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6.1 Newton’s First Law. pp. 138-140. NEWTON’S THREE LAWS OF MOTION. In his book, Principia, Newton described the three laws of motion. He did not discover all three laws, he explained them in a way that many people could understand. NEWTON’S FIRST LAW OF MOTION.
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6.1 Newton’s First Law pp. 138-140
NEWTON’S THREE LAWS OF MOTION • In his book, Principia, Newton described the three laws of motion. • He did not discover all three laws, he explained them in a way that many people could understand
NEWTON’S FIRST LAW OF MOTION • This is also known as the Law of Inertia • It says : An object in motion will remain in motion at a constant speed and in the same direction unless acted on by an unbalanced force. • An object at rest will remain at rest unless acted upon by an unbalanced force.
6.1 Law of inertia • Newton’s first lawsays that objects continue the motion they already have unless they are acted on by a net force. • If the net force is zero, an object at rest will stay at rest. • If an object is acted upon by unbalanced forces, its motion will change.
NEWTON’S FIRST LAW OF MOTION • In other words… • Unless you apply a force, things tend to keep on doing what they were doing in the first place. • Objects at rest (not moving) will not begin moving unless a push or pull is exerted on them.
6.1 Net force • Newton’s first law is often written in terms of the net force: • “An object at rest will stay at rest and an object in motion will continue in motion at constant velocity UNLESS there is a net force.” According to these vectors, in what direction is the net force?
6.1 Force changes motion • Forces can be used to increase or decrease the speed of an object, or to change the direction an object is moving.
NEWTON’S FIRST LAW OF MOTION • An object moving at a certain velocity will continue to move forever at that same speed and in the same direction unless an unbalanced force acts on it. • Inertia is related to mass. Mass is a measure of inertia.
NEWTON’S FIRST LAW OF MOTION • An object with a small mass has less inertia than an object with a large mass. • For example… • A tractor trailer has more mass than a motorcycle
6.1 Law of inertia • Inertia is the property of an object that resists changes in motion. • Objects with more mass have more inertia and are more resistant to changes in their motion. Which ball has more inertia?
NEWTON’S FIRST LAW OF MOTION • How do ABS (automatic brake systems) work in cars? • The system is controlled by a computer that prevents skidding by sensing when the wheels are about to lock.
ABS BRAKES EXPLAINED • The computer releases and reapplies the brakes up to 25 times a second. Instead of skidding out of control, the car slows down and stops safely.
Solving Problems A car drives along the highway at constant velocity. Find the car’s weight and the friction force if the engine produces a force of 2,000 newtons between the tires and the road and the normal force on the car is 12,000 N.
Solving Problems • Looking for: • …weight of car in newtons, force due to friction • Given: • …ForceN = 12,000N (up); • …ForceE = 2,000N (forward) • Relationships: • Newton’s 1st Law: • net force = zero at constant velocity; so ForceN = ForceW and ForceE = ForceF
FW = 12,000N FF = 200 N FE = 200 N FN = 12,000N Solving Problems • Solution • Draw a free body diagram. • There is no net force upward, so the weight of the car is an equal downward force of 12,000 N. • The forward engine force balances the friction force so the friction force is 2,000 N.
