In this study, we designed and developed an app called Ubiquitous-Physics (U-Physics) for mobile devices like tablet PC or smart phones to help students learn the principles behind a simple pendulum in Physics. The unique characteristic of U-Physics is the use of sensors on mobile devices to collect acceleration and velocity data during pendulum swings. The data collected are transformed to facilitate students' understanding of the pendulum time period. U-Physics helped students understand the effects of pendulum mass, length, and angle in relation to its time period. In addition, U-Physics was equipped with an annotation function such as textual annotation to help students interpret and understand the concepts and phenomena of the simple pendulum. U-Physics also generated graphs automatically to demonstrate the time period during which the pendulum was swinging. Results showed a significant positive correlation between interpreting graphs and applying formula. This finding indicated that the ability to interpret graphs has an important role in scientific learning. Therefore, we strongly recommend that physics teachers use graphs to enrich students' information content and understanding and negative correlation between pair coherence and interpreting graphs. It may be that most of the participants (vocational high school students) have limited skill or confidence in physics problem solving; so, they often seek help from teachers or their high-achieving peers. In addition, the findings also indicated that U-Physics can enhance students' achievement during a 3-week time period. We hope that this app can be globally used to learn physics in the future.