Calculating Efficiency
What does energy efficiency measure in a device or system?
What is how much useful energy is produced compared to the total energy input?
What two quantities do you multiply to calculate work done?
What are force and distance?
What is the formula for calculating power in physics?
What is Power = Work ÷ Time?
What is the gravitational potential energy of a 2 kg object lifted 3 m above the ground? (Use g = 9.8 m/s²)
What is 58.8 J?
GPE = m × g × h → 2 × 9.8 × 3 = 58.8 J
What is the kinetic energy of a 2 kg object moving at 3 m/s?
What is 9 J?
A kettle uses 1,000 J of energy and delivers 800 J of useful heat. What is its efficiency?
What is 80%?
(800 ÷ 1,000 × 100)
A person pushes a box with a force of 50 N over a distance of 4 m. How much work is done?
What is 200 J?
(50 × 4 = 200)
A machine does 600 J of work in 3 seconds. What is its power output?
What is 200 W?
(600 ÷ 3 = 200)
Where would an object have the highest amount of gravitational potential energy:
What is at the top of a hill?
Gravitational potential energy increases with height, so the higher the object is above the ground, the more GPE it has.
At which point on a rollercoaster does the cart have the greatest kinetic energy?
What is just before reaching the bottom?
Kinetic energy is greatest when speed is highest, which happens as the cart descends and converts gravitational potential energy into motion.
Why is no machine 100% efficient?
What is because some energy is always lost as heat, sound, or other forms of waste energy?
If a person pushes against a wall with a force of 100 N but the wall doesn’t move, how much work is done?
What is 0 J?
(No movement means no work is done.)
A person pushes a box with a force of 100 N over a distance of 5 m in 2 seconds. What is the power output?
What is 250 W?
First, calculate work:
(100 × 5)÷ 2 =250W
A ball with a mass of 500 g is lifted to a height of 2 m. What is its gravitational potential energy? (Use g=9.8 m/s²)
What is 9.8 J?
Convert mass: 500 g = 0.5 kg
Then:
GPE = 0.5 × 9.8 × 2 = 9.8 J
A ball with a mass of 500 g is moving at 4 m/s. What is its kinetic energy?
What is 4 J?
Convert mass: 500 g = 0.5 kg
A motor receives 2,500 J of electrical energy and produces 1,750 J of mechanical energy. What is its efficiency?
What is 70%?
(1,750 ÷ 2,500 × 100)
A machine does 600 J of work over a distance of 3 m. What force was applied?
What is 200 N?
(Work ÷ Distance = Force → 600 ÷ 3)
A motor has a power output of 150 W and operates for 10 seconds. How much work does it do?
What is 1,500 J?
Rearranged formula:
Work = Power × Time → 150 × 10
An object has 196 J of gravitational potential energy and is 4 m above the ground. What is its mass? (Use g=9.8 m/s²)
What is 5 kg?
196/(9.8x4)
An object has 72 J of kinetic energy and is moving at 6 m/s. What is its mass?
What is 4 kg?
A blender receives 12,000 J of electrical energy. It produces:
What is the efficiency of the blender?
What is 25%?
Only the kinetic energy is useful in this context:
Efficiency= (3,000/12,000) ×100=25%
A forklift applies a force of 1,200 N to lift a crate. The crate is raised by 250 cm. How much work is done in joules?
What is 3,000 J?
Convert distance:
250 cm = 2.5 m
Then: Work=1,200×2.5=3,000 J
A machine does 2,400 J of work with a power output of 120 W. How long did it operate?
What is 20 seconds?
Time = Work ÷ Power → 2,400 ÷ 120
An object with a mass of 3 kg has 294 J of gravitational potential energy. What height is it above the ground? (Use g=9.8 m/s²)
What is 10 m?
294 /(3 x 9.8)
An object with a mass of 2 kg has 50 J of kinetic energy. What is its velocity?
(Answer rounded to 2 decimal places)
What is 7.07 m/s (rounded to 2 decimal places)?