Can the direction of the speed of a body change to the opposite if a force constant in magnitude and direction acts on this body?

Yes maybe. If the body is thrown upwards, then only gravity will act on the body during flight (at low speeds and body dimensions, the force of air resistance can be neglected), which is always directed vertically downward. The body speed will be directed vertically upward and decrease. After stopping, the body will begin to fall, the speed will be directed vertically downward and increase.

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