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# Speeding up and slowing down - Answers

Speeding up and slowing down - Answers. f = ma Terminal velocity. What are the four forces acting on an aeroplane. Thrust, drag, lift, weight.

## Speeding up and slowing down - Answers

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1. Speeding up and slowing down - Answers f = ma Terminal velocity

2. What are the four forces acting on an aeroplane. Thrust, drag, lift, weight. • How do each of the four forces act on an airplane in flight? (Which direction does each force push or pull on the airplane in flight?)with motion, against the motion, upward, downward • Which force on an airplane is always directed toward the centre of the earth? weight • Which force is responsible for moving the airplane forward? thrust • Which force do the wings of the airplane generate? lift • For questions 7-10, predict what effect the following flight conditions would have on the plane. (Would the plane rise, fall, slow, or accelerate?) • Drag > Thrust slow down (decelerate) • Lift > Weight rise • Thrust > Drag accelerate • Weight > Lift fall • Define balanced and unbalanced. Balanced – when all the forces acting on an object are equal. Unbalanced – when the forces acting on an object are not equal. • During a flight when we are cruising at a specific altitude, the four forces are balanced. At which points during a flight are the forces unbalanced? Take off, climbing to cruising altitude, descending to land, landing. • Define Newton's First Law of Motion. An object will stay at rest or continue at a constant velocity unless acted upon by an external unbalanced force • In your own words, explain how Newton's First Law of Motion explains the motion of a cruising airplane. The plane will stay cruising at a constant speed and height unless another force acts upon it when a change in direction or speed will take place.

3. Force, mass, acceleration The diagram shows an athlete accelerating. The athlete has a mass of 65 kg and produces a constant forward force of 364 N. (i) Write down the equation that links acceleration, force and mass. F = ma (1 mark) (ii) Calculate the maximum acceleration of the athlete. Show clearly how you work out your answer. (2 marks) a = F/m = 364/65 = 5.6m/s2 (b) The athlete’s forward force is the same throughout the race. Explain why the acceleration of the athlete decreases to zero during the race. As the forward velocity increases the air resistance also increases and eventually the forces will be equal. (2 marks) • A car accelerates from 0 m/s to 10 m/s in 50 seconds.  What is the acceleration?(a=(v-u)/t) a=10/50=0.2m/s2 • A train decelerates from 30 m/s to 25 m/s in 2 minutes.   What is the acceleration?  (Answer to 2 s.f.) (a=(v-u)/t)a=5/180=0.042m/s2 • A car of mass 900 kg has an engine which exerts a force of 450N on the car. Assuming 100% efficiency, what is the minimum amount of time needed for the car to accelerate from 0 to 10 m/s?(f=ma) a=450/900=0.5 (a=(v-u)/t) t=10/0.5=20s

4. Terminal Velocity 0 seconds 10 seconds 20 seconds 30 seconds Fair = 400N Fair = 800N Fair = 1000N Fweight Fweight = 1000N Fweight = 1000N Fweight = 1000N Fweight = 1000N • Define the term terminal velocity. The velocity reached when all forces acting on an object are balanced • In each of the above diagrams what is the resultant force? A 1000N Down, B 600N Down, C 200N Down, D 0N Down. • Given that the acceleration of the parachutist is constant within all of the time frames, plot a speed time graph for the parachutist. (Use the graph paper over the page)

5. 320 (294) 280 240 200 160 Speed (m/s) 120 80 40 0 10 20 30 40 Time (s)

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