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ลำดับตอนที่ #2 : [[R.S.]]Physics Test::Chapter.4::28.8.09
The Summary of Physics for
Chapter 4:: Linear Motion
Chapter.4 :: Linear Motion [[Page.46]]
4-1 Motion Is Relative [[Page.47]]
- When we describe the motion of one object with respect to another, we say that the object is moving relative to the other object.
· For example an apple that is at rest (relative to the table it lies on) is moving at about 30 kilometers per second relative to the sun.
ð This means that when we compare an apple to the table, it’s not moving. But when we compare it to the sun, it’s moving.
- An object is moving if its position relative to a fixed point is changing.
· For example we say that a car is moving at 222 kilometers per hour, we mean that its movement relative to the road (we compare the car to the road which is a fixed point)
4-2 Speed [[Page.48]]
- Speed is how fast an object is moving.
- You can calculate the speed of an object by dividing distance covered by time.
- Miles per hour (mi/h), kilometers per hour (km/h), centimeters per day, or light-years per century are all legitimate units for speed.
- The speed at any instant is called the instantaneous speed.
· For example a car doesn’t always move at the same speed at all time, sometimes it slows down at a red light or speed up at green light. We can tell the car’s speed at any moment (instantaneous speed) by looking at the car’s speedometer.
- The average speed is the total distance covered divided by the time.
- Average speed doesn’t indicate variations in the speed that may take place during the trip. In practice, we experience a variety of speeds on most trips, so the average speed is often quite different from the instantaneous speed.
- If we know average speed and travel time, the distance traveled is easy to find.
4-3 Velocity [[Page.50]]
- Velocity is speed in a given direction.
- Speed is a description of how fast an object moves; velocity is how fast and in what direction it moves.
- A quantity such as velocity that specifies direction as well as magnitude is called a vector quantity. A quantity that requires only magnitude for a description such as speed is scalar quantity.
- Constant speed means steady speed no speeding up or slowing down.
- Constant velocity means both constant speed and constant direction.
· Constant velocity means motion in a straight line (the path doesn’t curve/ change direction) at constant speed.
- If either the speed or the direction (or both) is changing, then the velocity is changing
- In a care there are three controls that are used to change the velocity
· One is the gas pedal – used to maintain or increase the speed
· Second is the brake – used to decrease the speed
· Third is the steering wheel – used to change the direction
4-4 Acceleration [[Page.51]]
- Acceleration is the rate at which the velocity is changing.
- You can calculate the acceleration of an object by dividing the change in its velocity by time.
- The key idea that defines acceleration is change.
- So having good acceleration means being able to change velocity quickly and does not necessarily refer to how fast something is moving.
- Deceleration happens when the velocity is decreasing.
- Acceleration, like velocity, is a vector quantity because it is directional.
- The acceleration vector points in the direction the velocity is changing. If we change speed, direction, or both, we change velocity and we accelerate.
· When we accelerate in the direction of our velocity, we speed up.
· When we accelerate against our velocity, we slow down.
· When we accelerate at an angle to our velocity, we change direction.
- When the direction is not changing (straight-line motion is considered, acceleration may be expressed as the rate at which speed changes.
- Note that a unit for time appears twice in the above equation.
· For example, a truck is accelerating from zero to 5 km/h in 1 second; our change in speed is 5 km/h in a time interval of 1 s.
4-5 Free Fall :: How fast [[Page.53]]
- As an object falls, it gains some speed as it drops from a rest position. Gravity causes it to accelerate downward once it begins to fall. Meanwhile, air resistance also affects the acceleration of a falling object (to slow down).
- An object moving under the influence of the gravitational force only (not including air resistance) is said to be in free fall.
- The elapsed time is the time that has elapsed, or passed, since the beginning of any motion, in this case the fall.
