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Chapter 34 Force
LEARNING OUTCOMES
IN THIS SECTION, YOU WILL LEARN:
What a force is and what it does.
The unit of force.
How to name different forces and their effects.
What friction is.
What lubrication is and how it is used.
The force of weight and how it is calculated.
The relationship between the stretch in a spring and
the force causing it.
What is a force?
A force gives velocity to an object that is at rest or it changes the
velocity of a moving object.
This is the same as saying that a force causes a mass to accelerate.
Remember: An acceleration can be a change in speed OR a change in
direction OR both!
Measuring force
The force on an object is measured by multiplying the
mass of the object by the acceleration.
Force = Mass x Acceleration
The unit of force is the Newton N
The unit of force
If you look at the right hand side of the formula, mass has the unit of kg
and acceleration has the unit of m/s2, so the unit of force is kg.m/s2.
This unit is given the name of Newton, in honour of the famous scientist
Isaac Newton.
The symbol for the Newton is N.
Different kinds of force
and their effects
Type of force
Effects of the force
Push or pull
Starts an object moving or changes its velocity.
Weight
Pulls an object to the earth and the earth to the object.
Tension
Holds an object on the end of a string or a spring.
Friction
Tries to slow down or stop a moving object.
Electrical
Attracts or repels electrical charges.
Magnetic
Attracts or repels magnetic objects.
The force of friction
Friction is a force that tries to stop the motion of an object when the
object is in contact with another object.
Examples of Friction
Friction between our shoes and the ground stops our feet
slipping backwards.
Friction between car tyres and the road stops the car
skidding.
Friction is useful in these cases, but friction between objects
can also cause wear and damage.
Lubrication
When friction causes wear and tear a material can be placed between
the surfaces of the two objects to reduce friction. The material is called
a lubricant
Examples: putting oil in a car engine or putting grease on the hinge of a
door or gate.
The force of weight and how to calculate it
The weight of an object on the earth is the force of
attraction between the earth and the object.
The weight of an object is found using the force
formula F = ma.
The acceleration that an object gets from the earth
is the same for all objects and is approximately
10m/s2.
Calculation of weight
To find the weight of an object, use the force formula with the
acceleration 10m/s2.
Example: What is the weight of a book which has a mass of 1.5kg?
Solution:
F  m.a
 1.5 x10 kgm / s
 15 N
2
The relationship between the stretch in a spring
and the stretching force
When a spring or an elastic is stretched, its length increases. The
increase in the length of an elastic or spring is called the extension.
The bigger the pulling force, the bigger the extension or, in the language
of physics, the extension increases as the force increases.
Summary: Key points
A force causes an object that is not moving to move or changes the
velocity of an object.
The change in velocity can be a change in its speed or its direction or
both.
The unit of force is the Newton, symbol N.
Friction is a force that opposes the motion of objects when they are in
contact.
Summary (continued)
Lubrication is used to reduce friction between moving objects in
contact.
Weight is a force caused by the attraction between an object and the
earth.
Weight is calculated by multiplying mass by the earth’s acceleration
(10m/s2).
The extension in an elastic or spring increases with the stretching force.