Motion, Forces & Energy

• The SI unit for length is metre (m), for mass is kilogram (kg), for time is second (s), for force is newton (N), for energy is joule (J), for power is watt (W).
• The multiplication factor for giga (G) is 10^9, mega (M) is 10^6, kilo (k) is 10^3, centi (c) is 10^-2, milli (m) is 10^-3, micro () is 10^-6, nano (n) is 10^-9.
• A scalar quantity only has magnitude, while a vector quantity has magnitude and direction.
• Examples of scalar quantities are mass, distance, speed and energy.
• Examples of vector quantities are force, weight, velocity, acceleration and momentum.
• If the speed is changing, distance divided by time calculates the average speed.
• Velocity is the speed in a specified direction.
• The gradient of a distance - time graph is equal to the speed.
• To calculate the gradient of any straight line graph, divide the change in y by the change in x. To do this, you will need to draw a triangle (to increase accuracy, make sure the triangle is big)
• On a distance - time graph, if the speed is changing, you can find the speed at any instant by drawing a tangent and measuring its gradient.
• The area under a velocity - time graph is equal to the distance travelled.
• The gradient of a velocity - time graph is equal to the acceleration.
• Your weight is the gravitational force that the Earth exerts on you. The Newton III pair of your weight is the gravitational force you exert on the earth
• In a collision between two objects, the momentum of the whole system before the collision is equal to the total momentum of the whole system after the collision.
• Newton’s second law shows that Inertial mass is defined as the ratio of force to acceleration.
• The direction of a force which acts towards the centre of a circular path is called centripetal.
• An object moving at constant speed in a circle is accelerating because its velocity is constantly changing direction.
• In a collision between two objects, because momentum is conserved, the change in momentum of one object must be equal and opposite to the change in momentum of the other object.
• Force is measured in newtons, mass is measured in kilograms, and acceleration is measured in metres per second squared.
• Momentum is a vector quantity.
• We can use momentum to find the (average) force that one object exerts on an other during a collision because the force is equal to the change in momentum divided by the collision time.
• The principle of conservation of momentum tells us that momentum is conserved provided that no external force is acting.
• The two objects must exert equal (and opposite) forces on each other (Newton’s Third Law) because they experience the same change in momentum and the same collision time.
• Newton’s third law says that if Object A exerts a force on Object B, then Object B exerts an equal and opposite force of the same type on Object A.
• An object will move in a circular path at constant speed when the accelerating force is perpendicular to the direction of motion (the direction of the velocity).
• An object may have energy due to its motion, for example, a moving car has kinetic energy.
• A typical human reaction time is about 0.25 seconds.
• Reaction time is a measure of how much time passes between someone seeing something and reacting to it.
• The stopping distance of a vehicle is equal to the thinking distance plus the braking distance.
• Factors affecting a driver’s reaction time include distractions, alcohol, drugs and tiredness.
• An object may have energy due to its position in a gravitational field, for example, a raised weight has gravitational potential energy.
• Safety features such as crumple zones and air bags are designed to increase the deceleration time so that the size of the deceleration is reduced, which means that the damaging force is reduced (as F = ma ).
• Types of energy include kinetic, gravitational potential energy, chemical, thermal or heat, and elastic potential energy.
• The law of conservation of energy states that energy cannot be created or destroyed, but can be transferred from one form to another.
• In a car crash, you experience a large deceleration which means that the force acting on you must be large.
• The braking distance (d) can be calculated if you know the braking force (F) and the mass (m) and velocity (v) of the car. The loss in kinetic energy is equal to the work done by the braking force.
• The braking distance is proportional to the velocity squared, meaning if you are driving twice as fast, your braking distance will be four times greater.
• You can measure someone’s reaction time by getting them to catch a ruler which you hold vertically just above their thumb and forefinger and then release unexpectedly.
• The stopping distance is affected by the mass of the vehicle, the speed of the vehicle, the state of the vehicle’s brakes, the state of the road, the amount of friction between the tyres and the road surface, and the driver’s reaction time.
• During the driver’s thinking time, the car is travelling at constant speed so the thinking distance is equal to the speed of the vehicle multiplied by the reaction time.