Cranberry sorting

Cranberries are a tart berry grown in the New England region of the United States. Fresh, high quality berries are fairly hard whereas rotten berries are squishy. To help sort the good berries from the bad, farmers developed a bounce board separator, in which the cranberries are dropped from a fixed height. Good ones bounce, rotten ones don't, and the machine is configured such that there is a wall the berries must bounce over to qualify as fresh. You can see a video of such a machine in operation here .

A rotten cranberry will lose at least 90 % 90\% of its total energy during a bounce. If the cranberries are dropped from a height of 10 cm 10~\mbox{cm} , what is the minimum height in cm of the wall so that no rotten cranberries could ever bounce over it?

Details and assumptions

  • Neglect any air resistance.
  • You may assume the path of the cranberries are almost vertical.


The answer is 1.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

23 solutions

Anton Than Trong
Jul 23, 2013

We must assume the case where the rotten cranberry loses 90% of its total energy. Dropping from a height of 10 centimeters, it can only bounce up to 1 cm, which is our answer.

Daniel Liu
Jul 27, 2013

10 × 90 % = 1 10\times 90\%=\boxed{1}

Evan Chien
Jul 27, 2013

LOL 10% of 10 much?

Hahaha what a joke this problem LOL

Evan Chien - 7 years, 10 months ago
Victor Carnaúba
Jul 26, 2013

Easy just use EP= m.g.h M and G = K 10-90%= 1

Prince Raiyan
Jul 26, 2013

As the berries are dropped from 10 c m 10cm height, the good berries will surely bounce over 10 c m 10cm height... It's given that the rotten berries loss 90% of it's energy... So, it'll not bounce over,

90% of 10 c m 10cm = 10 c m × 90 100 = 1 c m 10cm \times \frac {90}{100} = 1 cm

Hence, the required answer is 1 \fbox {1} ...

Leonardo Cidrão
Jul 23, 2013

By the mechanic energy conservation principle, when the object falls, all potencial energy becomes kinetic energy. However, the object loses 90% of its total energy, so we can conservate the energy from the bouncing instant to the moment it reaches the highest height:

0.1(mgH)=mgh ---> 0.1(10^-1)=h ----> h= 1cm.

Potential energy after bouncing will be 0.1 of mgh = mg (0.1)h The height to which they rise is 0.1 10 = 1 cm is the answer.

Matt McNabb
Jul 22, 2013

The cranberry has some gravitational potential energy when it is released. After bouncing, at its new highest point it will have 90% less gravitational potential energy: although some energy was converted to kinetic and back, conservation of energy tells us that none got 'lost' apart from the 90% specified.

For distances that are small relative to the size of the earth, gravitational potential can be approximated by (mass * height * acceleration due to gravity). So losing 90% of its energy means that it loses 90% of its bounce height (mass and gravity are constants).

Jerry Yu
Jul 22, 2013

A rotten cranberry will lose 90% of its energy, so only 10% energy will be left. Energy is directly proportional to the height, which means that 10% energy left will result in only 10% of height reached. Thus, 10% of 10 cm is 1cm

Willian Guerreiro
Jul 22, 2013

(M V^2)/2=10 (MgH/100) cause 90% was lost during the fall.

so V^2=2g ,by the conservation of energy : mgh=(mV^2)/2 , h=2g/2g=1

Mark Foster
Jul 22, 2013

By the Law of Conservation of energy applied on the cranberries, we can write Ep = mgh and Ek = Ep , and after they bounce they lose 90 % of their energy. So their energy is 0,1 * Ep = mgx, where x is the minimum height of the wall. 0,1*mgh = mgx => x = 0,1 h = 0,1 * 10cm = 1 cm.

Let the energy at the initial point of drop be = E

As the energy is only with respect to its height , E = m g H (where m is mass of the object g is the acceleration due to gravity & H is the height from ground)

Now as the berry hits the ground and bounces back it loses energy as given n now let energy = E_(2)

e = m g h (h is the new height) Given e = 0.1 E => m g h=0.1 * m g*H

h = 0.1 * 10 = 1 cm

Michael Tang
Jul 22, 2013

A rotten cranberry loses at least 90 % 90\% of its energy during a bounce, so it retains no more than 10 % 10\% of its energy. This means that the cranberry will only bounce at most 10 % 10\% as high as it was dropped from, so if it is dropped from 10 10 cm, it can only bounce up to 10 10 % = 1 10 \cdot 10\% = 1 cm high. Thus, the minimum wall height is 1 \boxed{1} cm.

Tim Vermeulen
Jul 22, 2013

The height of the wall must be at least the height of the rotten cranberries that bounce highest, so the height of the wall is the height a cranberry which loses 90 90 % of its energy will bounce to. The potential energy of the cranberry is proportional to its height. Therefore, if it is dropped from 10 10 cm and it loses 90 90 % of its energy, it only has the energy left to reach 10 10 % of its initial height, which is 10 0.1 = 1 10 \cdot 0.1 = \boxed{1} cm.

We can use the ratio principle to do this problem.

We know that the cranberries are dropped from h = 10 cm

If the cranberries are lost at least 90 % of it's energy.... So that's mean that, the cranberries only have 10 % of it's initial energy to bounce.

So, the minimum height in cm of the wall so that no rotten cranberries could ever bounce over it is : 10 % x h = 10 % x 10 cm = 1 cm

Ashiqul Islam
Jul 22, 2013

mgh/mgh'=100/10 so,h'=h/10=(10/10)cm=1 cm

Wee Hau Chin
Jul 22, 2013

Since 90% of the energy is lost, 10% of the energy is conserved. Therefore, mgh = (1/10)mgh . By cancelling out the mg, we know that the minimum height that the cranberry can bounce is 1/10 of the original height which is 1/10(10) = 1cm.

Tan Li Xuan
Jul 22, 2013

As a rotten cranberry will lose 90% of its energy during a bounce,then the minimum height of the wall is 1 9 10 × 10 c m = 1 10 × 10 c m = 1 c m 1-\frac {9}{10} \times 10cm=\frac {1}{10} \times 10cm=1cm

Mayank Kaushik
Jul 21, 2013

Energy in it = mg10

only 100-90 = 10% energy it will left

so only 10% of 10 cm = 1 cm will be the height

After the bounce, the rotten cranberry loses at least 90% of its energy, and so it can only have at most 10% left. Ten percent of 10 cm is one centimeter. Explanations aside, that's about it.

Paul Peh
Jul 21, 2013

Potential energy = mass x gravity x height

Since 90 % of energy is lost, 10% is conserved.

10/100 x m x g x 10 cm( before bounce)= m x g x h ( after bounce) Therefore, h= 10/100 x 10 = 1 cm

Kyle Kibodeaux
Jul 21, 2013

Seeing that a tenth of energy is consumed, (1/10)(10cm)=1cm

Rajath Krishna R
Jul 21, 2013

When a berry is dropped from a height h1 total energy of the berry is mgh1.

90% of this energy is lost in one bounce, therefore energy remaining=0.1mgh1.

After bouncing if the berry rises to a height h2 then total energy of the berry is mgh2. According to conservation of energy,

                                                 mgh2=0.1mgh1
                                                   i.e h2=0.1h1

Plug in the value and get the answer.

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...