Motion, Forces, and Energy Revision (Chapters 3+4) What is force? The force exerted on a surface divided by the total area over which the force is exerted. What is the link between pressure & area? Pressure decreases as the area over which a force is distributed increases. The formula Pressure = N/m 2 = Pa (Pa = pascal) Force = N Area = m 2 Can you find the area by using the pressure & force? Yes. Area = force pressure. Can you find the force by using the pressure & area? Yes. Force = pressure X area. What is density? Its mass per unit volume. The formula Density = g/cm 3 Mass = g Volume = cm 3 What is Archimedes Principle? The buoyant force acting on a submerged object is equal to the weight of the fluid the object displaces. What is Pascal s principle? When force is applied to a confined fluid, the change in pressure is transmitted equally to all parts of the fluid. What is Bernoulli s principle? The speed of a moving fluid increases, the pressure within the fluid decreases. Changing Density If a submarine wants to make a dive, (point 1) the tanks are open so that water is taken into it. (point 2) the density of the submarine is greater than the buoyant force. The submarine s weight > the buoyant force.
If a submarine want to float, (point 1) the tanks are filled with water till its density is equal to the water. (poin2) meaning its weight equals the buoyant force. The submarine s weight = the buoyant force. If a submarine wants to make a rise, (point 1) compressed air is blown into the tanks which forces the water out. (point 2) the decrease density of the submarine makes its weight less than the buoyant force. The submarine s weight < the buoyant force. Question: look at the following graph then answer the questions: 1) What is the variable on the y-axis? Lifting Force. 2) What is the variable on the x-axis? Applied Force 3) What is the unit on the x-axis? Newton. 4) What is the unit on the y-axis? Newton. 5) How many lifts do we have? 3 lifts; lift A, B and C 6) For lift A: how much force is required to lift it when the applied force is 7500? 24000 N. 7) For lift C: how much force is required to lift it when the applied force is 5000? 22000 N 8) For lift B: how much force must be applied to lift a 4500 N lift? 5000 N. 9) For lift A: how much force must be applied to lift a 16000 N lift? 6000 N Question: A book applied a fore of 6 N on your desk, which occupied an area of 3 m 2. What is the pressure here?
Pressure = 6 3 = 2 Pa Pressure =? Force = 6 N Area = 3 m 2 Question: You calculated that water exerted a pressure of 8 Pa on 4 m 2 bottle. What is the force here? force = 8 X 4 = 32 N Force = Pressure X Area Look at the following densities. How would you know if they float or sink from first glance? Which floats & which sinks? Substance Density (g/cm 3 ) Wood 0.7 Water 1 Plastic 0.93 Glycerin 1.26 By taking water as my basis. Any substance > 1 g/cm 3 = sinks. Water is less dense here. Any substance < 1 g/cm 3 = floats. Water has more dense here. Floats = wood & plastic. Sinks = Glycerin. Based on the picture, answer the questions: A) B) Before we start (A), what is the object s weight? 10 kg. From (B) What is the weight of the object after being submerged? 7 kg.
What is the buoyant force against the object? 3 N. What is the dispersal liquid? 3kg What is the link between the dispersal here and the buoyant force? The buoyant force is equal to the dispersal's weight. What happened to the object s weight being decreased? Where did the rest of the weight go? When we submerged the clock, the buoyant force on the opposite direction pushed the object and that is why it appears to be here as 7. The dispersal liquid here is the missing weight of the object. That is why when we weigh them then added them together (object + dispersal) gives us the whole weight before the submerged. (normal weight) 10 = (submerged weight) 7 + (displaced weight) 3 Atmospheric pressure= 10.13 N/cm 2 at sea level. The standard barometric pressure = 1013.2 millibars at sea level. Work: (point 1) the amount of work done on an object can be determined by multiplying force times distance. (point 2) [the formula] Work = Force X Distance (point 3) if there was force applied on an object, but there were no appeared distance. Then the work is not done. (point 4) work's unit is Joule (J) Work (J) = Force (N) X Distance (m) Power: (point 1) the amount of work done on an object in a unit of time. (point 2) [the formula] Power = Work Time (point 3) work's unit is watt (W) Power (W) = Work (J) Time (s) (point 4) watt is a small unit of power, so we use one kilowatt (KW) = 1000 watts. (point 5) when we want to talk about vehicles' engines using power, we use horsepower. 1 horsepower=746 W (not an SI unit) Question: What is the mechanical advantage of the bottle opener you used when the input force applied on to it is 5 N and the output force is 7 N. Input force = 5 N output force = 7 N MA= 7/5 = 1.4 (no units) Question: A crane was working with 570 000 J and finished the work at 480 000 J, what is the efficiency of the crane? Input work= 570 000 J Output work= 480 000 J efficiency = 480 000 570 000 X 100% = 0. 8421 X 100% = 84 % Question: you weight a metallic cup being 1.5 g and you had its density as 1.29 g/cm3. Calculate the volume of that cup. Density = Mass Volume Volume =? Density=1.29 g/cm 3 Mass=1.5g
Volume = Mass Density Volume = 1.5 1.29 = 1.16 cm 3 Inclined + axe +wheel and axel Question: A man was pushing a shopping cart. He walked for 12 m and reached the top of the area which was 8 m height. What is the (ideal) mechanical advantage? Length of the incline plane = 12 m Height of the incline plane = 8 m Ideal MA = 12 8 = 1.5 (no units) Question: What is the mechanical advantage for the wheel and axle when the axle's radius is 2 cm and the wheel's radius is 10 cm. Radius of the wheel = 10 cm Radius of the axle = 2 cm MA = 10 2 = 5 (no units) Question: levers and pulleys have different types, what is the method of classifying or tell apart between each simple machine? Levers are based to the location of the fulcrum to the input and output forces. There are 3 types. While pulleys are based on fixed, movable. There are two types. The third is a joint between the two.