a) A body with constant speed can still have acceleration as in the case of circular motion.
In circular motion, there is a force acting perpendicular to the velocity, hence it changes its velocity (thus performing circular motion) but not its speed. Hence the speed remains constant.
b) Newton's third law states that "For every action, there is an equal and opposite reaction and they act on different bodies"
For eg, A rocket engine produces hot exhaust gases which push downward, this is the action and hence the rocket moves upward as the reaction.
Action and reaction is never on the same body, as stated by the third law of motion. When a ball is thrown on the wall, the ball puts a force on the wall and the wall puts a force on the ball. The action is on the wall and the reaction is on the ball, they are different bodies.
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