Measuring Time 15 June 2018 By the end of the lesson we will have been successful when we have: recapped the units of mass Used our ideas about time to estimate and make a variety of measurements Started to plan an experiment to investigate how a Pendulum works

Time Time can be measured using clocks, stop watches, computers and light gates. Time is measured in years, months, days, hours etc. We will measure short periods of time in seconds, which is abbreviated to s.

Time Estimate how long the light was switched off for. Explain how you estimated the time for which the lamp was off.

Measuring Time Aim To practise using a stop watch and to time a ball down a ramp. Diagram Before you start though…

Below are tables to show how the height of the drop of a ball affects the height of the bounce. 1. Write one good feature for each table in your books. 2. Which is the best table and why? Height of drop (cm) Height of the bounce (cm) Average bounce height (cm)   1st attempt 2nd attempt 3rd attempt 30 21 22 23 32 34 25 24 Height Bounce 30 21 36 25 42 31 Height of drop (cm) Height of the bounce (cm) 30 21 40 50 Height of drop Height of the bounce Average bounce height   1st attempt 2nd attempt 3rd attempt 30 21 22 23 32 34 25 24

A Perfect Table has a title Independent variable as header in the first column Units included Units included Dependent Variable listed as header Average Averages have the same number of decimal places as the initial readings Nice straight lines, drawn with a ruler

How to evaluate your results Accuracy = how close a result is to the correct /true result with no errors. Precision = how similar results are when they are repeated by the same group or person

Precision

Example: Dissolving Salt Two groups of pupils wanted to find our how much salt dissolved at different temperatures. The results are shown below. Which temperature has the most reproducible results? Temp °C Group 1 – mass of salt (grams) Group 2 mass of salt (grams) Average grams of salt dissolved 30 1.7 40 1.9 2.1 2.0 50 2.2 4.0 3.1 60 3.3 70 4.7 4.5 4.6 20 6.0 5.8 5.9

Why do we repeat readings Why do we repeat readings? Copy the box below, discuss with a partner and tick those that are a good reason to repeat. Are these reasons to repeat? To use scientific equipment So we can get more evidence Because the first reading might not be right To make the test fair To check results So we will get a better idea of the likely result Because readings are always a bit different so we need to do them more than once Because things change a little bit so we need to take an average So we can change the equipment Reasons = 6, 7, 8, 2, 5, 3

Pendulums were first used to make time measurements What is a pendulum? Where are they often found?

Pendulum - A pendulum is any mass which swings back and forth on a rope, string or chain. - They are often found in old clocks and other machinery

Pendulum If you pull the mass away from its rest position, so that the string is at an angle, and then let go, the mass will swing back and forth. The length of time it takes the mass to swing all the way over and back once is called the period of the pendulum.

Pendulum When the pendulum is at the top of it’s swing it is momentarily stationary. As the pendulum rises the kinetic energy is transferred back to potential energy. The speed increases as the pendulum falls and reaches a maximum at the bottom of the swing. As the pendulum falls the potential energy is transferred to kinetic energy. The speed of the pendulum decreases and falls to zero as it reaches the top of its swing, with the potential energy a maximum again.

or the height/angle of drop of the bob. What to record: We are going to investigate how one of the following variables changes the time period, either: the mass of the bob or the height/angle of drop of the bob. What to record: Use the sheet “investigating pendulums” to help you write up your experiment. By the end of the lesson we will have been successful when we have: Investigated a question in relation to how a pendulum works Demonstrated that we can draw a good table

Results Table Independent variable Time taken for 10 swings /s Time taken for one swing/ s

When you have finished… Think about… The relationship between your results Does your independent variable affect the dependent variable? Could you show your results in a graph?

Homework – Pendulums Complete the sheet “Evaluating Data in our Pendulum Experiment”