FULL QUESTION: Two balls are attached to the bottom two separate rods with a type of glue that melts once it reaches 30°C. One rod is made of 1Kg of Iron and the other of 3.5Kg of lead. Given that the energy of both rods is increase by 100J/s simultaneously, and they both start at 20°C.

(The Specific Heat Capacities are: Iron 450J / kg °C and Lead 130J / kg °C)


a) Which ball drops first.

b) The time taken until both balls have dropped.

Full Solution:

For part a) the first thing you must do is calculate the energy required to increase each rod by one degree Celsius. To do this you use the equation:

E = m × c × θ

And as θ=1°C

E = m × c

So for the Iron rod m=1Kg and c=450J/Kg°C, Therefore:

E = 450 × 1 = 450J

And for the Lead rod m=3.5Kg and c=130J/Kg°C, Therefore:

E = 130 × 3.5 = 455J

This means that the Iron rod takes less energy per degree to heat up, and, because each rod receives the same energy per second, the Iron rod will reach 30°C and therefore that ball will be the first to fall.

For part b), now you know that the ball attached to the iron rod drops first and thus the ball attached to the lead drops second, the time taken for both balls to drop will be the time taken for the ball attached to the lead rod to drop.

First you need to find the energy required to heat the lead rod to 30°C.

So we know that the rod needs to be at 30°C and starts at 20°C, therefore θ= 30 - 20 = 10°C.

Now the variable for the lead are m=3.5 Kg, c=130 J/Kg°C and θ=20°C.

Input these into the Specific heat capacity equation you get:

E = 3.5 × 130 × 10 = 4550J

This is the required energy for the lead rod to increase 10°C.

The question states that the rods receive 100J per second; therefore, all that is needed is to divide the required energy by the energy per second:


This gives the seconds required for the ball attached to the Lead rod to drop, which is also the time taken for both balls to drop and is the answer to part b.

George P. GCSE Physics tutor, GCSE Maths tutor

2 years ago

Answered by George, a GCSE Physics tutor with MyTutor

Still stuck? Get one-to-one help from a personally interviewed subject specialist


£18 /hr

Joseph B.

Degree: Accounting and Finance (Bachelors) - LSE University

Subjects offered:Physics, Maths+ 1 more

-Personal Statements-

“Previously I have found that all pupils learn differently, so need to be taught differently, thus, tailoring my lessons to suit individuals.”

MyTutor guarantee

£18 /hr

Alex R.

Degree: Physics (Masters) - Edinburgh University

Subjects offered:Physics, Music+ 3 more


“About Me: I am a Physics student at the University of Edinburgh, however I also study Computer Science. I believe there is a beauty to Science and Mathematics, however it requires hard work and understanding to be fully appreciated. I ...”

Kirsty M. A Level Maths tutor, GCSE Maths tutor, 13 Plus  Maths tutor...
£20 /hr

Kirsty M.

Degree: Computer Science (Bachelors) - St. Andrews University

Subjects offered:Physics, Maths+ 2 more


“ABOUT ME I am a computer science student at the University of St Andrews, and have always been enthusiastic about maths and science, to such an extent that I am a STEM ambassador, a title given to people who encourage school pupils to ...”

About the author

George P. GCSE Physics tutor, GCSE Maths tutor

George P.

Currently unavailable: for regular students

Degree: Physics With a years Professional Experience (Masters) - Exeter University

Subjects offered:Physics, Maths


“I am a first year Masters Physics student at my first choice, Exeter University. I am friendly, patient and enthusiastic and I enjoy working through problems and helping others to understand the concepts behind them. I am passionate ...”

MyTutor guarantee

You may also like...

Other GCSE Physics questions

What's the energy efficiency equation and how do I use it?

A rollercoaster carriage of mass 100kg has 45kJ of Kinetic Energy at the lowest point of its ride. Ignoring air resistance and friction between the wheels and the tracks, what is the maximum height above this point it could reach? [Take g as 10m/s/s)

Do batteries contain current, which comes out when they are in a circuit?

What is the difference between a scalar and a vector quantity?

View GCSE Physics tutors

We use cookies to improve your site experience. By continuing to use this website, we'll assume that you're OK with this. Dismiss