Weight and Mass

  • It is the force acting on mass due to gravity. It is the pull of gravity on the body – It is measured in Newtons (N) using a spring balance or forcemeter

π‘€π‘’π‘–π‘”β„Žπ‘‘ = π‘šπ‘Žπ‘ π‘  Γ— π‘”π‘Ÿπ‘Žπ‘£π‘–π‘‘π‘¦

W = mg

  • The gravitation field strength near the surface of the earth is 10Nkg-1. Therefore a mass of about 100g (0.1kg) has a weight of just less than 1N on the earth’s surface.
  • Its weight on moon is only 0.16N. The gravitational acceleration on the moon is not the same as on the earth. On the moon gravitational acceleration is 1.6Nkgs-1 while on earth is 10Nkg-1

Example: An astronaut and his spacesuit have a total mass of 120kg. What will be his

(a) Weight on earth (b) Mass on moonΒ  Β (c) Weight on moon


Differences between mass and weight

Β Β Β Β Mass Β Β Β Β Weight
  • Is a measure of the body’s inertia
  • Measured in kilograms using a balance
  • Depends on inertia
  • Is independent of the position of a body
  • Has only magnitude and no direction
  • force acting on mass due to gravity
  • Measured in newtons using a force meter or spring balance
  • Depends on mass
  • Varies according to the gravitational field in which the body lies
  • Has both magnitude and direction


  • Linear momentum is defined as the product of mass and velocity
  • It is a measure of an object’s resistance to stop.
  • The units are kgms-1

π‘šπ‘œπ‘šπ‘’π‘›π‘‘π‘’π‘š = π‘šπ‘Žπ‘ π‘  Γ— π‘£π‘’π‘™π‘œπ‘π‘–π‘‘π‘¦

  • For example, that a body of mass 2 kg travelling 3ms-1 has a momentum of 6kgms-1. A body of the same mass travelling at the same speed but in the opposite direction has a moment of 6kgms-1

    Example 1 A trolley has a mass of 30Kg. The trolley is moving at a constant (uniform) velocity of 2ms-1 to the right. Calculate the momentum of the trolley.

    Example 2 calculate the momentum of a cruise tuner of mass 20 000tonne when travelling at 6.0ms-1 (1 tonne = 1000kg)



  • It is the property of a mass which resists change from its state rest or uniform motion.
  • The inertia of an object refers to the reluctance of the object to start moving if it is stationery or reluctance of the object to stop moving if it is moving in the first instance.

Newton’s laws of motion

Newton’s first law

A body will remain at rest, or to move with a constant velocity, unless acted upon by a resultant force

  • The first law tells us two things i.e. if there is no resultant force acting on an object at rest, the object will remain at rest an if there is no resultant force acting on a moving object, the moving object will continue to move at a constant speed in a straight line, i.e. constant velocity.


  • Drivers need to wear seat belts when driving because of the effect of inertia. Without the seat belt, if the driver suddenly applies his car brakes, he will continue to move forward due to his inertia and crash into the windscreen.
  • Slide a glass full of water across a table to the right. The water in the glass will seem as though it is moving to the left of the glass. This is a result of inertia.
  • A foot ball will remain stationery forever unless moved by some external force


Newton’s second law

The acceleration of mass is proportional to the force on it, provided the mass stays the same

  • Newton’s second law tells us that when there is a resultant force acting on an object, the object will either slow down (decelerate) or speed up (accelerate)

𝐹 = π‘šπ‘Ž

Example 1 A boy pushed a box of mass 20kg with a force of 50N. What is the acceleration of the box?

Example 2 What is the force that acts on 2kg mass when a constant acceleration of 3m/s-2 is expressed?

Example 3 A car of mass 1.2 tonnes accelerates from 5ms-1 to 30ms-1 in 7.5s. Calculate the average accelerating force on the car.

Verifying Newton’s second law experimentally

  • Place a trolley on the top of sloped plank and friction compensates the slope. This means lift the plank till the trolley is just ready to roll i.e. the friction forces balance

    the gravitational forces. Any further force applied to the trolley will cause it to move.

  • Place a mass on the hook, which will cause the trolley to move down the slope, and its acceleration can be found from the ticker-tape.
  • Record this result. As the mass is subject to gravitational force, this must be what is causing the acceleration, a. This mass value must be multiplied by 10 to change into Newtons. Record this value as being the acceleration force, F.
  • Repeat the procedure at least six times, but change the mass which is causing the accelerating force so that a graph of acceleration can be plotted against force

Newton’s third law

Action and reaction are equal and opposite

  • If body A exerts a force on body B then body B will exert a force on body A of equal magnitude but in opposite direction.
  • Forces occur in pairs i.e. action force and reaction force
  • The action and reaction are equal in magnitude, act opposite to one another and act on different bodies.
  • A high jump will exert an action on the ground which reacts by causing the jumper to spring upwards.
  • As birds push down on the air with their wings, the air pushes their wings up and gives them lift thus making it possible for them to fly
  • A fish uses its fins to push water backwards. The water reacts by pushing the fish forward, propelling the fish through the water.
  • The baseball forces the ball to the left and the bat forces the ball to the right.

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