Be able to convert between different units, including hours to seconds

The idea

Unit conversion means replacing a quantity expressed in one unit by the same physical quantity expressed in another, using a fixed conversion factor. For time, 1 hour = 60 minutes and 1 minute = 60 seconds, so 1 hour = 60 × 60 = 3600 s. To convert hours to seconds, multiply the number of hours by 3600; to go the other way, divide by 3600. For example, 2.5 h = 2.5 × 3600 = 9000 s, and 45 min = 45 × 60 = 2700 s. Other common conversions include 1 km = 1000 m, 1 m = 100 cm, 1 kg = 1000 g and 1 kW = 1000 W. Always keep the unit attached to the number and check that the size of the answer is sensible: converting to a smaller unit gives a larger number.

What earns the marks

• Multiplies a time in hours by 3600 to express it in seconds.
• Divides a time in seconds by 3600 to express it in hours.
• Converts minutes to seconds by multiplying by 60.
• Applies a stated conversion factor such as 1 km = 1000 m or 1 kg = 1000 g correctly.
• Checks that converting to a smaller unit produces a larger numerical value.

Common mistakes and how to put them right

• Multiplies hours by 60 instead of 3600.

  Put it right: Use 3600 s in one hour because each hour contains 60 minutes of 60 seconds.

• Divides when converting from a larger unit to a smaller unit.

  Put it right: Multiply when the new unit is smaller.

• Drops the unit from the final answer.

  Put it right: Attach the correct unit such as s, m or kg to every converted value.

Exam tips

• Write the conversion factor down before calculating so the method is visible.
• Convert all quantities to the same unit before comparing or substituting them into an equation.
• Sanity-check the answer: a time in seconds should be much larger than the same time in hours.