the characteristics of inertia. As long as there is no external force, it can maintain its own state. On the contrary, if there is an external force, the partial equilibrium speed will change, and the acceleration is the physical quantity that describes the speed change. The direction of acceleration is a=F/m Note that acceleration is a vector, and the factors that determine acceleration are F and M, not V and T. When the magnitude and direction of acceleration of an object remain unchanged, the object moves at a constant speed. Such as free fall, flat throw and so on. When the acceleration direction and the initial velocity direction of an object are on the same straight line, the object moves in a straight line. Such as a vertical throwing motion. For example, the driver steps on the accelerator when going straight (that is, assuming that the traction force F provided by the car remains unchanged), while the steering wheel remains stationary, and the car moves in a straight line at a uniform speed. At this time, the acceleration and initial velocity are in the same straight line. The initial velocity of a uniformly accelerated particle is 1m/s, and after 2 seconds, its velocity becomes 3m/s, so the calculation method of this particle acceleration is: subtract the initial velocity from the final velocity and divide it by the time (2 seconds), which is the square of 1m/ s (m/s 2). It means that every time this particle passes through 1 s, its speed increases by 1 m per second. However, it should be noted that the acceleration is a vector, that is, this quantity indicates both the magnitude and the direction of the particle acceleration. Acceleration is the ratio of the change of speed to the time taken for this change. It is a vector. The relationship between the direction of acceleration and the direction of velocity: the direction of acceleration is the same as the direction of velocity change Δ v, and in linear motion, if the velocity increases, the direction of acceleration is the same as the velocity; If the speed decreases, the direction of acceleration is opposite to the speed. The speed of speed change is often different when different objects move (please pay attention to distinguish the difference between speed and speed change). Please note that acceleration and speed are not necessarily related. When the acceleration is large, the speed can be small, and when the speed is large, the acceleration can also be small. For example, a ball bouncing repeatedly on the ground, in a very short time, the speed is close to zero (or from the downward speed to zero, and then to the upward speed), but because the speed changes greatly in a short time, the acceleration is great. Another example is racing cars, which drive at a constant speed in a straight line. However, because it travels at a constant speed, the change of speed is zero, so its acceleration is also zero. When the acceleration is zero, the object is stationary or moving in a straight line at a uniform speed (relative to the same reference frame). Any complex motion can be regarded as the synthesis of numerous uniform linear motions and uniform acceleration motions. Acceleration varies with the choice of reference system (reference object), generally taking the ground as the reference system. When the angle between the direction of motion and the direction of acceleration is less than 90, it is acceleration and the acceleration is positive; On the contrary, it is negative. Especially when the angle between the direction of motion and the direction of acceleration is exactly equal to 90, the object neither accelerates nor decelerates, but moves at a uniform speed. Such as uniform circular motion. Force is the reason for the acceleration of an object, and the acceleration of the object is caused by external force, or force is the reason for the change of the speed of the object. Explain that when an object accelerates (such as free fall), the acceleration is positive; When an object decelerates (such as a vertical throwing motion), the acceleration is negative.