Class 5-Newtons Laws of Motion in general science
- 1. Chapter: Newton's laws of Motion
- 2. Newton'slawsofmotion:Introduction • Newton's laws of motion are three physical laws that together laid the foundation for classical mechanics. • They describe the relationship between a body and the forces acting upon it, and its motion in response to said forces. • They have been expressed in several different ways over nearly three centuries, and can be summarized as follows:
- 3. What is force? A force is a push or pull that one object exerts on another. Just like velocity and acceleration, force also is a vector that has a size and a direction. When you shoot a basketball or kick a soccer ball, you are exerting a force on an object. In fact, every push or pull you exert results in a force being applied to some object. The size of a force often is called the strength of the force. The direction of a force is the direction in which the push or pull is applied. For example, when you lift your backpack, you apply an upward force. In SI units, force is measured in Newton’s (N). Changing Motion What happens to the motion of an object when you exert a force on it? A force can cause the motion of an object to change. If you have played billiards, you know that you can cause a ball at rest to roll into a pocket by striking it with another ball. The force applied by the moving ball causes the ball at rest to move in the direction of the force.
- 5. Balanced Forces Force does not always change velocity. In Figure A, two students are pushing on opposite sides of a box. Both students are pushing with an equal force but in opposite directions. When two or more forces act on an object at the same time, the forces combine to form the net force. The net force on the box in Figure A is zero because the two forces cancel each other. Forces on an object that are equal in size and opposite in direction are called balanced forces.
- 6. Unbalanced Forces Another example of how forces combine is shown in Figure B. When two students are pushing with unequal forces in opposite directions, a net force occurs in the direction of the larger force. When forces combine to produce a net force that is not zero, the forces acting on the object are unbalanced forces. The net force that causes the box to accelerate will be the difference between the two forces because they are in opposite directions. In Figure C, the students are pushing on the box in the same direction. These forces are combined, or added together, because they are exerted on the box in the same direction. The net force that acts on this box is found by adding the two forces together.
- 7. Unbalanced Forces Change Velocity and hence Change Momentum When the forces acting on an object are balanced, the velocity of an object doesn’t change. If you and a friend push on a door from opposite sides with the same size force, the door doesn’t move. The net force is zero and the forces are balanced. But if you push harder, the door moves in the direction of your push. The velocity the door and hence the momentum, or any object, changes only when the forces on it are unbalanced.
- 8. Firstlaw • Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. • An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force. This law is often called "the law of inertia".
- 9. Secondlaw
- 10. Problem: What acceleration is produced by a force of 2000 N applied to a 1000-kg car? If the force is double, what will be the magnitude of the acceleration?
- 11. Prove thatF=ma Alternative statement of Newton's second law: The rate of change of momentum of a body is directly proportional to the applied force.
- 13. Thirdlaw • For every action there is an equal and opposite reaction. • This means that for every force there is a reaction force that is equal in size, but opposite in direction. That is to say that whenever an object pushes another object it gets pushed back in the opposite direction equally hard.
- 14. Example of third law
- 15. Problem1:Thevelocityofa 1000 kg Car goes from 10 m/s to 20 m/s in 5 s. (a) What is the force acting on it? (2000 N) (b) What is the origin of this force? (Reaction force of the road on the car’s tires) Problem 2:The brakes of a1000 —kg car exert 3000 // force. How long will the car take to come to a complete stop from the velocity of 30 mfs? (10 s)