6.1 Newtons first law of motion and inertia Newtons first law of motion No. Have you ever pushed a shopping cart in the supermarket? Is a force needed to keep the cart moving? 6.1 Newtons first law of motion and inertia Imagine that you are skating with your friend No. Is a force needed to keep you moving forward? A force is needed to accelerate you from rest. 6.1 Newtons first law of motion and inertia Newtons first law of motion: An object will remain at rest or in motion at a uniform velocity if no net force is acting on it. 6.1 Newtons first law of motion and inertia uFriction often tends to uFriction can never be eliminated completely. oppose motion decelerate a moving object Why does the shopping cart slow down and stop when we dont push it? A force, called friction, tends to slow down the cart. 6.1 Newtons first law of motion and inertia uFriction can be largely reduced. Objects may move at a nearly uniform velocity. Examples: linear air track bobsled 6.1 Newtons first law of motion and inertia When you push a shopping cart... Applied force applied force no net force friction The cart moves at a constant velocity. = friction net force = 0 6.1 Newtons first law of motion and inertia Applied force applied force friction net force applied force The cart accelerates. The cart decelerates. net forward force net backward force friction > friction < friction net force 6.1 Newtons first law of motion and inertia uNet force changes the state of motion of an object. i.e. it accelerates or decelerates the object. uWhen net force acting on an object = 0, the object is In summary, at rest moving at constant velocity 6.1 Newtons first law of motion and inertia Inertia The tendency of an object to maintain its original state of motion is known as the inertia of the object. The inertia of an object is measured by its mass. uSI unit of mass: kilogram (kg) 6.1 Newtons first law of motion and inertia uThe massive car has a greater inertia. more reluctant to change its motion. Why is it much harder to push a broken car than a bicycle? more massive lighter 6.1 Newtons first law of motion and inertia Car safety and inertia In a car crash test dummy driver dummy passenger 6.1 Newtons first law of motion and inertia The dummy passenger continues move forward after a crash. The dummy driver is protected by the seat belt. dummy driver dummy passenger Video: Car crash 6.1 Newtons first law of motion and inertia uSafety measures: seat belt headrest - protects the neck from injury 6.1 Newtons first law of motion and inertia air bags - inflate rapidly in a crash to offer greater protection 6.1 Newtons first law of motion and inertia Thats the end of Section 6.1 Check Point Key Ideas Previous Page Section 6.2 Exit