Year 11 Science Science 1.1 Demonstrate an understanding of aspects of mechanics in one dimension.
SLO’s You will be able to give reasons why phenomena, concepts or principles relate to given situations in the context of; • Distance, speed, interpretation of distance and speed time graphs, average acceleration and deceleration in the context of everyday experiences such as journeys, sport, getting going, etc. Using v =Δd/Δt and a= Δv/ Δt
• Mass, weight and the acceleration due to gravity, balanced and unbalanced forces, in the context of everyday experiences such as being stationary, moving at constant speed, accelerating, etc. The relationship F net = ma. • Force and pressure in the context of everyday experiences. The relationship P =F/A • Work and power, gravitational potential energy, kinetic energy, and the conservation of mechanical energy in free fall situations in the context of everyday experiences such as sports performance, dropping things, tossing balls, etc. The relationships Δ EP = mgΔh, EK = ½ mv2, W=Fd and P= W/t.
Key formula’s. Distance and speed. • Velocity = change in distance v= Δd change in time Δt • Acceleration= change in speed a= Δv change in time Δt
Mass and weight • Net force = mass x acceleration Fnet= ma Force and pressure • Pressure = force P= F area A Work and power • Potential energy = mass x gravity x height ΔEP = mgΔh
Kinetic energy= ½ mass x speed squared Ek = ½ mv2 • Work (energy)= force x distance W=Fd Ew = Fd • Power = work time P = W/t
Using a formula. • How do we know which formula to choose? • Read the question; look for the data given, find the equation which fits the data and the question. • Sometimes you will need to use 2 or 3 formulae. • What if we need to rearrange the formula? • Put the equation into a ∆ and look for the data point you want to calculate.
Writing it out. • When you answer any exam question; • Write out the formula. • Substitute the data. • Write out the answer with units. W=mg W= 50 x 10 W= 500N
Review % % X
The car starts at rest and travels a distance of 6m in 3s, what is its average speed? Formula; _____________________ Answer; ____________________________________________________________ The cars mass is 400g. The car then travels a further 28m at a constant speed of 4ms-1 for 7s, how much kinetic energy does the car have? Formula; _____________________ Answer; ____________________________________________________________ An athlete runs for 8s at a speed of 14ms-1, calculate their acceleration. Formula; _____________________ Answer; ____________________________________________________________
The athlete then does some weight training; he lifts a 100kg dumbbell, what is the weight force of the dumbbell? Formula; _____________________ Answer; ____________________________________________________________ The athlete lifts the dumbbell above his head 1.5m, what is the work being done? Formula; _____________________ Answer; ____________________________________________________________ While the dumbbell is in mid-air it has gravitational potential energy, calculate how much energy is created. Formula; _____________________ Answer; ____________________________________________________________
While he is holding the dumbbell, he is standing on both of his feet. His feet have an area of 459cm2 then combined weight of the athlete and the dumbbells is 160kg, what pressure is being exerted? Formula; _____________________ Answer; ____________________________________________________________ The athlete finishes off his workout by cycling; he accelerates at 1.5ms-2 over a distance of 25m, he and the bike have a mass of 70kg. Calculate the net force acting upon the bike when it is accelerating. Formula; _____________________ Answer; ____________________________________________________________
If it takes 5s to cover the distance of 25m, calculate the power output of the bike during this time. Formula; _____________________ Answer; ____________________________________________________________ Formula; _____________________ Answer; ____________________________________________________________ Formula; _____________________ Answer; ____________________________________________________________
Distance & Time • Distance is a measurement of how far apart two points are. • The unit for distance is the metre (m), or kilometre (km). • Displacement is a measurement of how far an object has moved from its starting point. • The equation is: displacement = final – initial distance distance • Time is a measurement of the duration of an event. • The unit for time is the second (s), or hour (h).
