Evan's Space

Wonders of Physics


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During impact of a free falling ball, the force on ground is greater than the weight of ball

In this post, it shows a free-falling ball from a height of 1.0 m. During the impact, the direction of the force on the ground is downwards and the force on the ground by the ball is greater then the weight.

As the ball is free-falling, the only force acting is its weight downwards. Hence a common misconception is to think that the force on the ground during impact is equal to the weight. This is wrong.

The normal force (force on the ball by the ground = stopping force on the ball by the ground) is greater than the weight.

The force on the ball by the ground is equal and opposite to the force on the ground by the ball. Hence the magnitude of the force on the ground is greater than the weight.

Similar concept can be applied if a man jumps off from a height. But in this case, the man’s leg will exert a stopping force over a short distance. That stopping force, once again, is greater than the weight of the man.


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Man Jumps Vertically Upwards, Pressure On Ground Is Greater During The Jump

This concept is similar to a 2016 O-Level Pure Physics Question P2 Q2, on why the pressure acting on the ground is greater during the jump, compared to when he is standing stationary on the ground.

During the jump, his leg will exert an upward force. This upward force (equivalent to normal force or force on the man by the ground) is greater than the weight of the man. Hence there is a net (resultant force) upwards, causing him to accelerate upwards.

That force on the man by the ground is equal and opposite to the force on the ground by the man. This is an action-reaction pair. Since the force exerted on the ground by the man is greater (greater than weight), the pressure exerted on the floor is greater.

(NOTE: Normal force and Weight is not an action-reaction pair)


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Light and sound wave diagram in different mediums with different density

Light and sound are both waves. So both carry energy from one place to another.

Light, which is part of the electromagnetic spectrum, is a transverse wave, It can travel through a vacuum at speed 3.0 x 108 m/s. As the light travels from an optically less dense medium (air) to an optically denser medium (liquid or glass), the light undergoes refraction and bends towards the normal due to a decrease in speed.

Light: Optically less dense medium to denser medium: 
– speed decreases
– wavelength shorter
– frequency remains constant

Sound is a longitudinal wave. It requires a medium to pass through and it cannot pass through a vacuum. Opposite to light, as the sound travels from a less dense medium (air) into a denser medium (water or solid), the speed increases.

Sound: Less dense medium to denser medium:
– speed increases
– wavelength longer
– frequency remains constant

Refers to the image below to understand how the waves behave in different mediums.
Click here to revise on the calculation of refractive index for light

light and sound


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Different kind of sounds

Students tend to think that ultrasound is a totally different kind of wave, or it is part of the electromagnetic spectrum, which is wrong!

The different kind of waves below are just different kind of sounds. So the properties of sound apply to all. Ultrasound is also sound wave, just that the frequency is greater than 20 kHz which is outside the range of audible frequencies for us.

different type of sounds

Radio Waves vs Sound – Similarities and Differences

Transverse Waves and Longitudinal Waves


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Which is better to cool the food?

ice in cooling food items

Other examples in our daily lives:

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In some supermarket, the seafood are placed outside of air-conditioned place. The seafood is kept cold by putting crushed ice covering the seafood to keep the them cold and fresh.

Refer to this Sci Physics question N2008P2Q6(b)

Capture1Capture2

Solutions: 
For the solid that does not melt, when thermal energy is absorbed from the surrounding  food, its temperature starts to rise. So it is not so effective at keeping the food cool.
For ice-pack, when thermal energy is absorbed from the surrounding food, it starts to melt. During melting process, a much larger quantity of thermal energy is absorbed from the food to melt per unit mass of ice, the temperature remains constant at 1oC, and the melting process is long. Hence ice-pack is more effective at keeping the food cool.

Related posts about ice:

Density of ice – Why ice floats on water?

Will whole lake be frozen during winter?

 


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Why lighting circuit should be connected in parallel?

Lighting circuit, in fact all circuits at home, should be connected in parallel.

Reasons being:
When one bulb is spoilt or switched off, the rest of the bulbs can still function normally at normal brightness. This is because the potential difference across each bulb in the branches remains the same.

Refer to series and parallel circuits summary.

If the lighting circuit is connected in series, when one bulb is spoilt or switched off, it will be an open circuit and no current can flow through the circuit. Hence all the bulbs cannot function.

Why lighting circuit should be parallel

Refer this post for the concept of earth wire and the fuse


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Important Concepts on Moments

When a system is in equilibrium, 2 conditions are met.

1) Principle of Moment (POM):
Sum of anticlockwise moment = Sum of clockwise moment

2) Net force = 0N; all forces are balanced:
Total downward forces = Total upward forces

Whenever a force passed through the pivot point, that force will not create any moment as there is no perpendicular distance from the force to the pivot. Hence this force will not be included in the calculation of moments.

2014 moments small


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Radio Waves vs Sound – Similarities and Differences

A common misconception by students is to associate sound waves produced by a sound system with radio waves. Signal is transmitted in the air via radio waves from the radio stations in the form of transverse waves. When sound which you hear that is produced from the speaker is sound waves which are longitudinal waves.

radio waves vs sound

 


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Thick Cables for High Power Electrical Appliances

Thick_cable_1Thick_cable_2Thick_cable_3Thick_cable_4Thick_cable_5

High power electrical appliances are those which draws high current.

The thinner the wire (smaller cross-sectional area), the higher the resistance of the cable which brings the current into the electrical appliances.

 

More thermal energy will be generated due to this higher resistance in the cable. The cable might be too hot or catch fire.

So it is advisable to have thicker cables for high power electrical appliance as thicker cables have lower resistance.