Centripetal/Centrifugal Force

Centripetal/Centrifugal Force
This content is targeted at KS2 students and aims to
give them an understanding of
centripetal/centrifugal force by using exciting
demonstrations and practical activities.
Teacher Information
Centrifugal force is the apparent force that 'pulls' a spinning object away from
the centre of rotation. It may also be defined as the equal and opposite
reaction of centripetal force. Shown below is the example of a ball attached to
a fixing by a string. The ball is spinning around the centre of rotation and the
centrifugal force is pulling the ball away whilst the centripetal force (in this
example, the tension on the string) is keeping the ball attached to the centre
of rotation.
Object travelling in
circular motion
Centrifugal force is technically only an 'apparent' force because it is actually
just a result of Newton's 1st law of motion. This law states that an object will
continue at a constant velocity and trajectory until it is acted upon by another
Trajectory of object
Object travelling in
circular motion
When an object is travelling in a circular motion it is constantly being acted on
by the centripetal force pulling the object towards the centre of rotation and
therefore being pulled away from its trajectory. Centrifugal force is just the
object trying to maintain its initial trajectory and therefore appearing to pull
away from the centre of rotation.
It is also important to note that for an object to maintain a fixed orbit (i.e. not
move further away or closer to the centre of rotation) the centripetal and
centrifugal forces must be equal. If the centrifugal force outweighs the
centripetal force then the object will move further from the centre of rotation
and vice versa.
This explanation of centrifugal force is far beyond what is necessary at KS2 but
is, nevertheless, very useful background information.
In BLOODHOUND SSC each wheel spins at roughly 10,000 revolutions per
minute (rpm) at top speed and the edge of the wheel experiences roughly
50,000 radial g (i.e. 50,000 times the normal force of gravity) due to the
centrifugal force. One of the main limiting factors for the speed of
BLOODHOUND SSC is that, if the car were to go any faster, the centrifugal
force would be sufficient to break apart the wheel.
This document is a 2 lesson plan which aims to give KS2 students an
introduction to centripetal/centrifugal force through a demonstration and
practical activities.
Lesson 1 brings in the basic concept of centrifugal force with a demonstration,
practical activity and follow up questions. Lesson 2 introduces the idea of
centripetal force and explores some of the real life applications of these forces.
Learning Objectives:
To understand that forces apply to a spinning object which 'pull' material
to the outside.
The learn that weight and speed affect centrifugal force.
To practice using and understanding the terms centrifugal/centripetal.
To understand why centrifugal force can cause a challenge for engineers,
especially in BLOODHOUND SSC, but can also be very useful.
Possible Differentiation
All students should understand that spinning an object causes material to be
pushed to the outside, away from the centre of rotation.
Most student should understand that factors such as weight and spin speed will
affect the centrifugal force.
Some students will be able to understand, define, and correctly use the terms
centrifugal and centripetal and apply them to real life situations.
Key Vocabulary
Centre of rotation
Lesson 1 – Centrifugal Force
Spinning Bucket Demonstration
This demonstration involves spinning a bucket of water above your head in a
circular motion and showing that the water is kept in the bucket by centripetal
force. On the BLOODHOUND BLAST page there is a video showing the
demonstration. It is possible to show the video to the class in place of the
demonstration if necessary.
A bucket with a handle.
A plastic sheet to catch any spilled water or, alternatively, the
demonstration can be performed outside.
A source of water.
Note: If the experiment is performed inside be sure that the room is tall
enough such that the bucket can be spun without it hitting the ceiling.
1. Fill the bucket to about a quarter full with water. If the experiment is
being performed by the children themselves, rather than as a
demonstration, then a smaller bucket with less water in is advised.
2. Start with small, low swings to build up some momentum and then
quickly swing the bucket vertically in a full circle.
3. Once a full rotation has been completed slowly bring the bucket to a
Questions for students:
1) Why did the water stay in the bucket?
2) What would have happened if the bucket had been spun very slowly?
3) Can you think of any everyday examples the same forces apply? (Think
about the play park.)
Blu Tack Practical
This practical will demonstrate the variables which affect the centrifugal force
of an object. The practical involves students spinning string with Blu Tack
attached to the end around their finger and experimenting with different
weights and spin speed to understand their effect on centrifugal force.
Blu Tack
Students should experiment by spinning string with Blu Tack of various weight
stuck to the end. They should experience the increase in centrifugal force as
more weight is added to the end of the string. It should also be highlighted to
the students that as more weight is added it took them more energy to get the
string spinning. The students can also experiment by spinning the string at
various speeds and should feel the forces increase as the speed increases.
Once students understand the concept of centrifugal force they can perform
the experiment again but this time recording their results in a simple table
such as the one below.
Did the centrifugal force increase or decrease?
Spin faster
Spin slower
More weight
Less weight
Extension: Perform the experiment again but this time observe and record the
shape of the blu tack at the beginning and end of the experiment. Did the blu
tack change shape? If so, why? How could you make it change shape more or
Plenary / Extension
Think of a real life example of centrifugal force in action. Draw a diagram of
your example and label it to show the direction of the movement of the object
and direction of the centrifugal force. Try to think of the most interesting
example that you can!
Hint: anything that spins involves these forces.
Lesson 2 – Centripetal force and real life examples
Fill in the blanks with the most suitable answer from the list of words below.
(Beware, there are some words there just to trick you!)
Last lesson we performed an experiment where we spun blu tack on a string.
We experienced a force on the blu tack which pulled it away from the ….......
This force is called …......... force. The faster we spun the blu tack the ….........
the force. The force was also affected by the weight of the blu tack used and
the force became stronger when the weight was ….........
Centre of rotation
Centripetal force
It can be difficult for students to understand that there must also be also be a
force pulling the spinning object in towards the centre of rotation. A good way
of demonstrating this is to show the result when this force is removed. This
can be done my spinning the string with blu tack attached and then letting go.
By letting go you are removing the centripetal force (in this example, the
tension on the string) and the blu tack and string will be thrown outwards.
Real life examples of centrifugal and centripetal force
Example 1:
Hayley designs roller coasters for a theme park and wants to include a loop the
loop in her latest creation.
1) Draw a loop the loop, with a roller coaster cart at the top of the loop, and
draw arrows to represent the direction of the centrifugal and centripetal
2) The theme park health and safety team think its very important that the
roller coaster cart stays on the tracks during the loop. If the cart is going
to stay on the tracks, and not fall down, which of the two forces you
have drawn must be bigger?
3) In order to keep the cart on the track, should the roller coaster be
designed to travel slowly or fast?
Example 2:
Satellites are used for everything from communication to predicting the
weather and there are thousands of them currently in orbit around the earth.
The speed that the satellites are moving at is causing some centrifugal force
pulling them away from the earth.
1) Which force is stopping them from moving away? (Hint: its the same
force that brings you back down to the ground after you jump up.)
2) What would happen if a satellite suddenly slowed down, decreasing its
centrifugal force?
Example 3:
The wheels of BLOODHOUND SSC spin roughly 10,000 times each second. This
creates a massive amount of centrifugal force which tries to pull the wheel
apart (engineers call this 'radial g force').
1) Circle the correct property that the engineers of BLOODHOUND SSC
looked for when designing the wheels.
Strong / Weak
Heavy / Light
Hard / Soft
Flexible / Rigid
2) What material should the BLOODHOUND SSC engineers use to make the
wheels? Explain your choice.