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Class 9 Physics (India)

Course: Class 9 Physics (India)>Unit 3

Lesson 3: Motion of objects in the influence of gravitational force of earth

Free fall 1 body - solved example

Let's solve problems on one body in free fall. Created by Mahesh Shenoy.

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• At I think we can use the equation which has time. We know that a=v-u/t, we know v and u and a, so we can calculate t and use the equation with t in it.
• You can do it like that, but it would be a time waste if we first solve for time and then use the equation while we have another for which we do not need time.
(1 vote)
• At he says that when an object speeds up it means the acceleration is positive but when its speed is decreasing, the acceleration is negative, but in the practice they consider upward movement as positive and downward as negative and I'm not able to understand it. Can someone please explain this?
• When u jump them there comes a time when u reach a maximum height and then start falling down. Why does this happen? This is because of acceleration due to gravity. Why do u slow down when u jump? Its because of g. So when u jump u go against g therefore it is negative. Similarly when u fall u seem to speed up some how, this is where u go with g therefore it is positive

Going up= going against acceleration due gravity = negative g

Falling = going with acceleration due to gravity = positive g

( g here means acceleration due to gravity)
• When to take “g” negative?
(1 vote)
• Whenever the direction of motion (taken positive) is against gravity(i.e. upwards), g is taken as negative. Negative value actually means nothing but something in an opposite direction

Hope that helped. Feel free to comment if you doubt persists :)
• What is a definition of Free Fall?
• Free fall means when there is only one force acting on the body which is the gravitational force.
• Physics is hard enough without switching up the variable letters. I f this is an AP physics lesson, then please use the symbols that the course uses.
(1 vote)
• @ he considered the acceleration is negative, but in previous videos they take the acceleration is negative and displacement is negative in downward direction as in projectile motion videos , Could anyone explain what is the difference?
(1 vote)
• maintain consistency in your videos and the practice questions. in the video, he says whenever something goes up, its acceleration due to gravity is negative because 'it slows down'. in the practice questions, however, the upward movement is positive. this has confused me to no ends as to what to consider between the two conventions.

i made so many mistakes in the practice numericals' positive and negative signs just due to this confusion.
(1 vote)
• In the spiderman example the initial velocity should also be zero since the object is jumping which means it is moving upwards from rest to motion. Pls clear my doubt.
(1 vote)
• Intial velocity wouldn't be 0 because he jumps with a velocity of 20m/s. That means he uses that 20m/s to escape the force of gravity to attain a certain hieght.
(1 vote)
• At he says that negative acceleration and deceleration are the same and when an object has a positive acceleration, it is speeding up. Is this true? If an object is moving towards the left (in a coordinate plane where right is positive) and is speeding up, then his acceleration would be negative, but it wouldn’t be decelerating.
(1 vote)
• In case of gravity if the object is speeding up the acceleration has to be positive since the the speed is increasing. And it the object is slowing down as in the spiderman case then the acceleration will be negative. Nowhere did the instructor talk about the coordinate plane here.
(1 vote)