Year 9 Textbook

St. Mary’s College Science DepartmentYear 9 ScienceExtension WorkDo you remember the scientist called ‘Arcimedes’? Hediscovered that when an object is put in water the‘upthrust’ force is equal to the weight of the water that hasbeen displaced.So if an object weighs more than the water it displaces itwill sink and if it weighs less than the displaced water, itwill float!You can investigate this more closely with your teacher.Don’t worry – you don’t have to know this bit for your End of Unit Test onForces!Homework 61. Copy and complete this paragraph…Weight is a _______ caused by the force of ______ pulling things downtowards the ground. When an object is placed in water there is a forcecalled _________ pushing up on the object. When an object sinks theupthrust is __________ than the weight and when it floats the upthrust is__________ to the weight.Extension Homework1. Look at the diagram. The weight of theobject in air is 40N. Its weight is wateris 25N.What is the upthrust force of thewater?Does this object float or sink?Extension Activity…You could investigate the surface tension in water too.Try this at home. Place a paper clip in a glass of water – it floats.Now put one drop of Fairy liquid into the glass of water. What happens?44

St. Mary’s College Science DepartmentYear 9 ScienceThe last thing you need to know in this topic is how to work out the resultantforce when there is more than one force in a situation.Imagine that Man Utd beat Liverpool in a match by a score of 3-1.Man Utd win the match but by how many do they win?The answer is that Man Utd win by 2 goals : 3 minus 1 = 2.This is the result of the match.What if two forces are acting against each other? Look at the diagram below.In the top diagram the forces are thesame size but in the opposite direction.We say that the forces are balanced sothe resultant force is 300 – 300 = 0.In the second diagram however theteam on the left have a force of 400Nso the resultant force is 400 – 300 =100N to the left.Let’s say that the woman ispushing the sofa with a force of200 Newtons and the frictionforce is 150 Newtons.So the resultant force is 200 –150 = 50 Newtons and thewoman can move the sofa.Look at these diagrams and how the resultant forces are calculated.45