Unit 1 B Newton's Laws of Motion

2 Classical Mechanics Describes the relationship between the motion of objects in our everyday world and the forces acting on them Describes the relationship between the motion of objects in our everyday world and the forces acting on them Conditions when Classical Mechanics does not apply Conditions when Classical Mechanics does not apply very tiny objects (< atomic sizes) very tiny objects (< atomic sizes) objects moving near the speed of light objects moving near the speed of light

3 Forces Usually think of a force as a push or pull Usually think of a force as a push or pull Vector quantity Vector quantity May be contact or field force May be contact or field force

4 Contact and Field Forces

5 Fundamental Forces Types Types Strong nuclear force Strong nuclear force Electromagnetic force Electromagnetic force Weak nuclear force Weak nuclear force Gravity Gravity Characteristics Characteristics All field forces All field forces Listed in order of decreasing strength Listed in order of decreasing strength Only gravity and electromagnetic in mechanics Only gravity and electromagnetic in mechanics

6 Newton’s First Law If no forces act on an object, it continues in its original state of motion; that is, unless something exerts an external force on it, an object at rest remains at rest and an object moving with some velocity continues with that same velocity. If no forces act on an object, it continues in its original state of motion; that is, unless something exerts an external force on it, an object at rest remains at rest and an object moving with some velocity continues with that same velocity.

7 Newton’s First Law, cont. External force External force any force that results from the interaction between the object and its environment any force that results from the interaction between the object and its environment Alternative statement of Newton’s First Law Alternative statement of Newton’s First Law When there are no external forces acting on an object, the acceleration of the object is zero. When there are no external forces acting on an object, the acceleration of the object is zero.

8 Inertia Is the tendency of an object to continue in its original motion Is the tendency of an object to continue in its original motion

9 Mass A measure of the resistance of an object to changes in its motion due to a force A measure of the resistance of an object to changes in its motion due to a force Scalar quantity Scalar quantity SI units are kg SI units are kg

10 Newton’s Second Law The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. F and a are both vectors F and a are both vectors Can also be applied three-dimensionally Can also be applied three-dimensionally

11 Units of Force SI unit of force is a Newton (N) SI unit of force is a Newton (N) US Customary unit of force is a pound (lb) US Customary unit of force is a pound (lb) 1 N = lb 1 N = lb

12 Gravitational Force Mutual force of attraction between any two objects Mutual force of attraction between any two objects Expressed by Newton’s Law of Universal Gravitation: Expressed by Newton’s Law of Universal Gravitation:

13 Weight The magnitude of the gravitational force acting on an object of mass m near the Earth’s surface is called the weight w of the object The magnitude of the gravitational force acting on an object of mass m near the Earth’s surface is called the weight w of the object F g = m g is a special case of Newton’s Second Law F g = m g is a special case of Newton’s Second Law g can also be found from the Law of Universal Gravitation g can also be found from the Law of Universal Gravitation

14 More about weight Weight is not an inherent property of an object Weight is not an inherent property of an object mass is an inherent property mass is an inherent property Weight depends upon location Weight depends upon location

15 Newton’s Third Law If two objects interact, the force F 12 exerted by object 1 on object 2 is equal in magnitude but opposite in direction to the force F 21 exerted by object 2 on object 1. If two objects interact, the force F 12 exerted by object 1 on object 2 is equal in magnitude but opposite in direction to the force F 21 exerted by object 2 on object 1. Equivalent to saying a single isolated force cannot exist Equivalent to saying a single isolated force cannot exist

16 Newton’s Third Law cont. F 12 may be called the action force and F 21 the reaction force F 12 may be called the action force and F 21 the reaction force Actually, either force can be the action or the reaction force Actually, either force can be the action or the reaction force The action and reaction forces act on different objects The action and reaction forces act on different objects

17 Some Action-Reaction Pairs F N and F N ’ F N and F N ’ F N is the normal force, the force the table exerts on the TV F N is the normal force, the force the table exerts on the TV F N is always perpendicular to the surface F N is always perpendicular to the surface F N ’ is the reaction – the TV on the table F N ’ is the reaction – the TV on the table F N = - F N ’ F N = - F N ’

18 More Action-Reaction pairs F g and F g ’ F g and F g ’ F g is the force the Earth exerts on the object F g is the force the Earth exerts on the object F g ’ is the force the object exerts on the earth F g ’ is the force the object exerts on the earth F g = -F g ’ F g = -F g ’

19 Forces Acting on an Object Newton’s Law uses the forces acting on an object Newton’s Law uses the forces acting on an object F N and F g are acting on the object F N and F g are acting on the object F N ’ and F g ’ are acting on other objects F N ’ and F g ’ are acting on other objects