Presentation on theme: "Forces. Newton’s First and Second Laws Newton’s 1 st law of motion deals with inertia An object at rest remains at rest, an object in motion maintains."— Presentation transcript:
Newton’s First and Second Laws Newton’s 1 st law of motion deals with inertia An object at rest remains at rest, an object in motion maintains its velocity, unless acted upon by an outside force Objects change their state of motion only when a net force is applied to the object Inertia: the tendency of an object to maintain its state of motion.
Inertia The tendency of an object to maintain its state of motion Inertia is related to the mass of an object – An object with more mass has more inertia – An object with less mass has less inertia Which is harder to push? A couch or a car? – Which has the most inertia? Why?
Inertia Which is harder to stop? A train or a car traveling at 40 mph? – Why? Use the concept of inertia to explain why seat belts are useful. A pickup truck is at rest. A basketball is in the bed of the truck, near the cab. – What happens to the ball when the truck starts to move? – Why?
Newton’s 2 nd Law of Motion Unbalanced forces cause an object to accelerate Unbalanced forces produce a net force F net = ma Greater net force causes greater acceleration Units of force – SI unit is the Newton – English unit is the pound
Newton’s 2 nd Law and Acceleration Acceleration depends on mass and net force Acceleration increases with more force Acceleration decreases with more mass
Gravity (12.2) Weight and mass are not the same thing Mass is a measure of matter Weight is the force of gravity acting on a mass
Weight When an object falls, it experiences acceleration due to gravity, this is called free fall acceleration (g) Weight = mass x free-fall acceleration This is an application of Newton’s 2 nd Law… F = maor W = mg Since weight is a force, units of weight are the same as units of force. Newtons (N) or pounds (lb)
Weight and Mass Weight of an object depends on the acceleration due to gravity On Earth, W = mg Earth W Earth = 66 kg x 9.8 m/s2 = 650 N ≈ 150 lb On the Moon, W = mg Moon W moon = 66 kg x 1.6 m/s2 =110 N ≈ 24 lb
Law of Universal Gravitation Gravity is a field force It is always an attractive force between two masses The force of gravity between two objects depends upon… How big the masses are How far apart they are
Newton’s Law of Universal Gravitation If the masses are bigger, the force of gravity between them is bigger If the distance between the masses is larger, the force of gravity is smaller
Free Fall When the only force acting on an object is gravity In the absence of air resistance, all objects fall at the same rate… Regardless of their masses
Terminal Velocity When air resistance acts on an object, it slows its rate of fall Eventually the object will stop accelerating Terminal velocity is the velocity of a falling object when the force of air resistance is equal to but opposite direction of force of gravity In other words, the force of air resistance balances the force of gravity
Are Astronauts Weightless? No Astronauts are in free fall But the earth is curving away from them as they fall So they don’t fall to Earth
Projectile Motion A projectile is a moving object that is under the influence of gravity only Examples – Baseball, bullet, a hopping toad, a long jumper – NOT: powered rocket, airplane, etc. Projectiles follow a parabolic path (trajectory) Projectile motion has both vertical and horizontal components (parts)
Projectile Motion has Horizontal and Vertical Components