- During each second of a free fall, the instantaneous speed of the object increases by an additional 10 meters per second. This gain in speed per second is the acceleration
The change in speed is in m/s and the time interval is in s, the acceleration is in m/s2, which is read as ‘meters per second squared.’ (cuz’ unit of time (second) occurs twice)
Free Fall Speeds of Objects | |
Elapsed Time (seconds) | Instantaneous Speed (meters/second) |
0 | 0 |
1 | 10 |
2 | 20 |
3 | 30 |
4 | 40 |
5 | 50 |
t | 10t |
- The acceleration of an object in free fall is about 10 m/s2
- For free fall, we use the letter g to represent acceleration because the acceleration is due to gravity. Average value for gravity is 10 m/s2; more accurately is 9.8 m/s2. (that’s why in calculating free fall speeds of object we multiply the elapsed time by 10)
- The instantaneous speed v of an object falling from rest after an elapsed time t can be expressed in equation form.
- The letter v symbolizes both speed and velocity.
- The average speed of any object moving in a straight line with constant acceleration line with constant acceleration is calculated the way we find the average of any two numbers: add them and divide by 2.
- When an object is thrown straight up, it continues to move upward for a while before stopping for a while after it reaches the highest point to change its direction from upward to downward. At that point, its instantaneous speed is zero. Then, it starts downward just as if it had been dropped from rest at that height.
· During the upward part of this motion, the object slows from its initial (starting) upward velocity to zero velocity; its speed decreases at 10 m/s2
· The instantaneous speed at points of equal elevation in the path is the same whether the object is moving upward or downward. But the velocities are different because they are in opposite direction.
· Each second, the speed/velocity changes by 10 m/s2
· The acceleration is 10 m/s2 downward the entire time, whether the object is moving upward or downward.
4-6 Free Fall :: How far [[Page.56]]
- For each second of free fall, an object falls a greater distance than it did in the previous second.
- At the end of time t, the object has fallen a distance d of
Free Fall Distance of an Object | |
Elapsed Time (seconds) | Distance Fallen (meters) |
0 | 0 |
1 | 5 |
2 | 20 |
3 | 45 |
4 | 80 |
5 | 125 |
t | |
- We used freely falling objects to describe the relationship between distance traveled, acceleration, and velocity acquired.
4-7 Graphs of Motion [[Page.57]]
- Equations, tables, and graphs are used to visually describe relationships such as velocity and acceleration.
- Speed-Versus-Time
· The straightness of the curve indicates a ‘linear’ relationship between speed&time
· This particular linearity is called direct proportion, and we say that time and speed are directly proportional to each other.
· If we double t, we double v; if we triple t, we triple v; and so on.
· The curve is a straight line, so its slope is constant – like an inclined plane. It shows that the acceleration is constant. (if the acceleration were greater, the slope would be steeper)
· On a speed-versus-time graph the slope represents speed per time, or acceleration.
- Distance-Versus-Time
· The curve shows that the relationship between distance traveled and time is nonlinear. The relationship shown here is quadratic and the curve is parabolic – when we double t, we do not double d; we quadruple it.
· The slope of this graph is very significant. The slope steepens (becomes greater) as time passes. This shows that speed increases as time passes.
4-8 Air Resistance and Falling Objects [[Page.59]]
- Air resistance noticeably slows the motion of things with large surface areas like falling feathers or pieces of paper. But air resistance less noticeably the motion of more compact objects like stones and baseballs. (that’s why when u release a coin and a feather at the same time, the coin reaches the ground first; but if u put both of them in a place with no air resistance, they would fall to the ground at the same time)
4-9 How Fast, How Far, How Quickly How Fast Changes [[Page.59]]
- When we wish to specify how fast something freely falls from rest after a certain time, we are talking about speed or velocity. The appropriate equation is v = gt.
- When we wish to specify how far that object has fallen, we are talking about distance. The appropriate equation is
- Velocity or speed (how fast) ;; distance (how far)
- Acceleration is a rate of a rate. It’s often confused with velocity, which is itself a rate (at which distance is covered). Acceleration is the rate at which velocity itself changes.
Ps. This is my first review sheet of the year > < !!
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first review sheet
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