text: can someone please help me with physics? i cannot understand this problem. please help me :( i would appreciate it a lot.  thank you much.  the total cross-sectional area of the load-bearing calcified portion of the two forearm bones (radius and ulna) is approximately 2.7 cm2 (that's squared). during a car crash, the forearm is slammed against the dashboard. the arm comes to rest from an initial speed of 80 km/h in 5.2 ms. if the arm has an effective mass of 3.0 kg, what is the compressional stress that the arm withstands during the crash? ok so your trying to find a stress. stress is force/area you alread have an area, so lets work on a force force is mass x acceleration you have a mass, so lets find an acceleration acceleration is the change in speed with respect to time dv/dt or delta v/delta t your delta v is 80km/hour and your delta t is 5.2 ms  lets put these in the proper units and solve: 80km/hr*(1000m/km)*(1hr/3600sec)=22.222m/sec time = 5.2ms(1s/1000ms)=.0052sec acceleration=4273.504m/s2  back to mass f=ma=3.0kg*4273.504m/s2=12820.5128 newtons  back to area 2.7cm2*(1m2/10000cm2)=.00027m2  stress then equals 12820.5128/.00027 47,483,380.740 pascals or 47.483 gpa