为什么Physics2D会为AddForce（）与速度产生不同的结果？

Question

Why does Gravity in Physics2D engine for Unity2D act different when these two following lines of codes are implemented alternately?

For example, I have attached to my player sprite a Player Controller C# script:

private float speed = 500f;
RigidBody2D playerChar = null;

And then to make my character walk :

Vector2 vec = new Vector2 (Input.GetAxis("Horizontal"), 0);
playerChar.AddForce(vec * speed);

And the Gravity is set to 50

Result 1: My character avatar falls down normally.

Meanwhile when I do:

Vector2 vec = new Vector2 (Input.GetAxis("Horizontal"), 0);
playerChar.velocity  = (vec * speed);

And the Gravity is still set to 50

Result 2: My character now takes a long time to fall (it slowly "floats" down).

Why is that?

Answer 1

This is because you're forcing the y component of the rigidbody's velocity to zero.

When you add force, it adds , it doesn't replace.

When you set the velocity, you're specifically setting it to a Vector2 that has ay value of 0, gravity then kicks in on the fixed update cycle and adds a small amount of gravity, causing your player to fall slowly. Then Update happens again and you force the y value back to 0 once more.