Assalamu alaikum
Thank you for your contact. I don't know any special dua to increase the physical strength. However, your aim is positive to protect the people from any possible harm. I give you one dua, the special names of Allah. You need to read them as much as you can and make dua from Allah for achieving your goal. But for the best effect, you will have to offer prayers as well.
Dua: Yaa Allah 2. Yaa Kareem 3. Yaa Awwal 4. Yaa AAkhir 5. Yaa Mujeeb
Wassalam