Newton s Laws Level : Conceptual Teacher : Kim Objectives i) Compare the concept of motion before and after Galileo s time ii) Describe and understand Newton s three laws iii) Understand the relationship between force, acceleration and mass iv) Understand the concept of net force What is a Force? - A force is any push or pull that is in direct or indirect contact of an object - Anything that can make an object move or change its velocity is a force -Give examples of force => Galileo on Motion & Newton s 1st Law - Before Galileo s time, people thought that the natural tendency of an object is to come to a rest position Ex) An object at rest, of course, has the tendency to stay at rest unless we give it a push. An object in motion will eventually come to a stop. Thus, to keep an object moving, we have to constantly apply a force to it - However, this idea did not explain other things, like the motion of the stars and planets. - One of the Galileo s greatest contributions to physics was demolishing the notion that a force is necessary to keep an object moving - What is Newton s 1 st law? => Restatement Newton s 1 st law -An object resting on a surface will remain at rest unless we apply a force on it
-If an object is moving at a certain speed, it will keep on moving at that speed unless we apply a force on it Ques1) If you give an object a strong push on a surface, the object will move for a moment and then stop. We can clearly see that there is a change in motion because the object was moving a certain velocity, then slowed down and eventually came to a stop. Yet there seem to no visible force acting on it after the push, which seems to violate Newton s 1 st law. Explain~! Newton s 1st Law = Newton s Law of Inertia An object continues in a state of rest or in a state of motion at a constant speed along a straight line, unless compelled to change that state by a net force. - Newton s 1 st law is stating that all objects have a natural tendency to resist change its state of motion - The resistance of an object to change its state of motion is called ( ) Q2) What characteristics of an object determine how much resistance it will have to change its state of motion? a) volume b) mass c) size d) color e) smell Hint: Which object will be easier to slow it down, a rolling boulder or a rolling giant balloon? Demo1) Place a light object on your chest and have someone hit it. Place a heavy object on your chest and have someone hit it. Which case will you feel more pain?
Q3) If an elephant was chasing after you, would it more advantage to you if you run away in a straight line or zig-zag? Justify your answer~ *~Observe video on vehicles violating Newton s 1 st law ~* Newton s 2nd Law - Review in acceleration acceleration = ( ) - What causes acceleration? => ( ), symbol = ( ) - The relationship between force, acceleration and mass => Force = ( ) ( ) F = ( ) or a = ( ) - The unit for force is ( ) F=ma or a = F m : Newton s 2nd law Q4) If a 2kg object is moving at an acceleration of 4m/s² on a frictionless surface, what is the horizontal force acting on the object? Q5) If you push a 10kg object with a horizontal force on 5N on a frictionless surface, what is the acceleration of the object?
Q6) If an object is pushed at a horizontal force of 5N on a frictionless surface and experiences an acceleration of 4m/s², what is the mass of the object? It is not always the case where only one force is acting on an object. If more than one person are pushing the object on the same line of action, then we simply add or subtract the forces F 1+F 2 F 2 F 1 F net = F 1+F 2 F net = F 1+(- F 2) = F 1- F 2 If multiple forces are acting on an object, we express as F net = F 1 + F 2 + F 3 + F N Newton s 2 nd Law When a net external force Fnet acts on an object of mass m, the acceleration a that results is directly proportional to the mass. The direction of the acceleration is the same as the direction of the net force F net = ma or a = F net m Q7) There is a 4kg block on a smooth surface. If you apply a horizontal force of 10N to the right and at the same time apply 2N to the left, which direction will the object move? What is the resultant force(=net force)? What is the acceleration of the block?
Q8) There is a 2kg block on a rough surface. The friction( f ) between the surface and the block is 4N. If you push the object with a horizontal force of 12N, what is the net force? What is the acceleration of the block? Q9) There is a 5kg block on a rough surface. The friction between the surface and the block is 6N. If you push the object with a horizontal force of 4N, what is the acceleration of the block? Q10) There is a 6kg block on a surface. The friction between the surface and the block is 3N. If you push the object with a horizontal force of 10N, and the wind is blowing at the same direction of the motion with force of 5N, what is the net force? What is the acceleration of the object? Q11) Two people are pushing a stalled car. The mass of the car is 1850 kg. One person applies a force of 275 N to the car, while the other applies a force of 395 N. Both forces are in the same direction. A third force of 560 N (due to friction) also acts on the car, but in the opposite direction. Find the acceleration of the car.