Q&A - PSLE Science

[Mathmaniac]

fristrom:

This is the part I am having trouble with. Assuming the two balls A and B:

- the balls have the same mass
- collision is elastic

Before collision:
A -> <- B

After collision:
<A>

The balls should just change direction without changing the speed, as speed a scalar quantity... my answer would be (1)

I don't understand why the speed can remain the same? :?

[fristrom]

Mathmaniac:

The balls should just change direction without changing the speed, as speed a scalar quantity... my answer would be (1)




I don't understand why the speed can remain the same? :?

For the two balls A, B:
KE_before = 0.5 (mass_A) (velocity_A)^2 + 0.5 (mass_B) (velocity_B)^2

assuming elastic collision,
KE_before = KE_after = 0.5 (mass_A) (velocity_A)^2 + 0.5 (mass_B) (velocity_B)^2

since mass of ball would not change due to collision, then, (velocity_A)^2 and (velocity_B)^2 would be the same before and after collision.

Velocity is a vector. With a change in direction, the sign would be changed before and after collision, but squaring it means the sign is immaterial.

Speed is scalar. As it has no sign, then, the numerical value should be unchanged.

It has been a long time since I do physics so my reasoning may not make much sense. I am not even sure at primary level, speed and velocity is the same thing...

[meimeitan]

It becomes complicated. Anyone can draw to the conclusion? :?

[Muffins]

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:

Hi from what I've learnt, the answer would be 3.

[Muffins]

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

:offtopic: :congrats: on your GMI!!!!!!

When you upgraded yourself, I upgraded myself :lol: :rotflmao:

[tianzhu]

Muffins:

:offtopic: :congratss: on your GMI!!!!!!

When you upgraded yourself, I upgraded myself :lol: :rotflmao:

Hi Muffins

Thank you.

Best wishes

[Mathmaniac]

fristrom:

Mathmaniac:

The balls should just change direction without changing the speed, as speed a scalar quantity... my answer would be (1)




I don't understand why the speed can remain the same? :?

For the two balls A, B:
KE_before = 0.5 (mass_A) (velocity_A)^2 + 0.5 (mass_B) (velocity_B)^2

assuming elastic collision,
KE_before = KE_after = 0.5 (mass_A) (velocity_A)^2 + 0.5 (mass_B) (velocity_B)^2

since mass of ball would not change due to collision, then, (velocity_A)^2 and (velocity_B)^2 would be the same before and after collision.

Velocity is a vector. With a change in direction, the sign would be changed before and after collision, but squaring it means the sign is immaterial.

Speed is scalar. As it has no sign, then, the numerical value should be unchanged.

It has been a long time since I do physics so my reasoning may not make much sense. I am not even sure at primary level, speed and velocity is the same thing...

While it is true about what you said regarding elastic collision, it is irrelevant here, especially in primary school context.

[meimeitan]

Hi


Actually I always feel warm as some of the families cannot afford to hire any tutors or join those famous or popular tution centres, here is the place where we can get help. Really, thanks a lot.

[atutor2001]

meimeitan:

Hi


Actually I always feel warm as some of the families cannot afford to hire any tutors or join those famous or popular tution centres, here is the place where we can get help. Really, thanks a lot.

Actually the answers we are getting here are equal if not better than top tuition centres - especially for mathematics.

[meimeitan]

Hi! Please help,


(A) Which of the following are possible reasons why aquatic plants and
animals cannot survive in a pond that contains mud particles?

a The water carries diseases.
b Sunlight cannot reach the aquatic plants
c The aquatic animals cannot get enough dissolved oxygen.
d There is not enough clean water for the aquatic plants and animals.

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

(B) The population of bullfrogs near a pond. The number of bullfrogs
incresed in the first few weeks. However, it stopped increasing the
following weeks. Which of the following events are possible causes
for the change in the population of bullfrogs from point A to point B?

a a great drop in the water level of the pond
b A great decrease in the number of fish in the pond
c A great decrease in the number of dragonfly nymphs in the pond
d A great decrease in the number of diseaseinfecting organisms that
kill the prey of the bullfrogs

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

Thanks.