كتاب الفيزياء الصف الحادي عشر نخبة منهج انجليزي الفصل الأول 2021-2022

كتاب الفيزياء الصف الحادي عشر نخبة منهج انجليزي الفصل الأول 2021-2022

FIGURE 1.1 A fast -movirg rain passes   railroad crossing ou can see that the train in Figure 1.1 is moving very fast by noticing that its image is blurred compared with the stationary crcssing signal and pole. But can you tell if the train is speeding up. slowing down, or zipping by at constant speed? A photograph can convey an object’s speed because the object moves during the expcmzre time, but a photograph cannot show a change in speed. referred to as xceleration Yet acceleration is extremely important in physics. at least as important as speed itself In this chapter. we lcxk at the terrns used in physics to describe an objecÜ rnotkn• displacement. vehcity, and rceleration We examine rnotion along a
straight line (one- dimensional motion) in this chapter and motion a curved path (rnoticm in a plane, or motion) in the next chapter. One of the greatest xivantages of physics is that its laws are universal. so the sanw general terrns and ideas apply to a wide range c.f situations. This. we can tse the same equatiors to the flight of a ball and the lift- off of a rocket into space from Earth to Mars. As you continue in this you will that almcst everything moves relative to other objects on some scale or other. whetlrr it a conrt plunging through space at several kilometers per secorxi the atoms in a seemingly sta- tionary c’ject vibrating millions times per The terms we introdtre in this chapter will be part your study the rest of the ami afterward

 

2.1 Three Dimensional Coordinat Systems

Having studied rnotion in one dimension. we next tackle nure complicated problems in two and three spatial dirnenskns. To descritr this rm.tiCM1. we will work in Cartesian cocr- dilutes. In a thrædirnensional Cartesian ccxrdinate we x- and y -axes to lie in the plane and the z-axis to vertically upward (Figure 22). The
three ccxrdmate axes are at to one as required fcy a Cartesian ccxydinate system convention followed with(llt exception in this is that Cartesian ccxr- dirute system is right -handed. This convention means that you can obtain the relative orientaticm of the three ccxrdinate axes using your right hand To determine the vxsitive directioru of the three axes. hold your right hand with the thumb sticking straight up aruå the index finger B’inting straight cwt; they will naturally have a angle relative to each other. Then stick out your middle finger so that it is at a right angle with tXth the index finger and the thumb (Figure 2.3) The three axes are assigned to the fingers as shown in Figure 2.3: thumb is x, index finger is y. and middle finger is z You can rotate your right hand in any direction. but the relative orientation of thumb and fingers stays the same. Ycnrr right hand can always be orientated in three dimensional space in such a way that the axes assignments on your fingers can brought into alignment with the ccxlrdinate axes shmvn in Figure 2.2. With this set of Cartesian ccx) rdinates a BSition can written in comvxment form as

in the horizontal plane. Such a case showm in Figure 2.5 a baseball tossed in the air

In this case. we can assign new coordinate axes such that the x-axis vxints along the hori zcmtal vrojection the trajectory and the y-axis is the vertical axis In this special case

the rnotm in three dimensions can in effect be described as a motion in two dirnersions A large clas of real-life problerns falls into this category. especially problens that involve ideal motion An ideal projectile is any object that is released with some initial velocity and then moves only under the influence of gravitational acceleratioru which is assumed to be cmstant and in vertical downward direction A basketball free throw (Figure 26) is a gcxd example of ideal projectile motion. as is the flight of a bullet or the trajectory of a car that becomes airbome. Ideal projectile motion neglects air resistance and wind speed, spin of the projectile, and other effects influencing the flight of real-life projectiles. For realistic situations in which a golf ball, tennis ball, or baseball moves in air. the actual trajectory is not well described by ideal projectile motion and requires a more sophisticated analysis We will discuss these cts in Section 2-5. but will rut go into quantitative detail Let’s begin with ideal projectile motion

with no effects due to air resistance or any other forces besides gravity. We work with two Cartesian components: x in the horizontal direction and y in the vertical (upward) direction. Therefore. the position
vector for projectile motion is and the velocity vector is

Given our choice of coordinate system. with a vertical y -axis. the acceleration due to gravity acts downward. in the negative y -direction; there is no acceleration in the horizontal direction

For this special case of a constant rceleration only in the ydirection and with zero acceleration in the x-direction. we have a free- fall problem in vertical direction and motion with constant velocity in the lu»rizontal direction- The kinematical equations for the x-direction are those for an object moving with constant velocity

Just as in Chapter l. we use the notation tho z Dx(t = O) for the initial value of the x-component Of the velocity. The kinematical equations for the y -direction are those for free-fall motion in one dimension

 

EXAMPLE

Many lecture dernonstrations illustrate that motion in the x -direction motion in y-direction are indeed Of each other. as asumed in the derivation of tip equations for projectile motion One popular demotbtration. called shcxt the monkey,- is shown in Figure 2.7. TIW demonstration is motivated by a story. A monkey has from the and has climbed a tree. zckeeper wants to shcxt the monkey with a tranquilizer dart in order to recapture it, but he knows that the monkey will let go of the branch it is lulding onto at the of the gun firing His challenge is therefore to hit the monkey in air as it is falling

PROBLEM

Where the need to aim to hit the falling monkey

SOLUTION

The must aim directly at the monkey. as shown in Figure 2.7. assuming that the time for the Of the gun firing to reach the monkey is negligible and the Of the dart is fast enough to cover the horizontal distance to the tree. As as the dart leaves the gun. it is in free fall, just like the monkey. Because txth the monkey and the dart are in free fall. they fall with the same acceleration. independent of the dart’s motion in the x-direction and Of the dart’s initial vel«ity. The dart and the monkey will meet at a directly the Bint from which the monkey

DISCUSSION

Any sharpshcxter can tell you that, for a fixed target. you need to correct your gun sight for the free-fall motion of the projectile on the way to the target- As you can infer from Figure 2.7, even a bullet fired from a highpwered rifle will not fly in a straight  line but will drop under the influence of gravitational acceleration. Only in a situation like the shocwthe-monkey demonstration. where the target is in free fall as as the projectile leaves the muzzle. can one aim directly at the target without making correc- tions the free -fall motion of the projectile