Newton’s First Law of Motion

Newton’s First Law “The velocity of an object will remain constant unless a new force acts on it” Law of Inertia

Law of Inertia If an object is moving, it will continue moving with a constant velocity (in a straight line and with constant speed) unless a net force acts on it If an object is at rest, it will stay at rest unless a net force acts on it

Inertia Inertia is the tendency of the motion of an object to remain constant in terms of both speed and direction The amount of inertia that an object has is dependent on the object’s mass The more mass an object has the more inertia it has

Amount of Inertia If an object has a large amount of inertia (due to a large mass) It will be hard to slow it down or speed it up if it is moving It will be hard to make it start moving if it is at rest It will be hard to make it change direction

Effects of Inertia Inertia causes a passenger in a car to continue to move forward even though the car stops – this is the reason that seat belts are so important for the safety of passengers Inertia is the reason that it is impossible for vehicles to stop instantaneously

Effects of Inertia Inertia is the reason that it is harder to start pushing a wheelbarrow full of bricks than to start pushing an empty wheelbarrow The filled wheelbarrow has more mass and therefore, more inertia Inertia is also the reason that it is harder to stop a loaded truck going 55 miles per hour than to stop a car going 55 miles per hour The truck has more mass resisting the change of its motion and therefore, more inertia

Newton’s Third Law of Motion

Newton’s Third Law of Motion “When one object exerts a force on a second object, the second one exerts a force on the first that is equal in magnitude and opposite in direction.” Sometimes referred to as the Law of Action and Reaction

Law of Action and Reaction Even though the forces are equal in magnitude and opposite in direction, they do not cancel each other This law addresses two objects, each with only one force exerted on it Each object is exerting one force on the other object Each object is experiencing only one force

Action & Reaction Examples A swimmer swimming forward The swimmer pushes against the water (action force), the water pushes back on the swimmer (reaction force) and pushes her forward A ball is thrown against a wall The ball puts a force on the wall (action force), and the wall puts a force on the ball (reaction force) so the ball bounces off

Action & Reaction Examples A person is diving off a raft The person puts a force on the raft (action force) pushing it, and the raft puts a force on the diver (reaction force) pushing them in the opposite direction A person pushes against a wall (action force), and the wall exerts an equal and opposite force against the person (reaction force) The Space Shuttle engines push out hot gases (action force), and the hot gases put a force on the shuttle engines (reaction force) so the shuttle lifts (there is no sling shot doing it)