Forces and Motion Notes

PHYSICS TOPIC 1 - FORCES AND MOTION This topic was in the first Year 10 and will be in the test on the week starting Monday 25th January 2016 UNITS, MOVEMENT AND POSITION PHYSICS TOPIC 1.1 Know all of the formulas below including the units and what they are used to calculate (BELOW). v = d/t a = v-u/t F = ma (Newton’s 2nd Law) W = m x g p = m x v Total p initial = Total p final (The initial and final momentum are equal) F = Change in p/t M = F x d (‘d’ is distance from pivot in this situation) Fa x da = Fc x dc v = 2 x 3.14 x r/t All of the units and letters for the formulas (ABOVE). d = distance (m), t = time (s), v = velocity (m/s), u = initial velocity (m/s), a = acceleration (m/s/s), m = mass (kg), W = weight (N), p = momentum (kg m/s), M = moment of turning force (Nm), F = resultant force (N) - Fa = Anti clockwise force (N) - Fc = clockwise force (N), r = orbital radius (m), T = orbital period (s), g = gravitational field strength (N/kg) PHYSICS TOPIC 1.2 A curving line with an increasing gradient on a DISTANCE - TIME graph shows acceleration. A curving line down shows deceleration on a DISTANCE - TIME graph. A graph can also show constant speed. This is when the line is straight; a straight line going up shows acceleration at a constant speed. Opposite for deceleration. PHYSICS TOPIC 1.3 The equation for speed/velocity is Speed = Distance/Time. The unit to measure speed/velocity is m/s. A short note - To convert m/s to km/hr multiply by 3.6. Displacement (vector) is distance travelled in a certain direction. Velocity is also a vector. It is speed in a particular direction. The Equation for velocity is Velocity = increase in displacement/time taken. Basically the same equation as Speed. PHYSICS TOPIC 1.4 [ Measure the acceleration of a falling mass with a ticker timers or data logger. ] - What it says on the syllabus. PHYSICS TOPIC 1.5 The equation for Acceleration is Acceleration = Change in velocity/time taken. The unit used to measure it is m/s/s. Acceleration is the rate at which an object changes its velocity. Deceleration - The object will have a smaller final velocity than the starting velocity. If you are finding the acceleration the answer should be negative. PHYSICS TOPIC 1.6 Velocity time graphs are different to Distance time graphs. In a velocity time graph the gradient or rise/run = size of acceleration, whereas in a distance time graph the gradient is the speed of the object. PHYSICS TOPIC 1.7 To find the instantaneous acceleration of an object at a specific point in the experiment, you need to draw/imagine a triangle under the line on the graph and put an A at the end of the line, a B at the end of the time axis and a C at the start of the time axis. You then need to find the gradient by doing AB/BC. PHYSICS TOPIC 1.8 To find the total distance travelled from a velocity time graph you need to calculate the area underneath the graph. This will tell you what the total distance travelled was. FORCES AND MOVEMENT PHYSICS TOPIC 1.9 Forces can make an object accelerate, decelerate or make it travel at a constant speed. Forces can even change the shape of materials. There are two different types of changes forces can make on an object one is temporary and the other is permanent. These are called elastic or plastic changes. Some materials such as glass are brittle therefore do not change shape easily. Others like plasticine and putty are not resilient but plastic; they change shape permanently even when small forces are applied. PHYSICS TOPIC 1.10 Forces between positive and negative charges is called electrostatic force. This is a type of force another one is gravitational. Below are some forces to remember when they occur. Air resistance is like friction, it tries to oppose the movement of objects through air. Objects that move through water experience a similar force called drag. It opposes the movement of objects through liquids. Upthrust is a force that acts on objects in air and liquid. It pushes the surroundings ( air or liquid ) up making the object go upwards despite the force of weight opposing it. PHYSICS TOPIC 1.11 AND PHYSICS TOPIC 1.12 If a quantity has size and direction it is a vector quantity e.g forces, displacements, velocity, acceleration,etc. When you add vectors the direction is important. If a quantity only has size then it is a scalar quantity e.g time, mass, energy, temperature, distance, speed,etc. When you add scalar units you just add the number together (the direction does not matter). Force is a vector quantity. PHYSICS TOPIC 1.13 To find the overall forces that act in the same direction, you just need to add them together. If the forces are acting in opposite directions you subtract the forces and the result is in the direction of the largest force. PHYSICS TOPIC 1.14 Friction is a force that opposes motion. The kinetic energy of the moving object is converted to heat as work is done by the friction force. Friction occurs when solid objects rub against each other and also when objects move through fluids (liquids and gases). Sometimes fluid friction is very desirable - for example, when someone uses a parachute after jumping from a