Scope of the presentation
 Scope of this presentation is to learn below
engineering concepts :
 Gravity
 Weight
 Mass
 Force
Note : All the above engineering concepts are interlinked
with each other.
Gravity
 Newton was the first person to seriously study
gravity
 Gravity is a force that attracts all objects toward
each other.
 The force of gravity is measured in units called
Newtons (N).
Gravity
 The strength of gravity between two objects
depends on two factors:
 1. Masses of the objects (If mass increases, force
also increases)
 2. Distance between the objects (If distance
increases, force decreases)
Gravity
 The greater the mass, the greater the force
 The greater the distance, the less the force
 Acceleration due to gravity = 9.8 m/s/s or 9.8 m/s2
Mass and weight
Mass Vs Weight
MASS
WEIGHT
Is the amount of matter in an is the force of gravity on an
object
object
Always remains constant
Weight of an object
changes if the gravity
changes
Does not depend on gravity
Weight=mass X Gravity
Measuring unit for Mass
 Mass is measured in
 grams and kilograms.
 1 paper clip= 1 gram
 1000g=1 kilogram
Force
 A force is a push or a pull.
 Force gives an object the energy to move, stop
moving, or change direction.
 When you write with a pen you exert a force.
When you peddle your bike, blow your nose, chew
your gum, or swimming in a pool, you are exerting
forces on other objects.
 We would never be able to move without exerting
forces on things
Newtons 2nd Law: Fnet = m a
 The acceleration an object undergoes is directly
proportion to the net force acting on it.
 Mass is the constant of proportionality.
 For a given mass, if Fnet doubles, triples, etc. in
size, so does a.
 For a given Fnet if m doubles, a is cut in half.
 Fnet and a are vectors; m is a scalar.
 Fnet and a always point in the same direction.
 The 1st law is really a special case of the 2nd law
(if net force is zero, so is acceleration).
What is Net Force?
F1
F2
F3
Fnet
When more than one
force acts on a body,
the net force
(resultant force) is the
vector combination of
all the forces, i.e., the
“net effect.”
Net Force & the 2nd Law
For a while, we’ll only deal with forces that are
horizontal or vertical.
When forces act in the same line, we can just
add or subtract their magnitudes to find the
net force.
32 N
15 N
2 kg
10 N
Fnet = 27 N to the right
a = 13.5 m/s2
Normal force directions
 Up
 You’re standing on level ground.
 Sideways
 A ladder leans up against a wall.
 At an angle
 A race car takes a turn on a banked track.
 Down
 You’re in a roller coaster at the top of a loop.
Find out the forces?
Weight (N)
Reaction
Force (N)
Answer for Question 1
1) The first force is weight, and weight always acts from
the centre of the object. (strictly it acts from the centre
of mass, for example a lollipop’s weight would be
labelled approximately at the centre of the sweet,
rather than the centre of the stick, because the sweet is
heavier than the stick)
2) Reaction force acts at 90 degrees to the surface at the
point of contact between the surface and the object.
Therefore it is drawn acting from the floor upwards, at
the middle of the cat.
Find out the forces?
Weight (N)
Reaction
Force (N)
Friction
(N)
Answer for Question 2
 The reaction force always acts at 90 degrees to the
slope of the surface at the point where the cat and roof
meet.
 Weight always acts down.
 Friction acts parallel to the surface, at the point of
contact between the object and the surface.
Find out the forces?
Weight (N)
Reaction
Forces (N)
Friction
(N)
Answer for Question 3
 This reinforces the ‘reaction force is at 90 degrees to
the surface at the point of contact’ as there are many
points of contact.
Find out the forces on WHEELBARROW ?
Weight (N)
Reaction
Force (N)
Friction
(N)
Lifting
Force (N)
Answer for Question 4
1) Weight acts from the centre of the wheelbarrow
2) Reaction Force acts from the point of contact between
the wheel and the ground, at 90 degrees to the ground.
3) Friction acts parallel to the surface, at the point of
contact between the object and the surface.
Find out the forces?
Weight (N)
Upthrust
(N)
Wind
Force (N)
Drag (N)
Answer for Question 5
1) Weight acts down form the centre of the boat
2) Upthrust acts like reaction force, except is the
equivalent for water.
3) Wind force pushes on the centre of the sail
4) Drag acts like friction, except for water. It opposes the
direction of motion, and occurs in water and air.
Imagine riding a motorbike on a still day, not even a
breeze. When you speed up, you feel a wind against
your face. This is the air pushing you back, and is
called drag. The same happens in water.
Find out the forces?
Weight (N)
Reaction
Force (N)
Driving
Force (N)
Drag (N)
Answer for Question 6
1) Weight, acting downwards from the centre
2) Reaction force, acting upwards at the wheels
3) Driving force, forwards direction - simply showing
this force is forwards is sufficient. See notes below for
further details on this force.
4) Drag – opposes motion
Find out the forces?
Weight (N)
Reaction
Force (N)
Force (N)
Answer for Question 7
1) Weight, from centre downwards
2)Reaction, from ground upwards
3)Wind and rain forces