BRAINCAKEForces
Revision notes for Forces in OCR Physics A (J249). Every lesson in BrainCake comes with R.E.C.I.P.E. recall steps, a quick check and spaced reviews.
- Measuring distance and timeTo find how fast something moves we need two measurements: the distance it travels, in metres (m), and the time it takes, in seconds (s).
- Calculating speed from distance and timeSpeed tells us how far an object travels each second.
- Converting units and calculating ratesSpeed is often given in kilometres per hour (km/h) or in centimetres per second, but the SI unit is metres per second (m/s).
- Vectors and scalars: distance, displacement, speed and velocityA scalar quantity has only a size (magnitude).
- Interpreting distance-time and velocity-time graphsOn a distance-time graph the gradient is the speed.
- Area under a velocity-time graphThe area enclosed between a velocity-time graph line and the time axis is the distance travelled.
- Average speed for non-uniform motionIn a real journey the speed is rarely steady.
- Speed, distance and uniform accelerationIn uniform motion an object moves at a constant speed, so distance = speed × time.
- Ways objects interactA force is a push or a pull that happens when two objects interact.
- Interactions between pairs of objectsA force never acts on its own.
- Forces as vectors and free body diagramsA vector is a quantity that has both size and direction.
- Newton's first lawNewton's first law says that an object stays at rest, or keeps moving at a constant (uniform) velocity in a straight line, unless a resultant force…
- Vector diagrams, resultant force and equilibriumWhen several forces act on an object we can replace them with one resultant force.
- Forces on an isolated object and terminal velocityTo understand how an object moves, we can treat it on its own, as an isolated object, and look only at the forces acting on it.
- Free body diagrams and resultant forcesA free body diagram shows one object on its own, with an arrow for every force acting on it.
- Balanced forcesForces are balanced when they cancel out, so the resultant force on the object is zero.
- Newton's second law calculationsNewton's second law says that the acceleration of an object depends on the resultant force on it and on its mass.
- Inertia and inertial massInertia is a measure of how difficult it is to change the velocity of an object.
- Momentum and collisionsMomentum is the product of an object's mass and its velocity: momentum = mass × velocity, or p = m × v.
- Force, mass, velocity and accelerationTwo equations link the quantities in a motion problem.
- Work done by a forceWhen a force moves an object, energy is transferred.
- Stored energy and energy transfersWork done is force × distance, so one newton-metre (N m) is the same as one joule.
- Power as the rate of energy transferPower is the rate at which energy is transferred.
- Newton's third lawNewton's third law says that when two objects interact, they exert forces on each other that are equal in size and opposite in direction.
- Circular motion and changing velocitySpeed only tells you how fast an object moves.
- Forces needed to stretch, bend or compressAn object only changes shape when it is squashed, pulled or twisted by more than one force.
- Elastic and plastic deformationWhen forces stretch, compress or bend an object, the object is deformed.
- Force and extension for a springWhen a spring is stretched, its extension is the increase in length: extension = stretched length minus original length.
- Linear and non-linear force-extension relationshipsA relationship is linear if its graph is a straight line.
- Calculating the spring constantThe spring constant, k, tells you how stiff a spring is.
- Work done in stretching a springWhen a force stretches a spring, it does work on the spring and energy is transferred to it.
- Gravitational fieldsEvery object that has mass has a gravitational field around it.
- Weight and massThe weight of an object is the force acting on it because of gravity.
- Acceleration in free fallAn object is in free fall when the only force acting on it is its weight, so it accelerates downwards.
- Using W = m gThe equation W = m g links three quantities: weight W in newtons (N), mass m in kilograms (kg) and gravitational field strength g in N/kg.
- Forces causing rotationA force can make an object turn about a fixed point called a pivot.
- Moments and balanceThe moment of a force is its turning effect about a pivot.
- Levers and gearsA lever is a rigid bar that turns about a fixed point called the pivot.
- Fluid pressure and forceA fluid is a liquid or a gas.
- Pressure, force and area, and hydraulicsPressure is the force acting on each unit of area in contact.