Question

9. A 6.0 kg object (at rest) is released from a height of 80 m. If air resistance is negligible and g = 9.8 N/kg, the kinetic energy of the mass when it is 20 m above the ground is:

(a) 4.7 × 103 J (b) 3.5 × 103 J

(c) 2.4 × 103 J (d) 1.2 × 103 J

Answer 1

According to the energy conservation law, the total energy at 20 m level its equal to the initial potential energy:

$$mgh_0 = mgh +K$$

where $h_0 = 80m$ is the initial height, $h = 20m$ is the current height, and $K$ is the kinetic energy at 20 m level. Expressing $K$, obtain:

$$K = mg(h_0-h) \\ K= 6.0kg \cdot 9.8 N/kg\cdot (80m - 20m) \approx 3.5\times 10^ 3J$$

Answer. (b) 3.5 × 103 J.