Q&A - PSLE Science

[meimeitan]

Hi! TianZhu and Starlight1968sg


Thanks for previous explanation. However, i quite confuse about the term of gravitational force. If a man lying on the ground, there should be no gravitational force acting on him. But, how come i cannot lying on the ground when i am on the moon. As i dun hv clear concept, then i dun know how to answer the following question.

Which of the following object(s) do(es) not have any gravitational force acting on it?

(a) a bird flying in the sky
(b) a man lying on the floor
a balloon rising into the air

Many thanks for your kind help.

[fristrom]

It has been a long time that I do physics… so please correct me if I am wrong.


From recollection, gravitational force is an universal attractive force between two objects, so all 3 cases would have gravitational force acting on the object.

"how come i cannot lying on the ground when i am on the moon" … because gravitational force is proportional to the mass of the object. As moon is much lighter than earth, the gravitational force is much smaller.

[meimeitan]

Hi Firstrom


thanks

[Muffins]

and by the way, meimeitan, you can lie down on the moon. The gravitational pull is ⅙ that of the Earth's, so you can still lie down on the moon....

[meimeitan]

Hi!


Ha ha ha .... :lol: Learning is with fun. Hope one day I can lie down on the moon.

[Muffins]

heehee… ya, when i was younger, i also wanted to be an astronaut… but now, since I have glasses…

[meimeitan]

Hi


never mind lah! U wear glasses so that u can solve more problems for me. Ha ha ha..... See! one more to come......

Some children are playing a game with two tennis balls. Each child rolls the tennis balls along the floor so that they collide. Which statement(s) is/are the effect of the force on the ball observed from the above activity?

(a) the ball stops moving.
(b) the shape of the ball is changed
(c) the speed of the ball is changed
(d) the direction of the ball is changed

(1) d only
(2) a and c only
(3) c and d only
(4) a, c and d only

Anyone who wears glasses also can pitch in to help. Thanks. :lol:

[fristrom]

Not sure whether I understand the question:


Some children are playing a game with two tennis balls. Each child rolls the tennis balls along the floor so that they collide. Which statement(s) is/are the effect of the force on the ball observed from the above activity?

Assuming that it mean:

Some children are playing a game with two tennis balls. Each child rolls the tennis balls along the floor so that they collide. What is the effect of the force on the ball?

In your list of possible answers, \"the shape of the ball is changed\" (i.e. b) is not used which suggests that the collision between the balls is elastic.

In elastic collision, the total kinetic energy of the two bodies after the encounter is equal to their total kinetic energy before the encounter.

Since both balls were moving before the collision i.e. total kinetic energy is not zero, then (a) cannot be correct.

The direction of the ball certainly changes after collision so we are down to either (1) or (2). Assuming that the balls are of equal mass, the speed of the ball should not changed because of the collision, as speed (as opposed to velocity) is a scalar quantity.

So (1) should be the correct answer... I think...

[tianzhu]

meimeitan:

Hi! TianZhu and Starlight1968sg


Thanks for previous explanation. However, i quite confuse about the term of gravitational force. If a man lying on the ground, there should be no gravitational force acting on him. But, how come i cannot lying on the ground when i am on the moon. As i dun hv clear concept, then i dun know how to answer the following question.

Which of the following object(s) do(es) not have any gravitational force acting on it?

(a) a bird flying in the sky
(b) a man lying on the floor
(c) a balloon rising into the air

Hi meimeitan

In PSLE science, students learned that gravity pulls all objects towards the Earth. Gravity is acting on you all the time. Even when you are standing still or lying on the ground, gravity still pulls you downwards. Without gravity, you would be floating in the air.

Yes, gravity or gravitational forces have effect on the three cases you mentioned.

Here a video, this may be some help to you.
http://www.cosmolearning.com/video-lectures/physics-in-primary-schools-forces-gravity-7090/

Best wishes

[meimeitan]

Hi!


Regarding the ball game, anyone can pitch in to help. Thanks.