The cart will definitely behave differently because more weight is added to it.
In order to successfully push a cart, the applied force must exceed the force of the weight of the cart along with its constituents and the frictional force between the tyres and the floor.
<em>When more weights are added (the 2 packs of water bottles), it means that more force would be needed to overcome the opposing forces to the movement of the cart in order to successfully push the cart again.</em>
Yes, Chayse's results are reasonable because all parts of the electromagnetic spectrum travel at the speed of light. By multiplying his results for frequency and wavelength, I can see that the velocity of the UV ray is close to the speed of light.