Danyal Education. namrcs. t,-j t acts to llnow. $$Prouems to Solve

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1 Chapten D namrcs.*l t,-j t acts to llnow Q: What are the effects of balanced and unbalanced forces on a body?. Balanced forces do not produce a resultant force. When balanced forces act on a stationary object, the stationary object will continue to be stationary. When balanced forces act on a moving object, the moving object will continue to move at its constant speed.. Unbalanced forces produce a resultant force. When unbalanced forces act on a stationary object, the stationary object will start to move with acceleration. When unbalanced forces act on a moving object, the object will move with either acceleration or deceleration (depending on the direction of resultant force) or change its direction of motion. Q: Describe the ways in which a force may change the motion of a body.. A force can make a stationary object move as well as to make a moving object stop.. A force can change the speed of a moving object.. A force can change the direction of a moving object. Q: Explain the effects of friction on the motion of a body.. Friction is caused by the interlocking of irregularities between two surfaces. It therefore always opposes motion.. It will cause an accelerating object to move with lower acceleration'. It will cause an object moving at constant speed to decelerate.. It will cause a decelerating object to move with higher deceleration. $$Prouems to Solve I. A box of mass 2 kg is being pushed horizontally across a smooth surface by a l0 N force as shown. 10N a. (i) What is the acceleration of the box? ( 2 marks ) 1 3l Chapter 3 Dynamics

2 (ii) Given that the box is initially at rest, what is its speed after 5 seconds? ( 2 marks ) b. (i) An opposing force of 6 N acts against the 10 N force when the speed of the box reaches the value obtained in a(ii). What is the effect of the 6 N force on the box? ( 1 mark ) (ii) Calculate the speed of the box after 3 seconds. ( 3 marks ) 2. A box of mass 2 kg achieves a constant speed of 4 m/s when a 10 N horizontal force is applied. a. What is the frictional force acting on the box? ( 2 marks ) b. Describe the motion of the box when the force increases from 10 N to 12 N. ( 3 marks ) 32 Chapter 3 Dynamics r

3 3. A stationary box of mass 2 kg is being pushed horizontally across a rough surface by a 10 N force. The frictional force is 4 N and there is an opposing force of2 N acting against the 10 N force as shown. j t0n Frictionalforce=4N a. Calculate the acceleration of the box. ( 2 marks ) b. Given that the opposing force is increased to 6 N, calculate the acceleration of the box. ( 2 marks ) c. Given that the opposing force is further increased to l0 N, calculate the acceleration of the box. ( 2 marks ) 2N -1 -'t) Chapter 3 Dynamics

4 4. A2kg mass moving at a constant speed of 5 m/s is being acted by a lon force and a 8N force simultaneously as shown. 8N 5 m/s l o a. When will the mass come to an instantaneous stop? ( 3 marks ) b. When will the mass move at a speed of 5 m/s in the opposite direction? ( 3 marks ) 5. Two perpendicular forces act on a particle as shown. 3N a. Use vector diagram to determine the magnitude and direction of the resultant force acting on the particle. ( 4 marks ) 4N l0n o 34 Chapter 3 Dynamics

5 b. Given that the mass of the particle is 0.5 kg, what will be the acceleration of the particle? ( 2 marks ) i I I I / c. What is the magnitude and direction of a single force which can be applied to the system to cause the particle to be at equilibrium? ( 2 marks ) 6. Three forces act on a particle as shown. t N i > 4N a. Use vector diagram to determine the magnitude and direction of the resultant force acting on the particle. ( 4 marks ) 2 \ ro' a 35 0hapter 3 Dynanics

6 b. What is the magnitude and direction of a single force which can be applied to the system to cause the particle to be at equilibrium? ( 2 marks ) 7. A uniform heavy rod is being kept in equilibrium by leaning against a rough wall and a smooth wall as shown. rough wall smooth wall a. Indicate on the diagram, the force of gravity, lv, on the rod. ( 2 marks ) b. Indicate on the diagram, the forces acted by the walls on the rod. ( 3 marks ) 36 Chapter 3 Dynanics

7 8. The maximum possible friction F between an object and a rough horizontal floor surface is given by F = N where p is the frictional coefficient of the floor surface and N is the weight of the object. The frictional coefficients of three types of floor surfaces are as follows: Floor type Frictional coeffi cient, p A 0.20 B 0.22 C 0.31 a. Which of the floor surface is the roughest? ( l mark) b. An object of dimensions 2.0 cm x 3.0 cm x 4.0 cm with a density of 2.7 glcm3 is placed on a type A horizontal floor. (i) Calculate the weight of the object. ( 3 marks ) (ii) Calculate the maximum friction between the floor and the object. ( 2 marks ) 5t Chapter 3 Dynanics

8 (iii) A horizontal force of 1.5 N acts on the object as shown. 1.5 N type A floor What is the acceleration of the object? ( 2 marks ) (iv) The object is adjusted such that the surface 2.0 cm x 4.0 cm is resting on the type A floor. What is the maximum frictional force between the floor and the object? Explain your (2marks) answer. 38 Chapter 3 Dynamics

9 .,i., '$ Misconceptions to Clarifr In an experiment, a coin and a feather are dropped in a long tube containing air as shown. coin---- e& - feather In another experiment, the coin and the feather are dropped in the same tube with the air totally pumped out. Which of the following statements about the two experiments are correct? (1) The final speed of the feather in the first experiment is slower than the final speed of the feather in the second experiment. (2) The coin and the feather in the second experiment hit the base of the tube at the same time. (3) The final speed achieved by the coin in both experiment is the same. A (1) and (2) only B (1) and (3) only C (2) and (3) only D (l), (2) and (3) ( ) 2. An object is hanging at rest by a spring as shown. spnng object Which of the following shows the correct free body diagram? AB C D () 39 Chapter 3 Dynanics

10 I 3. The wheel shown below is driven by a car engine accelerating towards the right. Which is the direction of frictional force on the wheel? wheel D direction of acceleration \_.i a\ A IC B surface of the road 4. A box accelerates down a rough slope as shown. TJ LJ () What is the effect of friction on the motion of the box? A The box continues to move with the same acceleration. B The box slows down. C The box moves with lower acceleration. D The box moves with higher acceleration. () 5. What is the effect of the air resistance on a falling object? A The speed ofthe object is reduced. B The acceleration of the object is reduced. C The distance travelled by the object is reduced. D The direction of motion of the object is changed. ( ) 6. An object is moving due east at a constant speed of 5 m/s before two equal and opposite forces of l0 N each act on the object at the same time. The object will A move with higher speed in the same direction. B move with lower speed in the same direction. C continue to travel at the same speed in the same direction. D change the direction of motion and travel in the opposite direction. ( ) 40 Chapter 3 Dynanics