9th Science NCERT Chapter 8
What is the product of an object's mass and velocity known as?
According to the Third Law of Motion, for every action, there is:
Why do we advise tying luggage kept on the roof of a bus with a rope?
Why does a cricket ball slow to a stop after being hit by a batsman?
What happens when two opposite forces of different magnitudes act on an object?
A truck starts from rest and rolls down a hill with a constant acceleration. It travels a distance of 400 m in 20 s. Find its acceleration.
An object of mass 1 kg travelling in a straight line with a velocity of 10 m/s collides with, and sticks to, a stationary wooden block of mass 5 kg. Calculate the total momentum just before the impact and just after the impact.
What is the momentum of an object of mass m, moving with a velocity v?
A hockey ball of mass 200 g traveling at 10 m/s is struck by a hockey stick so as to return it along its original path with a velocity of 5 m/s. Calculate the magnitude of change of momentum occurred in the motion of the hockey ball by the force applied by the hockey stick.
A bullet of mass 10 g traveling horizontally with a velocity of 150 m/s strikes a stationary wooden block and comes to rest in 0.03 s. Calculate the distance of penetration of the bullet into the block.
– First Law of Motion: An object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force. This is also known as the law of inertia【4:0†source】.
– Second Law of Motion: The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force, expressed as F = ma【4:4†source】.
– Momentum: The momentum of an object is the product of its mass and velocity, having the same direction as that of the velocity. Its SI unit is kg m s^-1. Momentum has both direction and magnitude .
– Third Law of Motion: To every action, there is an equal and opposite reaction, and they act on two different bodies. Action and reaction forces are always equal in magnitude but opposite in direction .
Define the momentum of an object. What is its SI unit?
Explain why it is advised to tie any luggage kept on the roof of a bus with a rope.
Why does a cricket ball slow down and come to rest after being hit by a batsman and rolling on a level ground?
A stone is thrown across a frozen surface of a lake and comes to rest after traveling a distance. What force causes the stone to stop?
Calculate the acceleration and the force acting on a truck rolling down a hill if it covers a distance of 400 m in 20 s with a mass of 7 tonnes.
Here are 3 DIY activities related to the chapter on the Second Law of Motion that students can easily do at home:
Balanced and Unbalanced Forces Demonstration:
- Materials Needed:Wooden block, two strings.
- Instructions:
Tie two strings X and Y to the opposite faces of the wooden block.
Pull only one string at a time to observe the direction in which the block moves.
Try pulling both strings with equal forces and note that the block does not move. This demonstrates balanced forces.
Finally, apply unequal forces through the strings to see the block move in the direction of the greater force, illustrating unbalanced forces.
Inertia Exploration with Carom Coins:
- Materials Needed:Carom coins (or similar objects).
- Instructions:
Stack the carom coins in a pile on a table.
Give a sharp horizontal hit at the bottom of the pile with another carom coin or striker.
Observe as the lowest coin quickly moves out, while the rest of the coins fall vertically due to their inertia.
Cart Motion Observation:
- Materials Needed:Plywood board, ball-bearing wheels, heavy object.
- Instructions:
Construct two carts using a plywood board and ball-bearing wheels as shown in the activity.
Place two children on separate carts and have them play catch with a heavy bag.
Observe if each child experiences an instantaneous force when throwing the bag.
Paint a white line on the cartwheels to observe the motion when the bag is thrown between the children.
These activities are fun ways to understand and experience the concepts of forces and motion detailed in the Second Law of Motion chapter. Enjoy learning through hands-on exploration!【4:0†source】【4:2†source】【4:3†source】.
Here are three real-world examples related to the topics covered in the chapter:
1. Inertia in Car Travel: When you are sitting in a moving car and the driver suddenly applies brakes, you may feel pushed forward. This is due to inertia, which is the tendency of objects to resist changes in their state of motion. The seat belt you wear helps reduce the impact by exerting a force on your body to slow down its forward motion.
2. Pushing a Motorcar: Imagine you and your friends pushing a motorcar. If you all push with the same force, the car moves at a constant speed. But if one person pushes harder, the car accelerates. This demonstrates Newton's second law of motion, where the acceleration of an object is directly proportional to the force acting on it.
3. Action-Reaction in Sports: In sports like cricket or hockey, when a player hits a ball, the ball reacts by moving in the opposite direction with an equal force. This exemplifies Newton's third law of motion, which states that for every action, there is an equal and opposite reaction.
By observing these daily occurrences, you can better understand the fundamental principles of motion and forces explained in your science chapter.
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