Work done is the distance a force acts over.
So, the work done here is 9.0N * 3.0m = 27 J
Answer:
10 kg
Explanation:
Assuming a frictionless surface, then force F=ma where F is the applied force, m is the mass and a is acceleration. Making m the subject of the formula then 
Substituting 100 N for the applied force F and 10 m/s^2 for acceleration a then the value of m will be 
Therefore, in terms of kilograms, the bookshelf weighs 10 Kg
Speed equals distance divided by time, so 350 divided by 2.5 equals 140 kilometers per hour.