CLASS 7-Motion & Time -CHAPTER NOTE

  An object is said to be in motion when its position changes with time and with frame of reference.

• Location of any object described by specifying a reference point. All kind of motions are relative to some frame of reference.
• For example, a body on the surface of the Earth may appear to be at rest, only because the observer (frame of reference) is also on the surface of the Earth. The Earth itself, together with both the body and the observer, is moving in its orbit around the Sun.
• Rest is opposite of motion means there is no change of position of the object.

 Distance 

• The total path length covered by an object is said to be the distance traveled by it.

 Displacement

•  The shortest path/distance taken in measurement from the initial to the final position of an object is known as displacement.
• Displacement is simply the distance between where the object started and where it ended up measured with a straight line.

 Distance and displacement are two quantities that may seem to mean the same thing but they have different meanings in physics.

• Distance is a scalar quantity that refers to "how much longer an object has moved" during its motion.

• Displacement is a vector quantity that refers to "how far an object has moved from initial position"; It shows the object's overall change in position.

 

Consider the motion shown in the diagram below. Rama walks 4 meters east, 2 meters south, 4 meters west, and finally 2 meters north

 

During the course of his motion, He has "covered 12 meters of ground" (distance = 12 m).

 Displacement, being a vector quantity, must give attention to direction. The 4 meters east cancels out  the 4 meters west; and the 2 meters south cancels out the 2 meters north. Vector quantities such as displacement are direction aware.

Uniform motion: 

• When any object covers equal distances in equal intervals of time, Then object is said to be in uniform motion.
• Acceleration of objects in uniform motion is zero.
• Distance time graph for uniform motion is always a  straight line.
• Slope of the distance-time graph represents the speed of the object.
• Speed for a uniform motion object is constant.
• Distance traveled by an object is determined by multiplying speed and time.
• The area under the speed-time graph gives the distance traveled by the object.

 Non-uniform motion: 

• Non-uniform motions where objects cover unequal distances in equal intervals of time.
• Acceleration of objects in non-uniform motion is non-zero.
• Distance time graph for non-uniform motion is the curve line.
• For non-uniform motion, the average speed of any object is obtained by dividing the total distance traveled by an object by the total time taken.

Speed: 

• The distance traveled by an object in unit time is known as speed. Its unit is m/s.
• Speed is a scalar physical quantity that can be expressed by magnitude only.

Speed is directly proportional to distance  traveled when time is constant: v ∝ s (t constant)

Speed is inversely proportional to time taken when distance is constant: v ∝ 1/t (s constant).

Combining these two rules together. We get the definition of speed in symbolic form.

v = s/t

Thus speed = distance/ time

  Instantaneous speed:

The speed is determined over a very small interval of time as an instant. The interval should be as close to zero as possible.

A car's speedometer shows the instantaneous speed of the car at every moment.

 Average speed:

Divide the total distance traveled by any object to the total time spent traveling. This will give you your average speed. 

 If A car traveled 150 km in 3 hours, 120 km in 2 hours, and 90 km in 1 hour, his average speed was about 60 kmph.

  ·         The distance-time graph is a form of line graph.

·         A distance-time graph shows how far any object has traveled in a given time. It is a simple line graph that denotes distance versus time  on the graph. Distance is plotted on the Y-axis and Time is plotted on the X-axis.

  

 

·         Above distance-time graph shows that object is at rest because the distance is not changing with respect to time.

         Moving with uniform speed

·         Above distance-time graph is showing object is in uniform motion because the equal distance is covered in equal interval time.

 

·         Above distance time graph showing two objects A & B are in uniform motion because equal distance is covered in equal interval time by each object.

·         Slope (angle with time axis) for object A is more than B. slope represents speed in distance time graph, hence speed of object A is more than speed of object B.

          

·        Above distance time graph showing object is in non-uniform motion because equal distance is not covered in equal interval time.

MEASUREMENT OF TIME

·        Simple pendulum: A simple pendulum is a device for measuring time.

·        The bob of the pendulum oscillates to and fro along the same path and passes through the mean position.

·        This type of motion is called oscillatory motion.

·        Oscillation: The complete to and fro motion of the pendulum constitutes one complete oscillation.

·        Time period: The time required to complete one complete oscillation is called time period of a simple pendulum.

·        The smallest time interval that can be measured with commonly available clocks and watches is one second.