The document defines a force as that which causes changes in speed or deformation of an object. It describes different types of forces including contact forces and forces at a distance. Gravity is defined as an attractive force between masses that decreases with increasing distance. Weight is the force of gravity on an object and is measured in Newtons. Archimedes' principle states that immersed objects experience an upward buoyant force equal to the weight of the displaced fluid. Forces such as gravity, buoyancy and weight are used to explain why objects fall, float or are in equilibrium in fluids.

1. UNIT 4:FORCES
AND THEIR
EFFECTS

2. WHAT IS A FORCE?
 The direct cause of changes in speed and deformation
that a body can experience is called force.
 The forces are represented by vectors. They’re
drawn with arrows.
 TYPES OF FORCES:
 By contact: If the bodies touch each other. Ex: Pull a
door.
 At a distance: If the bodies don’t touch each other. Ex: A
magnet.
 Forces are measured in Newtons.

3. GRAVITATIONAL PULL
GRAVITACIONAL INTERACTION
 Gravitational interaction is always attractive. Two or more
masses always attract each other.
 The intensity of this attraction decreases as the distance increases.
THE WEIGHT OF BODIES
 The weight (P) of a body is the force which the Earth (or other
celestial bodies) has on the body.
 It’s measured in Newtons.
 Mass (M) measures the inertia of a body or the
amount of matter it has. Its value is always
the same.
 Mass and weight are linked to the acceleration of gravity (g)
P= MxG

4. ARCHIMEDES’ PRINCIPLE
 All bodies submerged in a liquid or gas experience a
vertical upward force called upthrust (E), which is
equal to the weight of the volume of liquid displaced by
the body.
Factors that affect
buoyancy:
1. The bigger the body is,
the bigger the volume
of liquid is displaced.
2. The denser the liquid
is, the greater the
upthrust.

5. E<P
E>P
E>P E=P E<P
E=P

6. ACTIVITIES
1.Can you explain why bodies when they are
thrown up in the air, will eventually fall?
2.Find out the acceleration of gravity on Mercury
knowing that a body with a mass of 45Kg
weighs 117N on this planet.
3.Illustrate the forces that act on a body submerged
in water.
4. An object is first submerged in fresh water, and
later in salt water. In which of the two cases will
the upthrust be greater? Show this with a
drawing.

7. 5. Answer the following questions briefly:
a) Why does a body sink in a liquid?
b)Why does a body float in a liquid?
c)Why does it stay in equilibrium?
6. If the acceleration of gravity in Mercury is 2.8 m/
s2 and in Neptune it’s 11 m/s2 , work out:
a) A body’s weight with 20 Kg of mass on both
planets.