Force and Laws of Motion

This chapter explains the cause of motion through the idea of force and Newton's three laws of motion. You will distinguish balanced forces, which do not change the state of motion of an object, from unbalanced forces, which change its speed or direction. Building on Galileo's experiments, Newton stated the first law of motion: an object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force. This tendency is called inertia, and mass is the measure of inertia. You will define momentum as the product of mass and velocity, p = mv, and learn the second law, that the rate of change of momentum is proportional to the applied force, which gives F = ma, with force measured in newtons. The third law states that to every action there is an equal and opposite reaction. Finally, you will learn the law of conservation of momentum and apply it to the recoil of a gun and collisions.

Detailed Worksheet: Force and Laws of Motion Section A - Definitions (10 marks) 1. Distinguish between balanced and unbalanced forces with one example each. (2 marks) 2. State Newton's first law of motion. Why is it also called the law of inertia? (2 marks) 3. Define momentum. Write its SI unit and state whether it is a scalar or vector. (2 marks) 4. State Newton's second law of motion and define one newton. (2 marks) 5. State Newton's third law of motion and the law of conservation of momentum. (2 marks) Section B - Calculations and Applications (15 marks) 6. A force of 5 N gives a mass m1 an acceleration of 10 m/s^2 and a mass m2 an acceleration of 20 m/s^2. Find m1 and m2. What acceleration would the same force give if both masses were tied together? (3 marks) 7. A car of mass 1000 kg moving at 20 m/s is brought to rest in 4 s by applying brakes. Find the retardation and the braking force. (3 marks) 8. A bullet of mass 20 g is fired horizontally with a velocity of 150 m/s from a pistol of mass 2 kg. What is the recoil velocity of the pistol? (3 marks) 9. A truck starts from rest and rolls down a hill with constant acceleration. It travels 400 m in 20 s. Find its acceleration and the force acting on it if its mass is 7 tonnes. (3 marks) 10. A hockey ball of mass 200 g travelling at 10 m/s is struck by a hockey stick so that it returns along its original path at 5 m/s. Find the change of momentum of the ball. (3 marks) Section C - Diagrams (10 marks) 11. Draw a diagram of the activity with a coin placed on a card on a glass tumbler that is flicked. Explain the observation using inertia. (4 marks) 12. Draw a labelled diagram showing the forces acting on a block of wood on a table pulled by two strings in opposite directions, for the cases of balanced and unbalanced forces. (3 marks) 13. Draw a diagram to show action and reaction forces when (i) a gun is fired (ii) a swimmer pushes the water backwards. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain, using the laws of motion, why (i) passengers fall forward when a moving bus stops suddenly (ii) a fielder pulls his hands backwards while catching a fast cricket ball (iii) it is easier to stop a tennis ball than a cricket ball moving at the same speed. (5 marks) 15. Two objects of masses 100 g and 200 g move along a straight line in the same direction with velocities 2 m/s and 1 m/s. They collide, and after the collision the first object moves at 1.67 m/s. Find the velocity of the second object. Explain why momentum is conserved. (5 marks) 16. An engine pulls a train of 4 wagons, each of mass 2500 kg, along a straight horizontal track. After allowing for friction, the net force on the wagons is 20000 N. Find the acceleration of the wagons and the force exerted by wagon 1 on the remaining three wagons. Which law helps you find the force exerted by those wagons on wagon 1? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Write the formula, convert all quantities to SI units and give units with every answer.