Friction occurs because no surface is perfectly smooth. Rougher surfaces have more friction between them. Heavier objects also have more friction because they press together with greater force. Friction produces heat because it causes the molecules on rubbing surfaces to move faster and have more energy.
The Doppler effect is when the frequency of a sound changes as the position of you and the sound changes. For example, if a fire truck comes at you, the siren sound will increase, then, after it passes you, then the sound will decrease. The thing that just happened is the Doppler effect!
