P.1 Book 2 Section 3.2 Inertia and Newton's first law 3.2Inertia and Newton’s first law Traffic accident Is a force needed to keep a body moving? Inertia Inertia and mass Newton’s first law of motion Newton’s idea of force Check-point 2

P.2 Book 2 Section 3.2 Inertia and Newton's first law Traffic accident In serious traffic accidents, passengers are thrown out of cars, like the dummy in the picture. Why? This is due to inertia. Video 3.2 Not wearing a seat-belt

P.3 Book 2 Section 3.2 Inertia and Newton's first law 1 Is a force needed to keep a body moving? It seems that… we must continuously apply a force to keep an object moving; otherwise, it stops. I have to keep pulling otherwise the cart stops!

P.4 Book 2 Section 3.2 Inertia and Newton's first law 1 Is a force needed to keep a body moving? Ancient Greek philosopher Aristotle: ‘A force is needed to keep a body moving at a constant speed.’ In the 16 th century, Galileo challenged Aristotle’s idea and introduced the concept of inertia. Expt 3a Motion with and without friction

P.5 Book 2 Section 3.2 Inertia and Newton's first law Experiment 3a Motion with and without friction 1Set up the apparatus as shown.

P.6 Book 2 Section 3.2 Inertia and Newton's first law Experiment 3a Motion with and without friction 2Turn on the air pump. Give a soft push to the glider. 3Repeat step 2, but turn off the air pump as the glider passes by the middle point of the air track. Video 3.3 Expt 3a - Motion with and without friction

P.7 Book 2 Section 3.2 Inertia and Newton's first law 2 Inertia if there is no friction, a moving body can move at a constant speed. Similarly, A stationary object always remains at rest. Galileo’s law of inertia: Inertia is the tendency of a body to maintain its state of rest or of constant velocity. Expt 3a shows that…

P.8 Book 2 Section 3.2 Inertia and Newton's first law 3 Inertia and mass Greater mass larger inertia E.g. bowling Larger inertia of the bowling ball Knocks the pins down without affecting its route Video 3.4 Inertia and mass (1) Video 3.4 Inertia and mass (2)

P.9 Book 2 Section 3.2 Inertia and Newton's first law 4 Newton’s first law of motion Newton’s first law of motion states that: Every object remains in a state of rest or uniform motion (i.e. constant velocity) unless acted on by a net force, or an unbalanced force.

P.10 Book 2 Section 3.2 Inertia and Newton's first law 4 Newton’s first law of motion Net force is a result of adding up all forces acting on an object. Separate forcesNet force

P.11 Book 2 Section 3.2 Inertia and Newton's first law 4 Newton’s first law of motion Phenomena explained by the first law: 1Sitting still on a chair Weight W is balanced by normal reaction R. No net force At rest

P.12 Book 2 Section 3.2 Inertia and Newton's first law 4 Newton’s first law of motion 2Moving forwards during a sudden braking Braking force is applied on the car, not on passengers. Passengers remain in the state of uniform motion Moving forwards

P.13 Book 2 Section 3.2 Inertia and Newton's first law 4 Newton’s first law of motion There is friction! Force is needed to keep the book moving However, why does a book moving on a table stop after a while? If there is no friction, the book would keep moving (by Newton’s first law). Example 2 Explanation using Newton’s first law

P.14 Book 2 Section 3.2 Inertia and Newton's first law Example 2 Explanation using Newton’s first law Explain the following motions of a spacecraft using Newton’s first law of motion. (a)It is in uniform motion in space when its rockets are shut down. When rockets are shut down, net force acting on the spacecraft = 0 in uniform motion

P.15 Book 2 Section 3.2 Inertia and Newton's first law Example 2 Explanation using Newton’s first law (b)The speed and/or direction of the spacecraft changes when its rockets are fired. Rockets are fired net force  0 By Newton’s first law, the state of motion of the spacecraft changes. Speed and/or direction changes.

P.16 Book 2 Section 3.2 Inertia and Newton's first law 5 Newton’s idea of force Net force changes the state of rest or uniform motion of an object. Newton gave a meaning to force: net force  0 acceleration  0

P.17 Book 2 Section 3.2 Inertia and Newton's first law 5 Newton’s idea of force E.g. a cyclist’s motion Case 1 forward force = air resistance  no net force  constant velocity

P.18 Book 2 Section 3.2 Inertia and Newton's first law 5 Newton’s idea of force Case 2 forward force > air resistance  net force  0 (right)  speeding up

P.19 Book 2 Section 3.2 Inertia and Newton's first law 5 Newton’s idea of force Case 3 forward force < air resistance  net force  0 (left)  slowing down

P.20 Book 2 Section 3.2 Inertia and Newton's first law Check-point 2 – Q1 In which of the following situations is the net force acting on a car = 0 ? Waiting in front of a traffic light Moving at constant velocity Slowing down Speeding up

P.21 Book 2 Section 3.2 Inertia and Newton's first law Check-point 2 – Q2 A force of 1 N acts on a box and the box remains at rest. Find the friction acting on it. The box remains at rest. The net force acting on the box = ______ Friction acting on the box = _______ (towards _______) zero 1 N left

P.22 Book 2 Section 3.2 Inertia and Newton's first law Check-point 2 – Q3 A ping-pong ball and a bowling ball roll at the same velocity. Which one is harder to stop? Since the bowling ball has a larger _______, its _________ is larger and it is _________ to stop. mass inertia harder

P.23 Book 2 Section 3.2 Inertia and Newton's first law Check-point 2 – Q4 A boy is sliding down a gentle slope at a constant speed along a straight line. ANo net force is acting on the boy because his velocity is steady. B There must be a net force acting on the boy because he is moving downhill. Which of the following is correct?

P.24 Book 2 Section 3.2 Inertia and Newton's first law The End