Newton’s Laws of Motion

1. Newton’s Laws of Motion PSC 1121 Chapters 2-4

2. Newton’s Laws of Motion • Chap 2 – First Law – Law of Inertia • If you want to move something, you apply a force to it; you either push or pull it • Aristotle – 4th century BC – most famous Greek scientist classified motion into 2 kinds: • Natural – went up or down;heavy objects fall faster than light objects (ex.: boulder falls down, smoke goes up) • Unnatural – required forces – like pushing a cart • Ideas taken as facts for approx. 2000 yrs. • Galileo – 1500 • Expt at the Leaning Tower of Pisa – he dropped a heavy object and a light object and showed that they fell to the ground at the same time • He said a force is required to start an object moving, but once moving no force is required to keep it moving • Rolling balls expt. – reasoned that a ball moving horizontally would move forever if friction were entirely absent

3. Newton’s Laws of Motion • Tendency of things is to remain as they are: if moving, they tend to remain moving, if at rest, they tend to remain at rest • This property of objects is called inertia • Speed = distance covered per unit time = distance/time • if you run 6 meters in 1 second your speed is 6m/s • Velocity – vector quantity; tells you how fast and in what direction • A car travels at 60 km/h = speed • If it travels at 60 km/h to the north = velocity • Motion is relative • When we say a space shuttle moves at 30,000 km/h we mean relative to the Earth below, a racing car reaches a speed of 300 km/h, we mean relative to the track.

4. Newton’s Laws of Motion • The Law of Inertia • Isaac Newton was born on Christmas day in the year Galileo died. His laws of motion were built on Galileo’s findings • First Law – Every object continues in a state of rest, or in a state of motion in a straight line at constant speed, unless it is compelled to change that state by forces exerted upon it. • Objects at rest tend to remain at rest unless a force is applied to set them in motion • When an object is moving its tendency is to remain moving along a straight line path if no friction acts against it. • Net Force • Combination of all forces that act on an object • Ex: when you throw a basketball, you have 3 forces acting on it: force of gravity, air friction, pushing force you apply with your muscles (Fig. 2-10) • Newton = N = scientific unit of force • Forces are vector quantities

5. Newton’s Laws of Motion • Equilibrium for objects at rest • Net force = 0 • Forces acting upward on something at rest must be balanced by forces acting downward (Fig. 2-11) • The support force • A book lies at rest on a desk – it is in equilibrium, therefore net force = 0 • Force of gravity = weight of the book • Support force = the other force acting on the book to cancel out force of gravity; it is coming from the desk that supports the book • ex: bathroom scale; force 1 = your weight,downward pull of gravity, force 2 = upward support force of the floor

6. Newton’s Laws of Motion • Equilibrium for Moving objects • An object moving at constant speed in a straight-line path is also in equilibrium because two opposing forces are acting on it • It is known as dynamic equilibrium • A crate being pushed horizontally across a factory floor is in equilibrium if it moves at constant velocity • 2 forces acting on the crate – force of friction between crate and floor, and our pushing force • Force 1 = force 2, net force = 0 • Static equilibrium refers to objects at rest

7. Newton’s Laws of Motion • Chap. 3 – second Law – Force and Acceleration • Acceleration- developed by Galileo;experimented with inclined planes • Acceleration = change in velocity/time interval • Involves a change in velocity whether it is to speed up or slow down (negative acceleration) • It is a change in speed, change in direction, or change in both speed and direction • Ex: you are driving, in 1 sec you increase your velocity from 30 km/h to 35 km/h; in the next sec, you go from 35 km/h to 40 km/h; net result is you change velocity by 5 km/h each sec • Accel = 5 km/h/1 sec = 5 km/h-s • Force causes acceleration • Acceleration is directly proportional to net force • Acceleration ~ net force

8. Newton’s Laws of Motion • If you push with 25 N on an object, and somebody else pushes in the opposite direction with 15 N, the net force applied to the object is 10 N • The object will accelerate as if a single 10-N force acts on it • Direction of the object is always in the direction of the net force • Acceleration also depends on mass of object being pushed or pulled • Mass – measure of inertia • The greater the mass of an object, the greater the inertia; it takes a lot more force to get an elephant on wheels to accelerate than it would take to get a skateboard to move • Mass is a measure of how much material an object contains; measured in kilograms • Different from volume which is a measure of space, reported in cubic centimeters, cubic meters, or liters

9. Newton’s Laws of Motion • Ex: equal size bags of cotton and rocks may have equal volumes but unequal masses • Different from weight; weight depends on gravity • you would weigh less on the moon than on Earth because the moon’s gravity is weaker than Earth’s • Mass does not change if gravity varies • Mass and weight are directly proportional to each other; objects with large mass have large weight, objects with little mass have little weight • 1kg = 10 N (rounded off to 10 from 9.8) • 1 kg= 2.2 lb • Mass resists acceleration • For a given force acceleration is inverselyproportional to the mass • Acceleration = 1/mass

10. Newton’s Laws of Motion • Newton’s second Law • The acceleration produced by a net force on an object is directly proportional to the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object • Acceleration = net force/mass • a= F/m F= net force in N, m = mass in kg, a = acceleration in m/s2 • Ex: 1000 kg car pulled by a cable with 2000N of force. What will be the acceleration of the car? • A = F/m = 2000N/1000 kg = • 2000kg-m/s2/1000kg= 2 m/s2

11. Newton’s Laws of Motion • Friction is a force that affects motion • Friction occurs when one object rubs against something else • Always acts in a direction to oppose motion • When an object falls downward through the air, force of friction (air drag) acts upward • When you pull on a crate and it slides across a floor, both your force and opposite force of friction affect the motion – when you pull hard enough to match friction, net force on the crate is Zero and it slides at constant velocity • Objects in free fall have equal acceleration • Free fall is falling only under the influence of gravity; inclined planes slow down acceleration • Acceleration of free fall doesn’t depend on mass

12. Newton’s Laws of Motion • Why objects in free fall have equal acceleration • a= F/m • The ratio of weight to mass is the same for any object • All free falling objects have the same force/mass ratio and undergo the same acceleration at the same location • Effect of air drag on acceleration • Air drag depends on speed and surface area • For free fall, downward net force = weight – air drag • If air drag = weight, net force = 0, acceleration = 0 so object no longer picks up speed • Object has reached terminal speed

13. Newton’s Laws of Motion • Or if concerned with direction, like a sky diver falling, we say he has reached terminal velocity • Increasing surface area reduces terminal speed: a parachute greatly increases air drag and terminal speed can be reduced to 15-25 km/h

14. Newton’s Laws of Motion • Chap. 4 – Newton’s third Law: Action and Reaction • Force – interaction between one thing and another • force always occurs in pairs • Non-living things can exert forces • Ex: a truck crashes into a car; there is an interaction between the truck and the car – forces are equal in strength and opposite in direction and they occur at exactly the same time • When you push on a wall, there is an equal force pushing back on your hand

15. Newton’s Laws of Motion • Third Law- whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first • “To every action, there is always an opposed equal reaction” • Neither force exists without the other • Ex: car travelling on a road • Action: tire pushes on the road • Reaction: road pushes on tire • Objects of different masses • a = F/m

16. Newton’s Laws of Motion • Action and Reaction forces on different objects • Forces cancel only when they act on the same body • If you kick a football the action is force your foot exerts on the football- so the football accelerates; the reaction is the football exerting a force on your foot • You can’t cancel the force on the football with a force on your foot