Newton’s 1st and 2nd Laws Section 10.3 Standard 2: Forces Students know a force has both direction and magnitude. Students know when an object is subject to two or more forces at once, the result is the cumulative effect of all the forces. Students know when the forces on an object are balanced, the motion of the object does not change. Students know how to identify separately the two or more forces that are acting on a single static object, including gravity, elastic forces due to tension or compression in matter, and friction. Students know that when the forces on an object are unbalanced, the object will change its velocity (that is, it will speed up, slow down, or change direction). Students know the greater the mass of an object, the more force is needed to achieve the same rate of change in motion.

Father of Physics Sir Isaac Newton (1642-1727) “Truth is ever to be found in the simplicity, and not in the multiplicity and confusion of things.” Father of dynamics Published 3 Laws of Motion between 1684 and 1687 - Inertia - F = ma - Action/Reaction

Types of Forces There are 4 basic fundamental forces: Gravity Weakest of all forces Electricity & Magnetism Electrical charge, Magnets Weak Nuclear Force Nuclear Decay in Nucleus Strong Nuclear Force Strongest of all forces Keeps atoms together

An object at rest will continue to stay at rest Newton’s First Law What does Newton’s 1st law state? An object at rest will continue to stay at rest An object in motion will continue to stay in motion at constant velocity unless an outside force acts on it. Mr. Astronaut There is nothing to propel him in space (no air resistance to slow him down and no gravity to speed him up). What will happen to him? Mr. Archer By pulling the arrow back, he is balancing out the tension in the string. What happens when he lets it go? Mr. Curler In curling, the athlete moves with the stone so they are both moving at the same speed. When he releases the stone, there is almost no friction with the ice. What happens to the stone?

The tendency of an object to resist motion Inertia What is inertia? The tendency of an object to resist motion Think of driving in a car What happens when suddenly the car brakes? Inertia depends upon weight Heavier Object = Harder to start moving Heavier Object = Harder to stop moving Which would you rather try to stop when it is running at you?

Force = Mass x Acceleration Newton’s Second Law Force = Mass x Acceleration F = ma Mass is in kilograms Acceleration is in m/s/s Unit of force is in Newtons (N) What does this say about force? More Force to get it moving Less Force to get it moving More Force to stop it moving Less Force to stop it moving

Force = Mass x Acceleration F = ma Newton’s Second Law F Force = Mass x Acceleration F = ma m a This bus has a mass of 30,000 kg. How much force does it take to accelerate it 3 m/s/s? A bullet has a mass of 0.1 kg. How fast is it accelerating if it hits its target with 20 N of force? The engine in this motorbike exerts 12,000 N to make the bike accelerate at 15 m/s/s. How massive is the bike?

More F = MA Practice Three forces act on a box that is initially at rest as shown below. Determine the net force acting on the crate and describe the resulting motion of the crate. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec? What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec? What acceleration will result when a 12 N net force applied to a 3 kg object? A 6 kg object? A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass.