Principles and Laws of Motion

Transcription

1 minutes Teacher Notes: Ian Walter DipAppChem; TTTC; GDipEdAdmin; MEdAdmin (part) Program Synopsis This program begins by looking at the different types of motion all around us. Forces that cause motion are discussed and defined, with clear examples of the many types provided. The concept of balanced and unbalanced forces is clearly depicted. Newton s laws are defined and described with numerical examples, and motion on an inclined plane is discussed. Circular motion is described in terms of centripetal force with motion due to an unbalanced force, and simple formulae and quantities are explained. Projectile motion is explored from the standpoint of motion of an object in a gravitational field. The rectilinear equations of motion can be used to determine horizontal range, maximum height of trajectory and time of flight, together with horizontal and vertical components of the initial velocity. Momentum is defined, related to Newton s second law, and the concept applied to the motion of balls on a pool table. Finally, torque of a force is discussed.

2 Introduction This program introduces senior students to the fundamentals of force and its effects. Excellent graphics are provided for students to gain a sound understanding of the various types of force, and simple formula to calculate magnitudes of net forces are also provided. Forces acting in various situations are investigated and linked to the type of motion that each produces. Students will also look at examples of motion that describe each of Newton s laws and the use of mathematics and vector quantities in calculations. Program Rationale This program will help senior students to reinforce theory covered in the classroom on forces and motion. The graphics in the program are essential for visual learners in their understanding of the various types of force and motion and how they are related. This program aims to provide teachers and students with a comprehensive overview of motion and force in the world around us. Program Timeline 00:00:00 Introduction 00:00:58 Forces 00:03:26 Newton s Laws of Motion 00:08:48 Projectile motion 00:11:09 Circular motion and centripetal force 00:13:32 The pool table 00:16:43 Conclusion 00:17:38 Credits 00:19:02 End program Useful Resources Chapman, R et al. 2005, Heinemann Physics 12, 2nd edn, Fullick, P 2000, Heinemann Advanced Science 2nd edn, Heinemann, UK. Giancoli, DC 2000, Physics: Principles with Applications, 5th or 6th edns, Prentice Hall. Kane, JW & Sternheim, MM 1988, Physics John Wiley & Sons, New York. Lofts, G et al. 2005, Jacaranda Physics 1, 2nd edn John Wiley & Sons Australia Ltd, Brisbane. Related Programs All About Motion Displacement, Velocity and Acceleration Measurement and Uncertainty Understanding Scientific Measurement - 2 -

3 Program Worksheet Before the Program 1. Using the internet, or a suitable text, provide a 150 word report on Types of forces. Give a general definition of force and define the following forces, providing examples of their action. Balanced force, unbalanced force, centripetal force, gravitational force, magnetic force, electric force, and nuclear_force. 2. Review and summarise the equations of linear motion. Define and give the names and accepted symbols of the basic and derived quantities used in these equations; viz. length, time, velocity, speed and acceleration together with the common S.I. unit for each, and the accepted symbol for the unit

4 During the Program 1. A simple definition for force is: 2. State the unit of force in the S.I. system? 3. An object continues in its state of rest, or uniform motion in a straight line, unless compelled by an external force to change that state. Which one of Newton s laws is this? 4. What is the magnitude of the net force acting on a body moving with constant speed? 5. What is meant by the term equilibrium of a body under the action of forces? 6. Apart from F=ma which is used to describe Newton s second law, define this law in terms of the quantity momentum. 7. Define Newton s third law. 8. Projectile motion is an example of motion in two dimensions. What does this mean? 9. Give an example of an object moving at constant speed and accelerating. 10. An object is moving at constant speed in a circular path under the action of an unbalanced force. Name the force and state the direction in which it acts

5 11. You are whirling a bucket on a string around your body in a horizontal direction, and the string breaks. Describe the motion of the bucket after the string breaks? 12. A pool ball is rolling along the table and comes to rest. Explain this observation. 13. Explain what you understand by an elastic collision. Give an example of an almost perfect elastic collision. 14. Describe what is meant by the torque of a force? 15. Suggest a reason why a very long handled spanner is used to screw down the head onto the block of a car engine

6 After the Program 1. Complete the following table: Quantity Symbol Unit Symbol Force mass velocity acceleration For each of the following questions show your answer in the blank spaces provided. 2. A car of mass 1000 kg is resting on a plane inclined at 30 0 to the horizontal. Calculate the component of the weight of the car acting parallel to the plane. Take the value of the gravitational field strength, g, as 10 m s A car of mass 1000 kg travelling with an initial speed of 15 m s -1 has a constant acceleration of 0.50 m s -2 along a straight horizontal road. Do not consider air friction effects. a) Calculate the magnitude of the force producing this acceleration