plane. The arrow of the friction force will always point in the opposite direction to the direction of motion. Air resistance and drag are examples of friction. PHYSICS TOPIC 1.15 When the force pushing an object in its direction of motion is THE SAME AS the force opposing its motion it will stay at a constant velocity. When the force pushing an object in its direction of motion is THE SAME AS the force opposing its motion it will stay at a constant velocity. When the force pushing an object in its direction of motion is SMALLER than the force opposing its motion it will decelerate. Resultant force = Mass x Acceleration or F = ma The unit used to measure the resultant force is Newtons (N) or KiloNewtons (kN). When there is no overall force, either with no force or balanced force and constant velocity occurs this is a statement of Newton’s first law. When there is an overall force that causes an acceleration this is a statement of Newton’s Second Law. PHYSICS TOPIC 1.16 The weight of an object is the force that acts on it because of gravity. Weight = Mass x Gravitational Field Strength - This is used to find the weight of an object and the unit for this is N/kg. The value of g depends on how strong a planet’s gravitational field strength is.The gravitational field strength on Earth is 10N. PHYSICS TOPIC 1.17 When a object is falling through air, as it carries on falling the drag force increases. The faster the object move the larger the drag force becomes. Eventually both of the forces become equal and that is when a object reaches it maximum velocity also know as terminal velocity. Even if the object continues to fall it will not change speed. PHYSICS TOPIC 1.18 [ Be able to describe every part of different experiments to do with terminal velocity ] - What it says on the syllabus. PHYSICS TOPIC 1.19 The faster you travel the longer it takes to brake in a moving vehicle. The stopping distance = thinking distance + braking distance. There are many factors that effect the thinking distance some of them are: Vision of the obstacle ahead, the driver is under the influence of alcohol, if the driver is tired,etc. The higher the car’s speed, the further the car will travel during the thinking time. The braking distance is the maximum deceleration speed the car can have to brake. Vehicles with larger masses will have smaller rates of deceleration, therefore will travel further while braking. You can find the stopping distance by finding the area under a velocity time graph of the rectangle for thinking and the triangle for braking. Doubling the mass doubles the braking distance as the acceleration halves. Doubling the speed increases the braking distance by a factor of four. Thinking distance = speed x thinking time PHYSICS TOPIC 1.24 Newton’s third law states that the force that object A pulls of object B is the same as the force object B pulls on object A. Remember: Every force has an equal and opposite reaction force. The Earth pulling 1kg is the same as the force of gravity of 1kg pulling on Earth. MOMENTS PHYSICS TOPIC 1.25 Unbalanced moments cause objects to rotate. To increase the moment you can increase the turning force or the distance from the pivot point. Moments = turning force x distance from pivot or M = F x d. Make sure you can convert form unusual units. The unit used to measure moments is Nm. PHYSICS TOPIC 1.26 All objects have a centre of mass or centre of gravity and you can pretend this is where all of the mass in the object is contained. PHYSICS TOPIC 1.27 To calculate the unknown forces or distances that balance an object from stated forces and distances… Objects with different masses can be balances on a ruler as long as the heavier object is closer to the pivot point than the lighter object. If both objects are identical in mass and are placed at exactly the same distance from the pivot point then the ruler will be balanced. When an object is not turning around a pivot, the total clockwise moment and total anti-clockwise moment must be equal. The short way of remembering this is sum of the clockwise moments = sum of the anti-clockwise moments. PHYSICS TOPIC 1.28 From BBC Bitesize >> Calculating how to balance moments – Higher tier In your exam, you will be expected to calculate the force or distance that must be exerted on one side of a pivot in order to balance out the moments. Balanced moments Step 1: Work out the moment for which you have been given all of the information In this case it is the anti-clockwise moment. moment = force × perpendicular distance moment = 500 × 2 = 1000 Nm Step 2: Change the subject of the equation to calculate the force Remember that for the see-saw to be balanced, the total anti-clockwise moment must be equal to the total clockwise moment. Therefore the clockwise moment must be 1000 Nm. DEFORMATION OF SOLIDS PHYSICS TOPIC 1.29 As you apply force to any object you will change its size, tension forces will increase the size and compression forces will decrease the size. Extension means how much longer an object becomes after a force is applied compared to zero force. Experiments to show how the extension of a spring or rubber band changes with applied force.