Transcript Document
Newton’s Laws of Motion Newton’s First Law If there is no unbalanced force an object will move at constant velocity or remain at rest. Newton’s Second Law Fu = ma Newton’s Third Law Every force has an equal but opposite reaction force. Eg. Force of the rocket on the hot gases = - Force of the hot gases on the rocket Quantity symbol unit symbol Vector/ scalar distance time speed d t v metres second metres per second s s s displacement velocity s v metres metres per second acceleration a metres per sec per sec force friction F FR newton newton m d m/s ms-1 m m/s ms-1 m/s2 ms-2 N N v v v v v Balanced Forces Forces are balanced when they are equal in size but opposite in direction. If everyone pulls with the same strength there will be no movement. pull pull Forces are balanced. Buoyancy of water weight Forces balanced so no up down movement Bigger downward force boat moves down For balanced forces there is no change in movement. Force due to water buoyancy Force due to gravity weight Going down Going up Balanced Forces lift Terminal speed Air resistance thrust weight Object will be moving at constant speed or be at rest. Parachute weight is constant Sky diver Draw a speed time graph for a parachute jump No air resistance Just left the plane No speed, no air resistance. Downward unbalanced force – accelerate down weight Air resistance Speeding up, air resistance increasing. Unbalanced force is smaller so less acceleration. weight Air resistance Maximum speed ( terminal velocity) Air resistance = - weight so no unbalanced force No acceleration weight Air resistance The parachute is opened and air resistance increases. Upward unbalanced force, so the parachute decelerates and slows down weight Air resistance As speed decreases so does air resistance until the forces are balanced and terminal velocity is reached. weight Land !!! Speed is now zero Example 1. A car of mass 5000 kg is travelling at constant speed. The engine exerts a force of 20 N. (a) What is the magnitude of the friction forces? (b) The engine force increases to 100N. What is the car’s acceleration? Assume the friction forces stay constant. Tutorial Questions 51 to 58 SAQs up to 51 Purple book chp 2.1 Rockets 2. A rocket of mass 10 000kg was launched from the Earth. (a) If it had an initial acceleration of 2ms-2, what would the force of the rockets be? (b) As it moves away from the Earth, the acceleration increases, give three reasons why. Tutorial Questions 59 to 61 Lifts 3. A person of mass 60 kg stands on scales inside a lift. The lift accelerates at 4 ms-2 downwards. What will be the reading on the scales? Tutorial Questions 62 to 65 Purple book Chp 2.5 Connected Masses 4 . A force of 20 N is used to pull the blue box, mass 5 kg. Friction is 1N on each mass. The green box has a mass of 1.5 kg and the grey box has a mass of 2 kg. What is the acceleration of the blocks? What will be the tension between the green and blue boxes? Connected Masses 2 0.5 kg 1 kg 5. (a)What is the acceleration of the 0.5 kg trolley? (b)What is the tension in the string? Assume no friction. Tutorial Questions 66 to 70 SAQs up to 54 Purple book Ex 2.2 Forces at Angles 40 N 450 300 120 N 6. If the object has mass, 1500 Kg, what will its acceleration be? Tutorial Question 71 to 72 SAQs up to 56 Purple book Chp 2.4 Forces and Slopes 200 Fw 200 7. A mass of 10 kg is sliding down a slope. If it accelerates at 2ms-2, find:(a) The component of the weight acting down the slope. (b) The friction between the mass and the slope. Tutorial Questions 73 to 75 SAQs up to 62 Purple book Chp 2.4