7 b) Calculate the speed of the car after 60 s. 4. A cannon ball is fired from ground level at an angle of 30 0 above the horizontal with an initial speed of 100 m s -1. Do not consider air friction effects. Take the value of g as 10 m s -2. Calculate the maximum height reached by the cannon ball. 5. Calculate the magnitude of the centripetal acceleration of an object moving with a speed of 6.0 m s -1 in a circular path of radius 3.0 m

8 Suggested Student Responses During the Program 1. A simple definition for force is A simple definition for force is a push or a pull. 2. State the unit of force in the S.I. system? The unit of force in the S.I. system is the Newton. 3. An object continues in its state of rest, or uniform motion in a straight line, unless compelled by an external force to change that state. Which one of Newton s laws is this? An object continues in its state of rest, or uniform motion in a straight line, unless compelled by an external force to change that state. This is Newton s first law. 4. What is the magnitude of the net force acting on a body moving with constant speed? The magnitude of the net force acting on a body moving with constant speed is zero. 5. What is meant by the term equilibrium of a body under the action of forces? The term equilibrium of a body under the action of forces implies that there is no net force acting so the body may be stationary or could be moving with constant speed. 6. Apart from F=ma which is used to describe Newton s second law, define this law in terms of the quantity momentum. F=ma is used to describe Newton s second law but it can be defined in terms of the quantity momentum as the time rate of change of momentum. 7. Define Newton s third law. Newton s third law states that to every action there is an opposite and equal reaction. 8. Projectile motion is an example of motion in two dimensions. What does this mean? Projectile motion is an example of motion in two dimensions. The two dimensions for a projectile involve its horizontal motion and its vertical motion taken together. These two produce a curved path for the projectile under the action only of the gravitational force. 9. Give an example of an object moving at constant speed and accelerating. An example of an object moving at constant speed and accelerating is that of a body moving in a horizontal circle with constant speed. 10. An object is moving at constant speed in a circular path under the action of an unbalanced force. Name the force and state the direction in which it acts. An object is moving at constant speed in a circular path under the action of an unbalanced force. The force is called a centripetal force and the direction in which it acts is toward the centre of its circular path

9 11. You are whirling a bucket on a string around your body in a horizontal direction, and the string breaks. Describe the motion of the bucket after the string breaks? The motion of the bucket after the string breaks is tangential to the circular path. 12. A pool ball is rolling along the table and comes to rest. Explain this observation. A pool ball is rolling along the table comes to rest due to the frictional force acting between the ball and the table cloth in a direction opposing the motion of the ball. 13. Explain what you understand by an elastic collision. Give an example of an almost perfect elastic collision. An elastic collision is a collision in which both momentum and kinetic energy of the moving components of the collision are conserved. An example of an almost perfect elastic collision would be between two pool balls. 14. Describe what is meant by the torque of a force? The torque of a force is defined as the turning effect of a force. 15. Suggest a reason why a very long handled spanner is used to screw down the head onto the block of a car engine. Very long handled spanners are used to screw down the head onto the block of a car engine because greater torque can be applied. (For a shorter spanner, a greater force is required to produce the same torque)

10 Suggested Student Responses After the Program 1. Complete the following table: Quantity Symbol Unit Symbol Force F Newton N mass m kilogram kg velocity v metre per second m s -2 acceleration a metre per sec per sec ms -2 For each of the following questions show your answer in the blank spaces provided. 2. A car of mass 1000 kg is resting on a plane inclined at 30 0 to the horizontal. Calculate the component of the weight of the car acting parallel to the plane. Take the value of the gravitational field strength, g, as 10 m s -2. Weight component down the plane = mg sin q = sin 30 = 5000 N 0 3. A car of mass 1000 kg travelling with an initial speed of 15 m s -1 has a constant acceleration of 0.50 m s -2 along a straight horizontal road. Do not consider air friction effects. a) Calculate the magnitude of the force producing this acceleration. F = ma = = 500 N

11 b) Calculate the speed of the car after 60 s. u = 15 a = 0.50 t = 60 v =? Use; v = u + at = 15 + ( ) = 45 ms A cannon ball is fired from ground level at an angle of 30 0 above the horizontal with an initial speed of 100 m s -1. Do not consider air friction effects. Take the value of g as 10 m s -2. Calculate the maximum height reached by the cannon ball. u = 100sin 30 v = 0 y =? Use; 2 2 v = u + 2gy (100sin 30 ) 2 10 = + - y = 125m y 5. Calculate the magnitude of the centripetal acceleration of an object moving with a speed of 6.0 m s -1 in a circular path of radius 3.0 m. Use; 2 v a = r = = 12ms