Temasek Junior College
Integrated Programme
Year 2 Green Science
Forces and Pressure
Name:
Worksheet 5A: Types of Forces
Practical: Parts A & B: 75 min
SLS package (asynchronous): Part C
Class:
Date:
Learning Outcomes
At the end of this worksheet, you will be able to:
(a) Identify some examples of contact forces [e.g. frictional force, viscous forces including air
resistance (no treatment of the coefficients of friction and viscosity is required), normal
contact force] and non-contact forces [e.g. gravitational force, electrostatic force and
magnetic force]
Different types of forces
Forces can be categorized into contact and non-contact forces. Contact forces include friction,
viscous force and normal contact force while non-contact forces include gravitational force,
electrostatic force and magnetic force. In the first section of this worksheet, we are going to
identify some of these forces and observe their effects.
(A) Contact Force - Friction
There are two main types of friction: (i) static friction and (ii) kinetic friction. Static friction exists
between two surfaces that aren't moving relative to each other, while kinetic friction (also
known as dynamic/sliding friction) acts between objects in motion.
Static friction is what keeps a box from moving without being pushed, and it must be
overcome with a sufficient opposing force before the box will move.
Kinetic friction is the force that resists the relative movement of the surfaces once they're in
motion. For example, when we rub both our hands together, there is friction between our hands
and as a result, our hands get warm. The presence of friction cause heat to be dissipated.
Part A1 (15 min): Static Friction [Phone Book Friction]
THINK© – Trigger
As a class, watch the video titled [Phone Book Friction] by accessing the
following link https://tinyurl.com/hb4jv6d from 5:15 to 6:35 or by scanning
the QR code and answer the following questions.
Page 1 of 4
(a) Explain when static friction or kinetic friction exists between the pages of the book.
Static friction exists between the pages of the books when the pages of the books tend to
move with respect to one another but have not started moving / separating. (Moving apart
means that there is relative movement between the pages of the books.)
However, when the pages of the books start moving with respect to one another (start
separating), the friction that exist between them is kinetic friction.
(b) Explain whether static friction is small or large in magnitude.
Given that despite having several large trucks with ropes to pull the books apart in opposite
directions, the books remain to be unseparated, hence the static friction is large in
magnitude and possibly >5000 N.
I – Investigate (Pair work)
Now, it is your turn to experience the static friction that exist between the books. Using 2
books, interweave the pages of the books as shown below. Try separating the books between
you and your partner.
TH NK©
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(C) Non-contact force – Electrostatic force [SLS package (asynchronous)]
Electrostatic Force (Pair work)
THINK© – Trigger and Investigate
Step 1: Inflate and tie one balloon. Rub the balloon against your partner’s head. Slowly lift the
balloon upwards, away from your partner’s head. Observe what happens to your partner’s
strands of hair.
(a) Describe your observation.
When the balloon is being lifted, the strands of hair are attracted to the balloon and stand
up.
(b) Explain your observation.
During rubbing, charges move between the balloon and the hair such that eventually the
balloon and the hair becomes oppositely charged. Opposite charges attract and hence
the strands of hair are attracted to the balloon.
*Note: During rubbing, some electrons from the hair are deposited onto the balloon.
The balloon becomes negatively-charged while the hair becomes positively charged.
[Students not required to know which body becomes positively charged and which
becomes negatively charged]
Step 2: Now inflate 2 balloons. Rub both balloons with a polythene cloth for 3 minutes. Upon
rubbing both balloons, bring both the balloons together.
(c) Describe your observation. Explain why.
The balloons seem to be repelling one another. This is because both the balloons are now
of the same charge (negatively charged) and since like charges repel, both the balloons
repel one another.
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K© – Know [SLS package (asynchronous)]
THIN
Watch the video titled [Electrostatic Friction] by accessing the following link
https://tinyurl.com/v2j54g9
or by scanning the QR code and answer the following
questions.
(a) State the type of charges that exist and the observations between
like charges and unlike charges.
There are 2 kinds of charges, negative charges and positive
charges.
Like charges repel one another while unlike charges attract one
another.
(b) Hence, explain the observation shown by the 2 balloons in your
own experiment.
Since both the balloons acquired similar charges due to rubbing with the polythene
(same) cloth, both the balloons will repel one another as like charges repel one another.
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