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Physics Forces
Physics Spring Y12
| Question | Answer |
|---|---|
| Free body diagrams | Show forces acting upon single object. Direction of arrow = force direction. Length of arrow = magnitude (should be proportional). Starting point is where force is acting. |
| Starting point of free body diagram arrows | Need to start on object (no pushing into it). Weight from CoM. NCF from bottom of object. |
| What sort of arrows for flying squirrel gliding at constant velocity | Weight straight down. Drag diagonally up opposite to movement direction. Lift diagonally up in direction of movement. Arrows should reach equilibrium. |
| What is coasting in a car? | No driving force |
| What is the force forwards for car/someone dragging a sled/someone pushing a book? | Thrust |
| Force up on submerged object | Upthrust |
| Mass m on pulley dragging trolley mass M. How to graphically show g with gradient | Forces on hanging mass is mg - T. Therefore mg - T = ma T = Ma We don't know tension, so try to remove ma = mg - Ma Ma + ma = mg a = m/(m+M) x g If m independent and a is dependent |
| If one of the forces is completely unknown, how to resolve | Resolve perpendicular to that force so you don't have to factor it in |
| Graph for drag y vs speed x through different fluids | Through air, the line slopes upwards slowly. Through e.g. water, the line is straight, through origin, with much higher gradient |
| Force of drag in different viscosity of fluids | F ∝ v in viscous fluids F ∝ v^2 in non viscous fluids |
| v t graph for falling object | Gradually decreasing gradient At start, resultant force is mg. a = g At other points, resultant is mg - D. a = g - D/m At terminal v, both are zero |
| Moment of a force | The turning effect of a force about some axis or point moment = force x perpendicular distance of the line of action of force from axis or point of rotation moment = fx |
| Equilibrium | No net force and no net moment |
| Principle of moments | MUST MENTION IN EVERY QUESTION, INCLUDING CALC For a body in rotational equilibrium, the sum of the anticlockwise moments about ANY point is equal to the sum of clockwise moments about the same point |
| State and explain what happens to tension in chains on a swing if the swing is pulled to make larger angle with vertical | Weight same so T must be larger (sketch it out |
| Centre of gravity | Point at which the weight acts on an object |
| Normally centre of mass if the same as centre of gravity, but when isn't it? | Only for large enough objects that different point have different grav field strength acting |
| To experimentally find CoG for 2D object | Suspend object so free to rotate. Hang plumb line from same point and trace it vertically down. Suspend at different point and repeat. Find intersect |
| How to find centre of scalene | Draw lines from points to midpoint of opposite side: not angle bisectors, which give centroid. |
| If CoG is to the right, what to say | It creates a CW moment about the picot (say what the pivot actually is) and falls to the right |
| What is a couple | A pair of forces that act on an object in opposite directions separated by a distance. |
| Resultant force and moment on a couple | Resultant force = 0 (no linear acceleration) but resultant moment not 0 (will have angular a) |
| Total moment of couple = | F x d. The force of each times the distance between them. The torque/moment of a couple is equal to Fd about any point Both forces act in the same clockwise/anticlockwise |
| Beam suspended from two points | Taking one point as the pivot, the force exerted up at the other point must balance out moment from CoM |
| How to derive the pressure on a surface from a column of water equation | P = F/A P = mass of water x g / A P = volume x density x g /A P = height x density x g |
| Archimedes' principle derivation | Find pressure on face 1 and on face 2/ Find expression for force on each face. Upthrust given by deltaF = F2 - F1. Find DeltaF in simplified form eventually gives deltaF = mass of water x g upthrust = weight of water displaced |
| Drag | Frictional force experienced by an object travelling through a fluid PMT factors: speed of the object, and the object’s cross-sectional area. Drag is proportional to the square of the speed, and it also increases as the cross-sectional area increases. |
| What is archidemes principle | The upthrust exerted on a body immersed in fluid, whether partially or fully submerged, is equal to the weight of the fluid that the body displaces |
| ticker timer for terminal v | Optionally instead of markings on tube of fluid, instead can attach via pulley a tape to the steel bearing, connected to a ticker timer, which makes 50 dots a second on the tape as the ball sinks |
| Centre of mass | The centre of mass is the point through which the application of an external force produces only motion in a straight line, with no rotation (from PMT) |
| Upthrust and why | When an object is submerged in a fluid, it experiences an upwards force, called upthrust. This is because the pressure at the bottom surface of the object is greater than at its top surface |
Created by:
Pyrogearos2