Year 11 Physics Tutorial 84C2 Newton s Laws of Motion

Transcription

1 Year 11 Physics Tutorial 84C2 Newton s Laws of Motion Module Topic 8.4 Moving About 8.4.C Forces Name Date Set 1 Calculating net force 1 A trolley was moved to the right by a force applied to a cord attached to the trolley. A spring balance of negligible mass was connected between the trolley and the point of application of the force, as shown. masses on trolley In Experiment 1 the mass of the trolley was kept constant and the force applied was gradually increased. The acceleration of the trolley was measured for each force applied and the results recorded in Table 1 below. In Experiment 2 the force applied to the trolley was kept constant and the mass of the trolley was varied. The acceleration of the trolley was again measured and the results were recorded in Table 2 below. Friction was negligible. Table 1 Mass constant Table 2 Force constant No. Force applied (N) Acceleration produced (ms - 2 ) No. Mass (kg) Acceleration produced (ms - 2 ) a) Using the data listed in the tables, draw graphs of: i. a versus F for experiment 1 ii. a versus m for experiment 2 iii. a versus m 1 for experiment 2

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3 b) What is the shape of the graph of a versus F? c) How is the gradient of this graph related to the mass being accelerated? d) What is the value of the total mass being accelerated in experiment 1? Page 3

4 2 A mass of 4.5 kg is being moved along a horizontal, rough surface by means of a string attached to it. a) What is the net force on the mass? b) What acceleration of the mass would the net force produce? 3 A force of 40.0 N is applied to push an object up a smooth slope against the gravitational tendency of the object to slide down. The acceleration of the object is 2.0 ms - 2 and its mass is 4.0 kg. a) What is the size of the net force on the object? b) What is the size of the force which must be acting down the plane? Page 4

5 4 A car of mass 800 kg accelerates from 0 to 60 kmh - 1 at the rate of 2 ms - 2. a) If the acceleration is uniform, what net force acts on the car? b) This force would obviously be supplied by the car s engine. However, it is necessary for the output of the engine to supply a force greater than this why? 5 Sally finds that she has to push her baby Kim in a 10.0 kg pram with a force of 17.0 N to overcome the 5.0 N frictional force and give the pram an acceleration of ms - 2 (but not for very long!). What is baby Kim s mass? 6 The force with which the earth attracts objects (i.e. gravity ) is approximately 9.8 N on a 1.0 kg mass near earth s surface that is, F g has the value of 9.8 Nkg - 1. the a) If this was the only force acting on a 1.0 kg mass, what acceleration would it produce? b) If a second 1.0 kg mass was attached to the first, what force would now act on the combined masses? c) What acceleration would the combined masses have if the force in (b) was the only force present? d) Supposing 100 masses, each of 1.0 kg, were linked together. What would the force on them due to the earth s attraction? e) What acceleration would the 100 masses in (d) have if no other force acted? Page 5

6 f) What do the results of (a), (c) and (e) suggest about all falling objects? 7 A student pushes two blocks, masses as shown, along a smooth table with a force of 36.0 N. Block 1 pushes against block 2 but is not attached to it. a) What is the acceleration of the two blocks? b) What must be the net force on each block to produce this acceleration? c) Block 1 has a force of 36.0 N pushing it to the right. However, the net force on this block is considerably less than this. What other force causes this result? d) If the two blocks were connected by a string and pulled with a second string, as shown, would the acceleration of the blocks be the same? e) Write an equation of motion involving the tensions in the strings for each of the blocks in (d). f) Find the tension force in each string at X and Y. g) In what direction does the tension force of the connecting string (string1) act on block 2? Page 6

7 8 The force of gravitational attraction between two objects, such as the earth and a satellite, drops according to the inverse square law as the distance between the two objects increases that is, F G 1 (this F G is called 2 d the weight force). If a satellite of mass 1000 kg is launched from the earth s surface (distance d to the centre) and placed in orbit at a distance 2d, as shown: a) Determine: (i) the weight of the satellite on earth (remember the data from question6); (ii) the weight of the satellite in orbit; (iii) the force with which the satellite attracts the earth when in orbit. (iv) Is the mass of the satellite the same in orbit as it was on earth? (v) If air resistance on the satellite is negligible, is the weight force of the satellite also the net force acting on it? 9 In the situation below a scale of 1.0 cm = 4.0 N is used to represent the forces acting on a cart of mass 0.8 kg. a) What is the size of: i. F applied ; Page 7

8 ii. F friction? b) Determine the net force. c) What will be the acceleration of the cart? d) If the cart reached a velocity of 30 ms - 1 and the F applied was removed, what then would be the value of the car s acceleration? Set 2 - Miscellaneous Force Problems 10 A trolley is projected along a long, level surface by means of an unattached spring compressed between the trolley and a wall. The trolley is released from point 1 in the diagram and is pushed by the spring until it reaches point 2, at which stage there is no longer a push by the spring and the trolley continues its motion to the right. The average force exerted buy the spring is 40.0 N and the retarding forces total 5.0 N throughout the trolley s motion. Using this information, calculate:. a) the net force on the trolley between points 1 and 2; b) the acceleration of the trolley between points 1 and 2; c) the net force acting on the trolley as it passes points 1 and 2; d) the velocity of the trolley at point 2; Page 8

9 e) the velocity of the trolley as it passes point 5; f) whether or not the trolley will reach point A ball of mass 0.5 kg is thrown vertically upwards with an initial velocity of ms - 1 (i.e. positive 100 ms - 1 ). It is allowed to fall back to its original position. Air resistance can be regarded as negligible. a) What force acts on the ball during its upward movement? b) What force acts on the ball during its downward movement? c) What is the acceleration of the ball during its upward movement? d) What is the acceleration of the ball during its downward movement? e) What is the maximum height reached by the ball? f) How long would the ball take to reach its maximum height? Page 9

10 12 Frank, of mass 60 kg, is standing on a set of bathroom scale in a lift which is moving upwards with an acceleration of 2.0 ms - 2. What is the reading on the bathroom scales, which are marked in newtons: a) while the lift is accelerating b) when the lift is stationary at the next floor? 13 Tania was using motion principles to find the mass of a trolley using forces. She applied a force of F N to the trolley of mass m kg. She measured the acceleration produced as 6.0 ms - 2. She then hooked a second trolley of mass 4.0 kg to the first and round that she had to apply a force of size 4F N to maintain the same acceleration as in the original case. Note: friction can be assumed to be negligible. a) Write an expression for the force to cause motion in diagram (i) and diagram (ii), respectively. b) Hence find both the mass of the original trolley and the value of F. Page 10

11 14 A rider accelerates a motorcycle from rest at 8.0 ms - 2 for 3s, then travels at constant velocity for 3s, and finally brakes evenly over 4s, to bring the motorcycle to rest. The motion is in a straight like and acceleration is uniform. a) Draw a velocity versus time graph for the 10s interval. b) Find the distance travelled by the motorcycle in the first 6s. c) Find the acceleration of the motorcycle during the time the brakes were being applied. d) Calculated the force applied by the brakes if the rider and motorcycle have a combined mass of 300 kg. Page 11