ESL Academic English - FORCES
Vocabulary & Structures used to describe and explain
FORCES & MOTION:

What is a FORCE?
A FORCE is an INTERACTION between 2 OBJECTS.
How can 2 objects interact?
.... by pushing or pulling at each other
we say:
one object EXERTS A FORCE on another object
a force ACTS UPON/ON an object
ATTRACTS*
REPELS*
pushes
pulls

*to attract <> to repel (you may have heard these verbs used to talk about magnets)
to pull <> to push
the NOUN forms are:
attraction <> repulsion
the ADJECTIVES are:
attractive <> repulsive
ADJECTIVES we use to DESCRIBE a FORCE
an attractive force a repulsive force
a weak force a strong force

WHEN 2 objects exert a force upon each other, if one force is stronger and the other force is weaker, we can see the effect of the force:
FORCES can have 3 different EFFECTS (do 3 things):
1. CHANGE THE SPEED of an object (make it move faster/slower
2. CHANGE THE DIRECTION of a moving object
3. CHANGE THE SHAPE of an object (e.g. squeezing a sponge, changes its shape





CLASSIFICATION of FORCES:
There are 3 classes/types of forces
TYPE of FORCE
NAME of FORCE
details
Contact forces

ONLY happens when the 2 objects are TOUCHING
a push force
a pull force
e.g.
  • weight
  • the normal force
the 2 objects are IN CONTACT i.e. touching each other
Non-contact forces
also called:
action-at-a-distance forces**
  • the gravitational force / gravity
  • magnetism
  • the electrical force
these forces can affect objects at a distance i.e. when there is NO CONTACT
resistance forces

(to resist something = to try and stop/ or prevent it happening)
  • friction


  • air resistance


  • water resistance
  • The force of friction acts when 2 objects are touching and one of them is moving on top of the other (e.g. car on road surface)
  • The resistance of the air to an object moving through it
  • the resistance of the water to an object moving through it
In English we often use the word WEIGHT in everyday life BUT we often use it INCORRECTLY – when we should be using the word MASS (the amount of matter in an object) instead, because WEIGHT is a FORCE so we CANNOT measure it in kilos or grams!

HOW do we MEASURE forces?

with a force meter
using Newtons (N) as a unit of measurement

We measure a property e.g. height, temperature, mass, weight
using an instrument e.g. ruler, thermometer, bathroom scales, a force meter ..
and counting the nº of units of measurement e.g. metres, degrees Celsius, kilos, .....

BE VERY CAREFUL: METER and METRE are 2 different words with different meaning !!
one is an INSTRUMENT
the other is a UNIT of MEASUREMENT
Which is which???? (hint: look at the examples above)


DESCRIBING MOTION
an object can either be:
1. MOVING – it is IN MOTION
2. NOT MOVING – it is AT REST

DESCRIBING FORCES and MOTION

FORCES can be either:
1.BALANCED – this means the forces acting on the object are equal i.e. they balance out (cancel out)................. So if the forces are balanced,**
  • a stationary object DOES NOT MOVE
  • a moving object CARRIES ON MOVING AT THE SAME SPEED AND IN THE SAME DIRECTION
  • the RESULTANT/NET FORCE must be 0 N (zero Newtons)
2.UNBALANCED – one of the forces is stronger than the other
The RESULTANT/NET FORCE – UNBALANCED FORCES
When more than one force is acting on an object ....
  • if all these forces are acting in the same direction (e.g. pushing), then you ADD them together to get the resultant force
  • if these forces are acting in different directions (e.g. push force & friction) then you SUBTRACT them to find the resultant force.
It is THE RESULTANT/NET FORCE that makes the object change its speed, direction or shape.



GO TO: MOTION - Unbalanced Forces




Forces in motion:
Forces and Motion: Basics
Click to Run


LINKS

The 3 LAWS

Newton & The Laws of Motion


EXERCISES

Spotlight Science 8
Co-ordinated Science