FORCES AND MOTION
All sports involve forces. Athletes could not run without the force of friction to prevent their feet
slipping. In every ball game, a force applied to the ball makes it move, whether from the kick of a
footballer or the action of a tennis racket.
A force is a push or pull acting upon an object as a
result of its interaction with another object.
Forces can cause motion of an object.
Effects of Forces
Forces can affect bodies in a variety of ways:
o Changes in speed: forces can cause bodies to speed up or slow down
o Changes in direction: forces can cause bodies to change their direction of travel
o Changes in shape: forces can cause bodies to stretch, compress, or deform
Force is measured in NEWTONS ( N)
FORCES AROUND US
Forces act on us all the time, but we often ignore them as we have got used to them.
CONTACT FORCES- requires contact between the objects to act
NON CONTACT FORCES- requires no contact between the objects to act
GRAVITATIONAL FORCE
Gravity is the force of attraction between any two objects as a result of their masses.
Non- contact force
Amount of gravitational force is related to the masses of the objects as well as the distance
between them
Gravitational Field- region where the force is experienced
You are attracted to Earth by a gravitational force.
MASS
Mass describes how much matter is present in an object.
Mass always remains the same no matter where it is located.
Measured in Kilogram ( Kg)
WEIGHT
Weight is defined as:
The force acting on an object due to gravitational attraction
Planets have strong gravitational fields
o Hence, they attract nearby masses with a strong gravitational force
Because of weight:
o Objects stay firmly on the ground
o Objects will always fall to the ground
o Satellites are kept in orbit
Weight, mass and gravitational field strength are related using the equation:
This can also be written as:
W=m×g
g is known as the acceleration due to gravity or the gravitational field strength
o On Earth, this is equal to 9.81 m/s2 (or N/kg)
The weight that an object experiences depends on:
o The object's mass
o The mass of the planet attracting it
Mass (measured in kilograms, kg) is related to the amount of matter in an object
Weight (measured in newtons, N) is the force of gravity on a mass
o The weight of an object and the mass of an object are directly proportional
o The size of this force depends on the gravitational field strength (often called
gravity, g, for short)
Mass = ____Weight_______
Gravitational field strength
Gravitational field strength = ____Weight______
Mass
Worked example
NASA's Artemis mission aims to send the first woman astronaut to the Moon. Isabelle hopes to
one day become an astronaut. She has a mass of 40 kg.
Take the Earth's gravitational field strength as 10 N/kg, and the Moon's gravitational field strength
as 2 N/kg.
Comment on the difference between Isabelle's weight on Earth, and her weight on the Moon.
Answer:
Step 1: State the equation linking weight and mass
o The equation linking weight and mass is:
W=m×g
Step 2: List the known values
o The question instructs us to take the Earth's gravitational field strength as 10 N/kg
o The question also instructs us to take the Moon's gravitational field strength as 2 N/kg
o Mass, m = 40 kg
Step 3: Calculate Isabelle's weight on Earth
o Substituting the values of mass and Earth's gravitational field strength into the equation
gives:
W = 40 × 10 = 400 N
Step 4: Calculate Isabelle's weight on the Moon
o Substituting the values of mass and the Moon's gravitational field strength into the
equation gives:
W = 40 × 2 = 80 N
Step 5: Comment on the two values of weight
o Isabelle's weight is greater on Earth than on the Moon
o This is because the Earth has a larger gravitational field strength than the Moon, so
Isabelle's weight force (the force of gravity pulling down on her) is larger on Earth than on
the Moon
