Kinematics Chapter 3: LINEAR MOTION. http:// highered.mcgraw-hill.com/sites/0070524076/student_view0/interactives.html. straight-line path—linear motion http :// www.mhhe.com/physsci/physical/giambattista/forces/forces.htm http://www.mhhe.com/physsci/physical/giambattista/forces/forces.htm.
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Chelcie Liu asks his students to check their neighbors and predict which ball will reach the end of the equal-length tracks first.
The rules of motion involve three concepts:
Become familiar with them and be able to distinguish between them.
Here we'll consider only the simplest form of motion—that along a straight-line path—linear motion.
the shortest distance of the object from point O in a specific direction.Unit: metre (m)Type of Quantity: Vector quantity
The total length that is traveled by that object.Unit: metre (m)Type of Quantity: Scalar quantity
Displacement is a vector quantity
1. What is the average speed of a cheetah that sprints 100 m in 4 s? How about if it sprints 50 m in 2 s?2. If a car moves with an average speed of 60 km/h for an hour, it will travel a distance of 60 km.(a) How far would it travel if it moved at this rate for 4 h? (b) For 10 h?
3. In addition to the speedometer on the dashboard of every car is an odometer, which records the distance traveled. If the initial reading is set at zero at the beginning of a trip and the reading is 40 km one-half hour later, what has been your average speed?4. Would it be possible to attain this average speed and never go faster than 80 km/h?
The best way to imagine a situation with several physical quantities is by drawing a graph.
Here, the total distance travelled ( y) divided by the time taken ( x) is the gradient of the slope. This is also equal to the average speed of the object - remembering that
In this case, the speed is constant as the slope of the distance-time graph is constant.
By re-arranging the equation we can plot slopes of either distance, or time, on a graph to find their values. For example, we can see how to find the distance from a speed-time graph by rearranging to get:
The blue rectangle has an area equal to the speed multiplied by the time.
We can see from the equation above, that this is equal to the distance travelled.
The speedometer of a car moving to the east reads 100 km/h. It passes another car that moves to the west at 100 km/h. Do both cars have the same speed? Do they have the same velocity?
During a certain period of time, the speedometer of a car reads a constant 60 km/h. Does this indicate a constant speed? A constant velocity?
The instantaneous velocity of an object falling from rest can be expressed in shorthand notation as V = gt
Two balls are released simultaneously from rest at the left end of equal-length tracks A and B as shown. Which ball reaches the end of its track first?
3. An artillery shell is shot with an initial velocity of 20 m/s, at an angle of 60 degrees to the horizontal. At the top of the trajectory, the shell explodes into two fragments of equal mass. One fragment, whose speed immediately after the explosion is zero, falls vertically. How far from the gun does the other fragment land, assuming the terrain is level and that air drag is negligible?
4. A railroad flatcar of weight Wcan roll without friction along a straight horizontal track. Initially a man of weight w is standing on the car, which is moving to the right with a speed Vo. See Figure. What is the change in the velocity of the car if the man runs to the left (in the Figure) so that his speed relative to the car is Vrel?
An object, with mass m and speed V relative to an observer explodes into two pieces, one three times as massive as the other; the explosion takes place in deep space. (no external forces) The less massive piece stops relative to the observer. How much kinetic energy is added to the system in the explosion, as measured in the observer’s frame of reference?