Motion in Physics-Week Four Motion 1.pptx

Motion in Physics

1. YEAR 10 PHYSICS – MOTION 1

2. 1. What is density? 2. State the formula for calculating density? 3 . Calculate the density of an iron block of volume and mass 7.9g.   2 STARTER ACTIVITY

3. LESSON OBJECTIVE(S) 3 TOPIC: MOTION 1. LESSON OBJECTIVES At the end of this lesson, students should be able to:- 1. Define Motion and state types of motion? 2. Define Relative motion? 3. Define circular motion and give examples . 4. Define centripetal acceleration and also centripetal force . 5. Use the expression for centripetal acceleration and force to solve problems KEY WORDS i. Motion ii. Linear iii. Random iv. Rotational v. Circular vi. Acceleration vii. Centripetal viii. Centrifugal ix. Relative

4. MOTION 4 Motion is the change of position of a body with time. The branch of motion which deals with the description of how objects move without regard to forces causing their motion is called kinematics. The one that deals with why they move, that is what causes them to move as they do is called Dynamics.

5. TYPES OF MOTION 5 1. Random Motion This is the motion of objects or particles irregularly, haphazardly or disorderly, without preferred direction. The particles collide with one another and with the walls of their containers, if they are kept in containers. The Brownian motion is a description of random motion. Examples are: Motion of pollen grains suspended in water. Motion of chalk or dust particles suspended in air. c) Motion of smoke particles suspended in air etc.

6. TYPES OF MOTION 6 2. Translational Motion This is the movement of rigid bodies from one point in space to another, without rotating. Each part of the object undergoing this kind of motion, follows the same path. Examples are: A bee flying from one flower to another A car travelling from one station to another. An aircraft flying in space.

7. TYPES OF MOTION 7 3. Rotational Motion This is the kind of motion in which all points in the body move in concentric circles. The centre of these circles all lie on a line called the axis of rotation. Examples are: Rotation of the earth about its axis Motion of the wheels of a moving car. Motion of the blades of electric fan  

8. TYPES OF MOTION 8

9. TYPES OF MOTION 9 4. Oscillatory (Vibratory) Motion A body is said to undergo oscillatory motion when it moves to and fro, reversing the direction of its motion and returning regularly to its original position. The motion is periodic because it repeats itself at regular intervals. Examples are: Motion of swinging pendulum. Motion of diving board. Motion of the plucked strings of a guitar. Motion of the prongs of a tuning fork. Vertical motion of the mass on a spiral spring when disturbed.

10. RELATIVE MOTION 10 Relative motion can be defined as the motion of a body with respect to another. All objects moving on the earth surface have relative motion with respect to the earth which is assumed to be stationary. If a boy is standing on a moving train, his speed, with respect to the earth, is the speed of the train. If the boy in the train runs inside the train towards the direction of the train, then his speed with respect to the earth is the speed of the train plus the speed of the boy.

11. EXAMPLE 1 11 Two cars X and Y travelling in opposite directions along the same highway at uniform velocities 110kmh -1 and 90kmh -1 respectively pass each other at a certain point. The velocity of X relative to Y at the time they pass each other is A. 200kmh -1 B. 100kmh -1 C. 40kmh -1 D. 20kmh -1

12. SOLUTION TO EXAMPLE 1 12

13. Circular Motion 13 An object moving with a uniform speed along a circular path is said to undergo uniform circular motion. Examples of circular motion are: A stone tied to a string and whirled round a horizontal circle. A stone tied to a string and whirled round a vertical circle. A satellite e.g the moon moving round the earth. The earth moving round the sun.

14. Circular Motion 14 The magnitude of the speed of the object is constant but its direction changes in a uniform manner. Thus the velocity of the object is changing. This change in velocity, brought about by the change in its direction constitutes acceleration.

15. Circular Motion 15 This acceleration is directed towards the centre of the circular path and is called centripetal acceleration. The magnitude of the centripetal acceleration a is given by Where v = uniform speed r = radius of circular path The velocity V at any point is directed along the tangent at that point.  

16. Circular Motion For a body to accelerate, there must be a force causing the acceleration. Thus this acceleration that is directed towards the centre of the circular path is caused by a force which also acts towards the centre of the circular path (inwards). This force is called centripetal force. F r

17. Centripetal Force F r Centripetal force is that inward force required to keep an object moving with a constant speed in a circular path. Where m = mass of moving object v = speed of the moving object r = radius of the circular path  

18. Centrifugal Force Since centripetal force keeps the object moving in a circular path, there must be an equal and opposite force which balances the object from falling inwards. This is in agreement with Newton’s third law of motion, which states that action and reaction are equal and opposite. Thus centrifugal force is an equal but opposite force which balances the centripetal force to keep the object moving in a circular path.

19. Angular Speed and Velocity = angular displacement r = radius of circular v = linear velocity s = length of arc moved t = time = angular speed/velocity. It is also called angular frequency. Now if the object moves from P to Q and the radius r sweeps through an angle , in time t, then = t  

20. Angular Velocity This is the ratio of the angle swept/turned through to the time taken. But the linear velocity v is given by ----------------(1) Note that s tends to a straight line when becomes very small as in this case. If the angle is measured in radians then s = r ----------------(2) = The radian is defined as the angle subtended by the arc of a circle whose length is equal to the radius of the same circle. 360 = 2 radian = one complete revolution/cycle/oscillation  

21. Angular Velocity

22. Angular Velocity

23. Angular Acceleration  

24. Example 1 A body is whirled round a circular path at constant speed. Which of the following diagrams correctly indicates the direction of the body’s velocity (v) acceleration (a)?

25. Example 2 A stone is whirled round a circular path of radius 15cm. If the stone makes 30 oscillations in 10seconds, calculate the angular speed of the stone. [ ] A. 9.42rads -1 B. 12.56rads -1 C. 18.84rads -1 D. 62.80rads -1 E.141.30rads -1  

26. Solution 1 oscillation = 1 cycle = 360 30 oscillations = 30 cycles = 30 360 Angular speed, = angular displacement measured in radians. t = time in seconds. In radians, 360 = 2 For 30 oscillations = 30 2 Substituting into = 6 6 3.14 = 18.84 rads-1 (C)  

27. Class activity 1 A body of mass 12kg moves with a speed of 10ms -1 in a circular path of radius 5m. Calculate the magnitude of the force acting on the body. A. 24N B. 30N C. 125N D. 240N E. 335N

28. Solution Since the body moves in a circular path, the force acting on the body is centripetal force. F r . F r = m = mass of the body = 12kg v = speed of the body = 10ms-1 r = radius of circular path = 5m F r = = = = 240N D