Suppose a particle is moving along x-axis with constant velocity.A force directed along y-axis starts acting on the particle.As the particle accelerates along y-direction its velociy increases along y-direction..So the net velocity increases.But why is it written in textbooks that speed remains constant?And it is also written that kinetic energy remains constant.But that is increasing.
Looking at what you have said your logic looks okay. But, maybe the question is sayin somethin different like velocity is always perp. to force (like mangetic force for ex.). If full question is stated it can be said who is correct here.
As the particle was moving along x-axis and then a force started acting on it which along y-direction so the force is perpendicular to the velocity of the particle..But you can see that both velocity and speed change.And yes,the velocity will be constant for x-direction.But the net velocity has to increase.So kinetic energy has to increase and so it cannot be constant.I were asking if I were true.