500kg object jumping

To determine Force ( measured in newtons ) you use Newton's second law Force = mass * acceleration so to answer this question you need to know the acceleration of the jump in order to find the force.  However, the question you gave gives no way of solving for acceleration so I can't see how you would solve the problem. Try rechecking the place where you got the question from.

The 500 kg object doesn't have to jump. The acceleration due to gravity is 9.81 m/sec^2. Since 1 newton =1 kg.m/s^2, it follows that 500 kg object will "feel" the force of: 9.81 m/s^2 x 500 kg=4,905 newtons(kg.m/s^2).