Batterjee Medical College General Physics Chapter 3 The laws of motion 35 37 كلية البترجي الطبية

Chapter Three:
The laws of motion
intended Learning Outcomes:
In this chapter we will
• Explain Newton’s laws of motion • Solve mathematical problems related to Newton’s laws.
3.1 Forces
• A force is commonly imagined as a push or a pull on some object, perhaps rapidly, as when we hit a tennis ball with a racket. We can hit the ball at different speeds and direct it into different parts of the opponent’s court. This means that we can control the magnitude of the applied force and also its direction, so force is a vector quantity, just like velocity and acceleration.
• Forces can be used to change the speed of the object or the direction of the object. • The unit of force in the metric system is:
Newton (N) where 1N = 𝑘𝑔.𝑚 𝑠 2
3.2 Mass and Inertia
Imagine hitting a golf ball off a tee with a driver (the golf club). If you’re a good golfer, the ball will sail over two hundred yards down the fairway. Now imagine teeing up a bowling ball and striking it with the same club (an experiment we don’t recommend). Your club would probably break, you might sprain your wrist, and the bowling ball, at best, would fall off the tee, take half a roll, and come to rest. From this thought experiment, we conclude that although both balls resist changes in their state of motion, the bowling ball offers much more effective resistance. The tendency of an object to continue in its original state of motion is called inertia. Although inertia is the tendency of an object to continue its motion in the absence of a force, mass is a measure of the object’s resistance to changes in its motion due to a force. The greater the mass of a body, the less it accelerates under the action of a given applied force. The SI unit of mass (Scalar quantity) is the kilogram.
• Inertia is the tendency of an object to resist changes in its state of motion.
3.3 Newton’s First Law (Law of Inertia)
In the absence of unbalanced forces (or external forces), an object at rest remains at rest and an object in motion continues in motion with a constant velocity.
• If the forces on an object are equal and opposite, they are said to be balanced, and the
object's state of motion doesn't change.
• If the forces on an object are not equal and opposite, they are said to be unbalanced, and
the object's state of motion will change.
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