Q&A - PSLE Science

[RYKC_GOH]

interesting topics:?

[Mathmaniac]

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:

Let me try. When there's a collision, the ball will move faster or slower, ie (c) is correct. Depending on where it's hit, the direction will change too, ie (d) is correct. It will not stop abruptly and the shape will definitely not change.

So my answer is (3).

[meimeitan]

thanks a lot.

[fristrom]

Mathmaniac:

meimeitan:

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

Let me try. When there's a collision, the ball will move faster or slower, ie (c) is correct. Depending on where it's hit, the direction will change too, ie (d) is correct. It will not stop abruptly and the shape will definitely not change.

So my answer is (3).

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)

[tianzhu]

Hi meimeitan


It’ll better if more information is provided in the question.

The question mentions about some children playing a game with two tennis balls; we may infer that they are of similar mass. However, there could be different scenarios.

1)Two balls of same mass and same speed but opposite direction of movement collide head on in a collision.

2)Two balls of same mass and different speeds but opposite direction of movement collide head on in a collision.

3)When a moving tennis ball hits another moving tennis ball at an angle.

I’ll try to answer this question with the information within PSLE science syllabus.

In PSLE Science, students learned that a force can make a moving object

-Move faster/slower
-Change its direction of movement
-Stop moving
-Change the shape of an object.

If we are talking about the immediate activity after the collision, we should be able to see c) the speed of the ball is changed and (d) the direction of the ball is changed

Therefore, I am more inclined to go for option (3)

Best wishes

[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: