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Forces • Aidhm • What are forces • What are they measured in
Do objects move on their own? No. Something must cause it to move A force is something that causes an object to accelerate
Forces • Cause an object at rest to move • Cause a moving object to change its direction (velocity)
Isaac Newton Unit of force is called the Newton after Isaac Newton
Different Types of Forces • Pushing • Pulling • Weight • Friction • Magnetic Can you think of any other types?
What type of force is this? Pulling
Can a ball move without a force been applied to it? Yes: due to the force of gravity
Friction Friction is the force which opposes motion between two objects in contact Puck on ice
The advantages and disadvantages of friction Advantages • Walking • Holding objects • Wheels of cars and bikes • Disadvatages • Slows things down • Wastes energy • Gases in atmosphere effect surface of space shuttle
How do we reduce friction? Lubrication Oil: reduces friction in cars Synovial fluid: in joints in the body
How would you measure friction? Newton meter Block of wood Friction between block and table
Moon walk http://www.youtube.com/watch?v=OSJlL4wqLGo&feature=related
Weight Do astronauts have the same weight on the moon as they do on the earth? No The force of gravity is not the same on the moon as on the earth The weight of an object is the force of gravity on it Weight is a force. So what is it measured in? Newtons
Would you weigh the same if you stood on Mount Everest and stood at normal ground level? No. Weight depends on distance between two objects (person and the centre of the earth)
Mass versus Weight • 1.Weight of an object is the force of gravity on it • 2.Measured in Newtons • 3.Changes • depending on the force of gravity (moon and earth) • Depending on distance between objects (person on Mount Everest) 1.Mass is the amount of matter in an object 2.Measured in kilograms 3.Never changes no matter where you are
Are you finding the mass or weight of an object using scales? You are finding the mass
Formulae you need to know Force = Mass × Acceleration Newtons m/s2 kg When you drop an apple, its speed will increase 10metres in 1 second. Its acceleration is 10m/s2 Weight = Mass × 10 m/s2 Newtons Kg
(a) What is the weight of an object whose mass is 2 kg 5 kg 9 kg 500 g (hint 1000 g = 1 kg) (b) What is the mass of an object that has a weight of 30 N 4.5 N (c) What force is needed to give an object of mass 3 kg the following accelerations 3m/s2 5 m/s2 (d) What force is needed to give an object an acceleration of 4 m/s2 when they have the following masses 7 kg 8kg
What acceleration is needed to give an object A force of 12 N, mass of 3 kg A force of 15 N , mass of 3 kg What mass is an object if A force of 20 N and travels at an acceleration of 10 m/s2 A force of 50 N and travels at an acceleration of 25 m/s2
Elastic/Stretching Forces When a force is put on an elastic object it changes shape When the force is removed it returns to its original shape
Hookes Law The extension of a spring is directly proportional to the force that is stretching it http://www.youtube.com/watch?v=NfedS1tbV2o At 4 mins onwards
When icy weather is expected, the Co. Council or Corporation often spread sand on the road. Why do they do this? • If you slide down a rope, your hands can get burned. What causes this? • A spring has a 3N weight hanging from it and the total length of the spring is 20cms. When the 3N weight is replaced by an 8N weight, the total length of the spring is 40cms. What would the length of the spring be when there is no weight on it?