A person pushes a crate with a horizontal force of 200 N over a horizontal distance of 5 m. How much work do they do on the crate?

40 J 205 J 1000 J 200 J

A 2 kg ball is moving at 6 m/s. What is its kinetic energy?

12 J 6 J 72 J 36 J

A 1 kg book is placed on a shelf 2 m above the floor (g = 10 m/s²). What is its gravitational potential energy relative to the floor?

2 J 10 J 20 J 200 J

A roller-coaster car (mass 500 kg) starts from rest at the top of a 40 m hill. Ignoring friction, what is its speed at the bottom? (g = 10 m/s²)

~14.1 m/s ~20 m/s ~28.3 m/s ~40 m/s

Why does the speed of a roller-coaster car at the bottom of a hill in a real park not quite reach the value calculated assuming no friction?

Gravity is weaker at the bottom of a hill than at the top The mass of the car increases as it accelerates Friction and air resistance convert some of the potential energy into thermal energy, so less is available as kinetic energy Conservation of energy breaks down at high speeds

An electric motor does 6000 J of work in 20 seconds. What is its power output?

120 000 W 300 W 30 W 3000 W

An engine receives 5000 J of fuel energy and delivers 3500 J of useful mechanical work. What is its efficiency?

35% 100% 70% 143%

Why can no real machine achieve 100% efficiency?

Because engineers are not skilled enough to design perfect machines Friction between moving parts always converts some input energy into heat, which is wasted Because energy is destroyed inside machines Because power is measured in Watts, not Joules

A crane lifts a 2000 kg steel beam 15 m in 30 seconds (g = 10 m/s²). What power does the crane's motor deliver (assuming 100% efficiency)?

1000 W 100 000 W 10 000 W (10 kW) 30 000 W

Car A travels at 20 m/s and car B travels at 40 m/s. Both have the same mass. How do their kinetic energies compare?

Car B has twice the kinetic energy of car A Car B has four times the kinetic energy of car A Both cars have the same kinetic energy because they have the same mass Car B has eight times the kinetic energy of car A

A person carries a 20 kg box horizontally across a room for 10 m at constant height. How much gravitational work do they do on the box?

1960 J — weight times distance 200 J — mass times distance 0 J — no work is done against gravity because the displacement has no vertical component 980 J — half the weight times the distance