D/t graph. Distance (m) Time (s)
Speed & Velocity • An object has speed when it travels a distance in a time interval. • At any moment in time, a moving object has instantaneous speed. Since this is difficult to calculate, we usually use average speed. • Since velocity is speed in a given direction,the equation for average velocity is: final distance – initial distance velocity = final time – initial time ∆d vav= ∆t • The unit for speed or velocity is metres per second (ms-1) or kilometres per hour (kmh-1). • Pg 8
Distance/time Graphs • Time always goes along the horizontal (x) axis • Distance always goes up the vertical (y) axis • A flat line (slope = 0) meansthe object is stationary • A slope means the object ismoving. The slope gives thespeed of the objectA straight line indicatesa constant speed. • A curved line means theobject is accelerating(speeding up or slowingdown) • Pg11 d t d t d t
What are these objects doing? d d d d t t t t
Speed/time Graphs • Time always goes along the horizontal (x) axis • Speed or velocity always goes up the vertical (y) axis • A flat line (slope = 0) meansthe object is travelling at a constant speed(no acceleration) • A slope means the object isaccelerating. The slope givesthe acceleration of the object • A curved line means therate of acceleration is increasing or decreasing • Pg14 v t v t v t
2cm 3cm 4cm Each is 0.1s 2/0.1 = 20ms-1 3/0.1 = 30ms-1 4/0.1 = 40ms-1
Area under graphs • The area under a velocity/time graph can be used to calculate the total displacement of an object • For a simple straight line, simple calculate the area of the rectangle or triangle under the line • For more complex shapes, split them into rectangles and triangles and add together the areas for each shape to calculate the total displacementeg. total displacement = ½ base x height v t
Area under the graph- 1 shape V ½ base x height t
Area under the graph- 2 shapes V ½ base x height Then add them together Base x height t
Acceleration • An object changing itsspeed is said to be accelerating. If the acceleration is: • positive (eg. 2ms-2) = object speeding up • negative (eg. -2ms-2) = object slowing down • The equation is: final speed – initial speed acceleration = time taken ∆v a = ∆t • The unit for acceleration is metres per second squared (or ms-2). • Pg 9
Deceleration • So deceleration is simple an object slowing down. • The same formula is used, but the result will be a negative number. • Pg 13, 14
Forces • A force is a push, pull, or twist. • Forces can change the: • speed • direction, and/or the • shape of an object. • The unit for force is the Newton (N) • When a force acts on an object (the action force), an opposing force appears as a reaction force (eg. gravity or friction). eg. driving force friction force (from engine) (from road/air) Pg 16
Thrust. • The force that moves objects in the direction of the force. • The greater the thrust, the faster the movement. Thrust
Friction • Friction is a force that opposes motion. • Friction always works in the opposite direction to thrust. • Friction between an object and air or water is called drag. Friction Pg 17
Friction • Friction is a force that opposes motion. • It is created when objects rub against each other, releasing energy as heat. • Friction between an object and air or water is called drag. • Friction can be: • useful – eg. brakes, tyres • undesirable – eg. engine wear • Friction can be reduced by using: • lubricants • bearings Pg 17
Support • The force that pushes upwards, so objects do not fall to the centre of the Earth. Support
Gravity • The force that pulls objects toward the centre of the Earth. • It is a constant force at 10ms-2. Gravity
Balanced Forces • When the action and reaction forces are equal in size and opposite in direction, they are balanced. • If the forces on an object are balanced it will either remain stationary, or continue moving at a constant speed. • eg. 450N 450N Net force = 0N Pg 18
Unbalanced forces. • When the action and reaction forces are different in size or direction, they are unbalanced. • If the forces on an object are unbalanced it will accelerate in the direction of the resultant or net force. 500N 450N Net force = 50N
Force, Mass & Acceleration • When an unbalanced force acts on an object, it accelerates in the direction of the net force. • The equation is: force = mass x acceleration Fnet = ma • The same equation also applies to the effect of gravity on mass (ie. weight). weight force = mass x acceleration due to gravity Fw= mg = m x10N (if mass is in kg) • The unit for weight force is the Newton (N). Pg 19
Gravity, Mass & Weight 600N 100N • Since weight is the result of the force of gravity acting on mass, weight can change depending on the force of gravity • On Earth, a person with mass 60kg would weigh 600N • On the moon, a person with the same mass would only weigh 100N • F=mg
Force and lifting. • Force is needed to lift an object. • This force equals the weight of the object. • The formula is; Force = mass x gravitational acceleration F = mg • E.g. A mass of 5kg is lifted 2m off the ground F = 5 x 10 F = 50 N • Work is also done to lift the object W = F x d W = 50 x 2 W = 100J 5kg
Forces and the shot-puts. Experiment 1; lifting the shot-put (F=mg) Experiment 2; work and the shot-put (W=Fd) Experiment 3; dropping the shot-put (F=ma) We also need to measure the area of the impact craters.
Force and pressure • Pressure is created when a force is applied to an area. • The formula is; Pressure = force / area P = F A • The greater the force and the smaller the area, the greater the pressure. 10N P = 10/8 P=1.25Nm-2 P= 10/2 P= 5Nm-2 8cm 2 2 cm2 Pg 20