Chapter 11 Forces Newton’s Laws of Motion (1 st 2) Gravity Newton’s 3 rd Law

Newton’s Laws of Motion 1 st 1 st : An object at rest will remain at rest or an object in motion will remain in motion at constant speed unless a net force acts upon it. 1 stExamples: 1.A book at rest 2.A person restrained by a seat belt after a car suddenly stops 3.The engine of a car being the net force to cause it to move

Newton’s Laws, Cont. 2nd: The greater the force of an object, the greater the acceleration. The greater the mass of an object, the greater the force needed to accelerate the object. Examples: 1.More force is needed to pull a wagon full of bricks rather than an empty wagon. 2.Lighter cars use less fuel. 3.More force can cause balls to be thrown farther.

Newton’s Second Law (cont) F=ma m= F/a a=F/m Units for force: Newtons! One newton is equal to the force required to accelerate a 1 kg object 1 m/s 2

Section 2: Gravity Define: force of attraction between 2 objects Depends on: –Mass of the objects –Distance between the 2 objects

Falling Objects Falling Objects Free Fall: the motion of a body when only the force of gravity is acting on it. The acceleration of a falling object due to the force of gravity between the object and the Earth is: 9.8 m/s m/s 29.8 m/s 2 The final speed or time that it takes for objects to fall can be calculated: V f =att=v f /ause 9.8m/s 2 for (a)

Air Resistance Do all objects continue to accelerate until they hit the ground? Air resistance opposes downward motion. Some objects are affected more than others depending on their shape. Once an object stops accelerating, it reaches terminal velocity. Gravity and air resistance are equal at this point. terminal velocityGravity and air resistance terminal velocityGravity and air resistance

Projectile Motion Projectile Motion and Gravity Projectile Motion Projectile motion is the curved path of an object that is thrown 2 components – vertical and horizontal vertical and horizontalvertical and horizontal The downward accelerations of a thrown object and a falling object are identical (see page 359). You should aim above targets for objects in projectile motion.

Weight and Mass Weight is the measure of the force of gravity on an object. Measured in newtons Equation: F w =ma(a=acceleration due to gravity) Remember! Mass is the amount of matter in an object and cannot change… (measured in kg)

Newton’s Laws, cont. 3 rd 3 rd : For every action, there is an equal but opposite reaction – all forces come in pairs. 3 rdExamples: 1.Stepping off a canoe while pushing, will cause the canoe to move back. Stepping off a canoe Stepping off a canoe 2.Pushing on a desk 3.The reaction engine of a rocket rocket

Summary of Newton’s Laws 1.Inertia 2.F=ma 3.Action-Reaction forces Action-Reaction forcesAction-Reaction forces

Momentum A quantity that is the product of the mass and velocity of an object Momentum=mass x velocity or p=mv p=mv Momentum is conserved. Momentum is transferred. Examples: