Force, Momentum and

Impulse

12

12.1 Introduction

ESBEO

In Grade 10 we studiedmotion but not what caused the motion. In this chapter we will learn thata net

force is needed to causemotion. We recall whata force is and learn about how force and motion are

related. We are introduced to two new concepts, momentum and impulse, and we learn moreabout

turning forces and the force of gravity.

See introductory video:VPkgi at http://www.everythingscience.co.za

12.2 Force

What is aforce? ESBEQ

A force is anything thatcan cause a change to objects. Forces can:

• change the shape of anobject


• accelerate or stop an object


• change the direction of a moving object.

A force can be classifiedas either a contact force or a non-contact force.

FACT

There is a popular story

that while Sir Isaac New-

ton was sitting under an

apple tree, an apple fell

on his head, and he

suddenly thought of the

Universal Law of Grav-

itation. Coincidentally,

the weight of a small ap-

ple is approximately 1 N.

FACT

Force was first described

by Archimedes of Syra-

cuse (circa 287 BC - 212

BC). Archimedes was a

Greek mathematician,

astronomer, philoso-

pher, physicist and

engineer. He was killed

by a Roman soldier

during the sack of the

city, despite orders from

the Roman general,

Marcellus, that he was

not to be harmed.

A contact force must touch or be in contact with an object to causea change. Examples ofcontact

forces are:

• the force that is used topush or pull things, likeon a door to open or close it


• the force that a sculptor uses to turn clay into apot


• the force of the wind toturn a windmill

A non-contact force does not have to touch anobject to cause a change. Examples of non-contact

forces are:

• the force due to gravity, like the Earth pulling the Moon towards itself


• the force due to electricity, like a proton and anelectron attracting eachother


• the force due to magnetism, like a magnet pulling a paper clip towardsitself

The unit of force is the newton (symbol N). This unit is named after Sir Isaac Newton whofirst defined

force. Force is a vectorquantity and has a magnitude and a direction.We use the abbreviation F for

force.

This chapter will often refer to the resultant force acting on an object. The resultant force is simply the

vector sum of all the forces acting on the object. It is very important toremember that all the forces

must be acting on the same object. The resultant force is the force that hasthe same effect as all the

other forces added together.