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Laws of Motion

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Physics Part I

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CBSE Gujarat Board Haryana Board

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Class 10 Class 12
Figure 5.17 shows the position-time graph of a body of mass 0.04 kg. Suggest a suitable physical context for this motion. What is the time between two consecutive impulses received by the body? What is the magnitude of each impulse?

Between x = 0 and x = 2 cm, ball will be rebounding between two walls.

After every 2 s,

Magnitude of the impulse received by the ball = 0.08 × 10
–2 kg m/s

The given graph shows that a body changes its direction of motion after every 2 s.

Physically, this situation can be visualized as a ball rebounding to and fro between two stationary walls situated between positions 
x = 0 and x = 2 cm.

The slope of the position-time
 graph reverses after every 2 s, the ball collides with a wall after every 2 s.

Therefore, ball receives an impulse after every 2 s.


Mass of the ball, m = 0.04 kg

The slope of the graph gives the velocity of the ball.

Using the graph,

Initial velocity, (
u) = fraction numerator left parenthesis 2 space minus space 0 right parenthesis space straight x space 10 to the power of negative 2 end exponent over denominator left parenthesis 2 minus 0 right parenthesis end fraction space equals space 10 to the power of negative 2 end exponent space m divided by s
Velocity of the ball before collision, u = 10–2 m/s

Velocity of the ball after collision, v = –10–2 m/s 

Here, the negative sign arises as the ball reverses its direction of motion.

Magnitude of impulse = Change in momentum 

                                 = | mv - mu |

                                 = | 0.04 (v - u) |

                                 = | 0.04 (-10-2 - 10-2) |

                                 = 0.08 × 10-2 kg m/s 

State Galileo’s law of motion.

Galileo’s law of motion states that, a body continues to move in the same direction with constant speed, if no force is acting on the body. 

What is Aristotle’s law of motion?

Aristotle’s law of motion states that an external force is required to keep the body in motion.

Define inertia.

The property by virtue of which the body cannot change its state of rest or uniform motion in a straight line, unless an external force is acting on the body is called as Inertia. 

When the branches of an apple tree are shaken, the apples fall down. Why?

The apple fall from an apple tree when it shaken because of inertia of rest. Apple is in a state of rest and when the tree is suddenly shaken, apples still tends to remain in it's same state of rest whereas branches move. 

So, the apples fall down. 

Is Aristotle’s law of motion now correct?

No. Aristotl'e law of motion is false. 

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