JC Science: Force, Work and Power

Force, Work and Power

Force A force is something that causes an object to change velocity or accelerate. The newton (symbol N) is the unit of force.Forces can be measured using a spring-balance (the proper name is a newton-meter). Friction Friction is a force that opposes motion between two objects in contact. Friction can be a nuisance (e.g. it slows things down when you don’t want it to when driving a car or riding a bike).Friction can also be useful (e.g. brakes on a bike, and to enable us to walk). Experiment: To demonstrate friction and how a lubricant can be used to reduced friction. Attach a spring balance to a wooden block and use it to pull the block over the bench. Note the reading on the spring-balance. Now put some oil or grease under the block and repeat. Notice that the force required to pull the block is much less. Hooke’s Law The extension of a stretched string is directly proportional to the force that is stretching it This means that if the force on the string doubles so will the extension; if the force triples then the extension will also triple, etc. Experiment: To investigate the relationship between the extension of a spring and the force that is stretching it. 1. Use a newton-meter to stretch a spring and note the extension and the force used. 2. Repeat for lot of different forces. 3. Plot a graph of force used against extension. 4. Result: Your graph should be a straight line. 5. Conclusion: The graph is a straight line through the origin which shows that the extension is directly proportional to the applied for {Strictly speaking stating Hooke’s Law is not on the syllabus but it did get asked by mistake one year so it’s best to learn it just in case it happens again. You do however have to know the experiment.} Weight and Mass Mass is a measure of the amount of matter (“stuff”) in something. The weight of an object is a measure of the gravitational pull on it. Weight is a force (due to gravity) The mass of an object doesn’t change from one place to the next because it still has the name amount of stuff inside (the same number of atoms), but the weight of the object will be much less on a planet smaller than Earth (or on the moon) while it will weigh much more (it will be much ‘heavier’) on a bigger planet. In fact if you could go to one of the bigger planets you would not even be able to stand up because you weigh so much! Maths Problems Weight (in newtons) = Mass (in kilograms) × 10 e.g. a student of mass 60 kg has a weight of 600 N. Note: There are 1000 grams in a kilogram (kg). A little more on Mass and Weight There is a very important and often misunderstood difference between mass and weight. Mass is the amount of matter present in the object. It doesn’t change, i.e. the amount of ‘stuff’ in your phone doesn’t change, regardless of where you bring it (even if you bring it to the moon it will still have the same number of atoms in it.) Another way to look at it is to compare weight, which is caused by gravity, to magnetism. Bringing a magnet from one place to another won’t change the amount of stuff (mass) in it, but if you bring the south end of a magnet closer to the north pole of another magnet it will experience a greater attraction to the north pole. It will appear to be heavier. The force of attraction between two magnets depends partly on the strength of the two magnets. Similarly the force of gravitational attraction between two objects depends partly on the mass of the two objects. ‘Weight’ is our shorthand way of talking about gravitational attraction. So the gravitational attraction between you and the Earth is greater than the force of attraction between you and the moon. Weight is the force with which the object is attracted to the Earth (or other planet) by gravity. Weight (in newtons) = Mass (in kilograms) × 10 The number 10 is an indication of the size of the force of gravity. It would be too much of a coincidence for it to be exactly 10. It turns out to be about 9.8, and because the force of gravity is less on the moon than on the Earth, this number is smaller for the moon (it’s about 1.6). Energy, Work and Power Energy is the ability to do work Energy The unit of energy is also the joule. Work done = Force × distance Work The unit of work is the joule (J). Power Power is the rate at which work is done The unit of power is the watt (W)