Using delta U_grav = m g (delta h), what is the change in gravitational potential energy for a mass of 5 kg moved upward by 2 m (g ≈ 9.8 m/s^2)?

• A. 49 J
• B. 196 J
• C. -98 J
• D. 98 J

Answer: (D)

Lifting a mass against gravity increases its gravitational potential energy, because you’re doing work to raise it. The change in gravitational potential energy is given by delta U = m g delta h. Here the mass is 5 kg, gravity is about 9.8 m/s^2, and the height increases by 2 m, so delta h = +2 m. Compute: delta U = 5 × 9.8 × 2 = 98 J. The positive sign shows the energy increases by 98 joules when you lift the mass. If you had lowered it by 2 m, delta h would be -2 and delta U would be -98 J, energy decreasing as you descend.