Noadswood Science, 2012

 To understand the effect of a force on an object Monday, September 14, 2015

Rockets  Watch the demo of the water rocket - what forces are involved with it?  Can you explain why it accelerates up and why it eventually slows and begins to fall back to Earth?  How do you represent the forces?

Forces  The rocket accelerates upwards as the force propelling it up is greater than the force pulling it back down (the thrust of the rocket exceeds the force of gravity) - as rocket fuel is expelled one way, there is an equal and opposite force on the rocket in the opposite direction (giving it lift)  Eventually when the rocket has expelled all its fuel it will slow (gravity's force becomes proportionally larger) until a point where gravity becomes the dominant force and then it will drop back to Earth (unless it has escaped Earth's gravity)  Force can be represented by pairs of arrows (the bigger the arrow, the bigger the force)

Forces  What is a force?  A force is a push or a pull (measured in Newtons (N))  Gravity  Reaction force (e.g. from a surface, pushing you back up)  Thrust (push/pull)  Drag (resistance or friction which slows objects down)  Lift (e.g. from an aeroplane wing (where air flows over the top of the wing quicker than the bottom causing a pressure difference))  Tension  Forces are represented by arrows (always in pairs) which point the way the force is acting - the bigger the arrow, the bigger the force (balanced forces have equally sized arrows) Unbalanced forces - acceleration Balanced forces – steady speed

Newton  Newton developed the idea of the three laws of motion…  First law – balanced forces mean no change in velocity  Second law – a resultant force means acceleration  Third law – reaction forces

Newton’s First Law  First Law - balanced forces mean no change in velocity  As long as the forces are balanced then the object will stay at a constant velocity (this could be 0mph or 100mph)!  To go at a steady speed there must be zero resultant force Car moving at a steady speed (100mph) (balanced force) Car stationary (0mph) (balanced force)

Newton’s Second Law  Second Law - a resultant force means acceleration  If there is an unbalanced force then the object will accelerate in that direction  The size of the acceleration is determined by Force = mass x acceleration  Acceleration can be in the form of: starting; stopping; speeding up; slowing down; and changing direction Car accelerating (unbalanced force) m F a

Newton’s Third Law  Third Law - reaction forces  If object A exerts a force on object B, then object B exerts the exact opposite force on object A  Push against a wall, and the wall will push back against you just as hard (opposing forces must be there, else the wall would fall down)!

 Place the following in order of how fast they are travelling (from slowest to fastest) - what information do you need to work out how fast something is travelling? SR-71 BlackbirdLamborghini Daytona 675 Snail Olympic CyclistBulletSpace Shuttle Light

 Speed (and velocity) are measurements of how fast you are going (velocity includes direction) – to work out speed you need to know the distance travelled and the time it took… SR-71 Blackbird 2500 mph Lamborghini 200 mph Daytona mph Snail 0.05 mph Olympic Cyclist 30 mph Bullet 1500 mph Space Shuttle 17’000 mph Light 670,616,629 mph

Speed (s) Distance (d) Time (t) Speed = Distance  TimeTime = Distance  SpeedDistance = Speed x Time

 Jack ran 100m in 12 seconds. What speed was he traveling at?  Jack then ran 100m again, but this time it was much more windy, and it took him 15 seconds. What was his new speed, and why was this different?  My car was going at 50mph for 1 hour. How many miles did I travel  My car was going at 50mph, and I traveled 20 miles. How long did this take me? Speed Distance Time

 Jack ran 100m in 12 seconds. What speed was he traveling at? Speed = 100  12 = 8.34m/s  Jack then ran 100m again, but this time it was much more windy, and it took him 15 seconds. What was his new speed, and why was this different? Speed = 100  15 = 6.67m/s (more air resistance)  My car was going at 50mph for 1 hour. How many miles did I travel Distance = 50 x 1 = 50 miles  My car was going at 50mph, and I traveled 20 miles. How long did this take me? Time = 20  50 = 0.4 hours (24 minutes) Speed Distance Time

 Distance-Time graphs are used to show three pieces of information: -  Distance  Time  Speed (distance ÷ time)  Complete a rough graph of the following journey (plot the time (seconds) on the x-axis and the distance (meters) on the y-axis (x is across))  I left school to walk to the shop – I walked slowly for 10 minutes, covering a distance of 1500m. At the shop I stopped to talk on the phone for 5 minutes. Having realised I was about to miss my bus I ran for 2 minutes to the bus-stop which was 1800m away…

 Distance-Time graphs are used to show three pieces of information: -  Distance  Time  Speed (distance ÷ time)  Key information: -  10 minute (600 seconds) walk covering 1500m  5 minute (300 seconds) stationary covering 0m  2 minute (120 seconds) run covering 1800m  *Total time = 17 minutes (1020 seconds); total distance = 3300m

 Work out the speed during the different sections (walking; stationary and running)  Annotate your graph – what does the graph show?

Steady speed (2.5 m/sec) Steady speed (stationary) (0 m/sec) Steady speed (15 m/sec) Speed Distance Time 1500 ÷ 6000 ÷ ÷ 120

 In a distance-time graph the gradient = the speed  A flat section is where the object has stopped moving  The steeper the graph the faster the speed  However distance-time graphs can also show acceleration and deceleration: -  Steepening curve = speeding up (gradient increase)  Leveling-off curve = slowing down (gradient decrease) The gradient shows the speed in a distance-time graph: - (vertical ÷ horizontal)

 Annotate the following graph, explaining what is being shown – include the distances covered, the speed (was this constant speed / accelerating / decelerating) and the overall time for the journey…

Steady speed (15 m/sec) Speed Distance Time 300 ÷ 20 Steady speed (0 m/sec) 0 ÷ 20 Accelerating (5 m/sec) 100 ÷ 20 Decelerating (3.33 m/sec) 100 ÷ 30 Steady speed (16.67 m/sec) 500 ÷ 30 *The gradient shows the speed, e.g. on return journey the gradient = 500 ÷ 30 = 16.67msec -1

 How could you collect your own data for a distance-time graph with someone in the class running a 100m race (we want to see the changes in speed during different parts of the race)?  Measure the distance (100m!) and then every 10m have someone standing with a stop watch  Time how long it takes the runner to cover each 10m, then we can graph and note the speed differences between the start and end of the race…