Forces and Laws of Motion in Science

Updated on July 13, 2025 | By Learnzy Academy

What is Force?
Force is a push or pull acting on an object that can change its state of motion, speed or direction, and shape or size.
SI Unit of Force: Newton (N)

Types of Force:
Balanced Force – Does not change the state of motion, net force is zero.
Unbalanced Force – Causes motion or change, net force is not zero.

Newton’s Laws of Motion:

  1. Newton’s First Law of Motion (Law of Inertia):  An object remains at rest or in uniform motion in a straight line unless acted upon by an external force.
    Inertia is the tendency of a body to resist change in its state.
    Objects with more mass have more inertia.
    Examples: A book stays at rest until pushed. Passengers jerk forward when a car suddenly stops.
  2. Newton’s Second Law of Motion:  The rate of change of momentum of an object is directly proportional to the applied force and takes place in the direction of the force.
    Formula: F = m × a
    Where F is force, m is mass, and a is acceleration.
    Example: A 2 kg object with acceleration of 3 m/s² will have force F = 2 × 3 = 6 N.
  3. Newton’s Third Law of Motion:  For every action, there is an equal and opposite reaction.
    Examples: A gun recoils backward when fired. While walking, your foot pushes the ground backward, and the ground pushes you forward.
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List of question on "Forces and Laws of Motion"

  1. How much momentum will a dumb-bell of mass 10 kg transfer to the floor if it falls from a height of 80 cm? Take its downward acceleration as 10 m/s².
  2. Akhtar, Kiran and Rahul were riding in a motorcar that was moving with a high velocity on an expressway when an insect hit the windshield and got stuck on the windscreen. Akhtar and Kiran started pondering over the situation. Kiran suggested that the insect suffered a greater change in momentum as compared to the change in momentum of the motorcar (because the change in the velocity of the insect was much more than that of the motorcar). Akhtar said that since the motorcar was moving with a larger velocity, it exerted a larger force on the insect. And as a result the insect died. Rahul while putting an entirely new explanation said that both the motorcar and the insect experienced the same force and a change in their momentum. Comment on these suggestions.
  3. An object of mass 100 kg is accelerated uniformly from a velocity of 5 m/s to 8 m/s in 6 s. Calculate the initial and final momentum of the object. Also, find the magnitude of the force exerted on the object.
  4. An object of mass 1 kg travelling in a straight line with a velocity of 10 m/s collides with and sticks to a stationary wooden block of mass 5 kg. Then they both move off together in the same straight line. Calculate the total momentum just before the impact and just after the impact. Also, calculate the velocity of the combined object.
  5. A bullet of mass 10 g travelling horizontally with a velocity of 150 m/s strikes a stationary wooden block and comes to rest in 0.03 s. Calculate the distance of penetration of the bullet into the block and also calculate the magnitude of the force exerted by the block on the bullet
  6. A hockey ball of mass 200 g travelling at 10 m/s is struck by a hockey stick so as to return it along its original path with a velocity of 5 m/s. Calculate the magnitude of change of momentum that occurred in the motion of the hockey ball due to the force applied by the hockey stick.
  7. According to the third law of motion when we push on an object, the object pushes back on us with an equal and opposite force. If the object is a massive truck parked along the roadside, it will probably not move. A student justifies this by answering that the two opposite and equal forces cancel each other. Comment on this logic and explain why the truck does not move.
  8. Using a horizontal force of 200 N, we intend to move a wooden cabinet across a floor at a constant velocity. What is the friction force that will be exerted on the cabinet?
  9. What is the momentum of an object of mass m, moving with a velocity v? (a) (mv)² (b) mv² (c) ½ mv² (d) mv
  10. An automobile vehicle has a mass of 1500 kg. What must be the force between the vehicle and road if the vehicle is to be stopped with a negative acceleration of 1.7 m s–2?
  11. A 8000 kg engine pulls a train of 5 wagons, each of 2000 kg, along a horizontal track. If the engine exerts a force of 40000 N and the track offers a friction force of 5000 N, then calculate: (a) the net accelerating force and (b) the acceleration of the train.
  12. A stone of 1 kg is thrown with a velocity of 20 m s–1 across the frozen surface of a lake and comes to rest after travelling a distance of 50 m. What is the force of friction between the stone and the ice?
  13. A truck starts from rest and rolls down a hill with a constant acceleration. It travels a distance of 400 m in 20 s. Find its acceleration. Find the force acting on it if its mass is 7 tonnes
  14. A batsman hits a cricket ball which then rolls on a level ground. After covering a short distance, the ball comes to rest. The ball slows to a stop because (a) the batsman did not hit the ball hard enough. (b) velocity is proportional to the force exerted on the ball. (c) there is a force on the ball opposing the motion. (d) there is no unbalanced force on the ball, so the ball would want to come to rest.
  15. When a carpet is beaten with a stick, dust comes out of it, Explain
  16. Why is it advised to tie any luggage kept on the roof of a bus with a rope?
  17. An object experiences a net zero external unbalanced force. Is it possible for the object to be travelling with a non-zero velocity? If yes, state the conditions that must be placed on
  18. Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest?
  19. Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.
  20. In the following example, try to identify the number of times the velocity of the ball changes: “A football player kicks a football to another player of his team who kicks the football towards the goal. The goalkeeper of the opposite team collects the football and kicks it towards a player of his own team”. Also identify the agent supplying the force in each case.
  21. Which of the following has more inertia: (a) a rubber ball and a stone of the same size? (b) a bicycle and a train? (c) a five rupees coin and a one-rupee coin?