PSLE 2012 - Science

[shengp]

Active kid:

777:

Can't remember. Only know that i put option 2. Wat is option 4 ?

I also put (4) --The air in the balloon had lost heat to the material of the balloon.

i put 3 i think... :whut:

[shengp]

found the paper manageble. section A: okay, but tricky :imcool:

Section B: damn hard sia! :gloomy:

[creamyhorror]

RVHS:

Aslo for Yahoo answer
the frivction exists everwhere between surfaces
but if there is no other force in the horizontal direction and it rests on the horizontal surface then there is no frictional force acting on it

I haven't seen the question, but I'll just lay out some enrichment material (for the people who are interested in physics):

If there's an object sitting on a flat platform, and the flat platform and object are moving sideways at a constant velocity together, then I would say there isn't any horizontal frictional force acting on the object.

However, if the platform is speeding up (accelerating), then there is definitely a frictional force acting on the object. That force is what causes the object to speed up along with the platform. Without that force, the object would slip backwards (due to its inertia).

It's like sitting on top of a car: if the car is cruising at a constant speed, you can sit easily. There's no friction pushing you sideways. If the car starts to speed up, friction between your bottom and the car surface is holding you there and pushing you forward, so that you can speed up along with the car. If the frictional force is insufficient, you'll slide backwards because your body isn't keeping up with the car, and possibly fall right off.

[Fat Mama]

I think the qn the kids discussing is :


Is there friction acting on a stationary box resting on a horizontal surface.

Anyone can help?

[creamyhorror]

Fat Mama:

I think the qn the kids discussing is :


Is there friction acting on a stationary box resting on a horizontal surface.

If the surface is stationary, then I would say no, there's no friction acting on the box. If the surface starts to move, then friction starts to act on the box.

It's a tricky question for primary school, I think. These are things that are clarified in secondary physics.

[Fat Mama]

Both box and surface is stationary. So any friction acting on the box?

[ultimateschooler]

The force of friction is a force acting from a surface in the opposing direction of the body’s tendency to slide. It is due to both the normal force pushing from a surface against a body and a condition of the surface called the coefficient of friction. Dependent on whether the body is at rest or moving there is a different coefficient of friction. Static friction is used when the body is at rest and is defined by

Ffriction=Nμs

where Ffriction is the force of Friction, N is the normal force, and μs is the coefficient of static friction. Kinetic friction is used when the body is sliding along the surface and is defined by
Ffriction=Nμk
where μk is the coefficient of kinetic friction

[ultimateschooler]

Static friction > 0.

[Campbell]

Regarding the question concerning mist - actually the answer is very easy.

Ans: The tiny droplets or fog, quickly evaporate, cooling the surrounding air.

[RVHS]

ultimateschooler:

The force of friction is a force acting from a surface in the opposing direction of the body's tendency to slide. It is due to both the normal force pushing from a surface against a body and a condition of the surface called the coefficient of friction. Dependent on whether the body is at rest or moving there is a different coefficient of friction. Static friction is used when the body is at rest and is defined by

Ffriction=Nμs

where Ffriction is the force of Friction, N is the normal force, and μs is the coefficient of static friction. Kinetic friction is used when the body is sliding along the surface and is defined by
Ffriction=Nμk
where μk is the coefficient of kinetic friction

The force of friction is a force acting from a surface in the opposing direction of the body's tendency to slide[/color]
if someone try to push but box still stationary, then there is friction
so for A, no friction as no push or pull force.