# Year 8 Physics Knowledge Organiser Forces 2 Motion

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Year 8 Physics Knowledge Organiser Forces 2 Motion Speed • The speed of an object tells you how long it takes an object to cover a distance. The unit for speed is m/s (metres per second). • Speed is calculated by dividing distance by the time • If the speed of an object is increasing, then it is accelerating. If the speed is decreasing it is decelerating. Distance Time Graphs • A distance time graph has the time on the x axis and the distance on the y axis. • If an object is stationary (not moving) the line will be horizontal. • If the line is diagonal the object is moving at a constant speed. • If the line has a larger gradient (steeper), it means the object is moving faster. • If the line is going back towards the x axis the object is returning to its starting point. Key Terms Definitions Speed How fast an object is moving, regardless of direction Velocity How fast an object is moving, taking direction into account as well Gradient How steep the line on a graph is. Stationary Not moving Acceleration A measure of how quickly the speed of an object is increasing Deceleration A measure of how quickly the speed of an object is decreasing Equation Meanings of terms in equation v = speed (m/s) s = distance (m) t = time (s) Acceleration and Deceleration • When an object is accelerating, the distance time graph will curve upwards. • When an object is slowing down an object will curve towards the horizontal. Higher gradient = faster speed Lower gradient = lower speed Acceleration Deceleration

Year 8 Physics Knowledge Organiser Forces 2 Motion Key Terms Definitions Displacement The distance an object moves from where it started. This is measured in metres. It is a vector quantity, because it includes the direction an object moved Gradient How steep the line on a graph is. Equation Meanings of terms in equation and units Acceleration • Acceleration is the measure of how quickly velocity changes. It is a vector quantity, because direction is included. • Acceleration can be calculated by measuring how fast the speed changes and dividing this by the time taken for the speed to change. The units are m/s 2 Velocity-Time Graphs • A velocity time graph shows how the velocity of an object changes over a journey. • Velocity is on the y axis and time is on the x axis • If an object is stationary (not moving) the line will be horizontal and will be on the x axis. • If the line is horizontal and above the X axis, the object is moving at a constant velocity • If the line is diagonal the object is accelerating at a constant rate. • If the line has a larger gradient (steeper), it means it is accelerating at a faster rate • If the line is going back towards the x axis the object is decelerating at a constant rate s = distance (m) v = speed (m/s) t = time (s) Calculating Acceleration from a velocity time graph • Acceleration is the measure of how quickly velocity changes. It is a vector quantity, because direction is included. • Acceleration can be calculated by measuring how fast the speed changes and dividing this by the time taken for the speed to change Calculating Displacement from a velocity time graph • Displacement is a vector quantity, it shows the distance travelled. • To calculate displacement you need to calculate the area under the line. • The example below shows some velocity time graphs in graph number 1 the area is a rectangle so the area can be calculated (4 x 15=60 m) • Graph 2 is a triangle so the area can be calculated (0. 5 x 6 x 15=45 m) 1 2 3

Year 8 Physics Knowledge Organiser Forces 2 Motion Key Terms Definitions Newton’s Second Law Objects accelerate if there is a resultant force acting on them. The amount of acceleration is proportional to the magnitude of the resultant force and inversely proportional to the mass of the object. (see equation) Equation Meanings of terms in equation and units Newton’s Second Law • Newton’s second law states that the velocity of an object will only change (accelerate or decelerate) if there is a resultant force acting upon it • Newton's second law states that acceleration is force divided by mass. • The equation shows that the size of the acceleration depends on the mass and the force applied. • For instance, if a resultant force of 20 N acts on the 10 kg object below, the acceleration will be 20 / 10 = 2 m/s 2. • But with this 20 kg object, the same resultant force only causes 20 / 20 = 1 m/s 2 acceleration. Newton’s Second Law • The equation that links force, mass and acceleration is shown in the box on the right • This means that if the force on an object is doubled then the acceleration on the object will also double. • We can say that force and acceleration are directly proportional. F = resultant force (N) m = mass (kg) a = acceleration (m/s 2)