1. 1 Physics: Chapter 4 Forces & the Laws of Motion Topics:4-1 Changes in Motion 4-2 Newton’s First Law 4-3 Newton’s Second & Third Laws 4-4 Everyday Forces

2. 2 Changes in Motion  Force = a push or a pull  Force causes a change in motion Forces can start motion Forces can stop motion Forces can change the direction of motion  Forces act through contact A push or a pull is a good example; contact between objects must occur  Forces can act through a distance AKA field forces Gravity & magnitism are good examples of a field force Field forces important in particle physics; nuclear strong & weak forces responsible for the nuclear interactions

3. 3 Force Diagrams  Force diagram = a diagram of the objects involved in a situation & the forces exerted on the objects.  Magnitude of applied force influences the magnitude of motion change.  Free-body diagram = diagram of the forces acting on a single object (simplifies diagram to understand what is happening) Disregard any forces not acting on that object

4. 4 How to draw a free-body diagram Ex: car being towed  1. Isolate & identify forces acting on car; draw simple shape for car  2. Draw/label all vectors acting on the car Forces act from a central point in object  3. Include magnitude & direction of vectors drawn  4. Using vector analysis, the resultant external force acting on object can be calculated

5. 5 Newton’s First Law of Motion  An object at rest remains at rest, & an object in motion continues in motion with constant velocity unless acted upon by an external force. When a net external force acting on an object is zero, the object’s acceleration is zero  Inertia = tendency of an object to maintain current state of motion

6. 6 Net External Forces  External force – a single force that acts on an object as a result of the interaction between the object & its environment.  Net external force – the vector sum of all forces acting on a body; aka the resultant force. To determine the resultant force, apply knowledge of vector addition  Equilibrium is reached when net external force is zero Can an object be moving and still be in equilibrium?

7. 7 Net External Force Problem:  Problem: a book is placed on a drafting table which has an angle of 35. Find net external force acting on book & determine if the book will remain at rest.  Knowns: F g = 22N F f = 11N F n = 18N F net = x  Solve: draw diagram & solve using vector addition equations  Look at Sample Problem 4A, page 132 for solution

8. 8 Inertia, Mass, & Equilibrium  Mass is a measurement of inertia. Remember: inertia is tendency to resist change in motion Imagine a golf ball and a basketball are both struck by a golf club with equal force. Which accelerates faster?  The greater an object’s mass, the less it will accelerate when a force is applied.  Remember objects at rest or moving with constant velocity are in equilibrium & no outside force can be acting on the system The vector sum of all forces acting on object is zero

9. 9 Newton’s Second Law  The acceleration of an object is directly proportional to the net external force acting on object and inversely proportional to the object’s mass. F = ma F is net external force, m is mass, a is acceleration

10. 10 Newton’s Third Law  If 2 objects interact, the magnitude of the force exerted on object 1 by object 2 is equal to the magnitude of the force simultaneously exerted on object 2 by object 1, & these forces act in the opposite directions. For every action there is an equal & opposite reaction. Consider hammer driving a nail into wood If forces are equal & opposite, why is the system not in equilibrium (shouldn’t the nail not move?)  Because forces always act in pairs (action-reaction pair); forces act on different objects! Use free-body diagrams to determine what forces are acting on what object & determine what movement may be happening.

11. 11 Weight  Weight – magnitude of the force of gravity acting on an object  Direction of gravity is downward toward Earth’s center  Using Newton’s 2 nd Law, F=ma, we can determine the F gravity or F g Use acceleration due to gravity or 9.81m/s 2  Example: Fg for a 100 kg object is F g = (100kg)(9.81m/s 2 ) = 981kgm/s 2 or 981N  Mass is a property of matter, but weight is not b/c it depends on the force of gravity

12. 12 The Normal Force  Normal force – contact force exerted by one object on another in a direction perpendicular to the surface of contact  This force is always  to surface of contact, but not always  to F g For example, if a box is on an incline the F n is  to incline surface, but F g is at some angle to surface

13. 13 Friction – the force that opposes motion  Types of friction Static friction – the force exerted on a motionless body by its environment to resist an external force Kinetic friction – the force exerted on a moving object; this is always less than static friction Fluid friction – the force exerted on an object by a fluid surrounding it, most common type is air resistance  Friction depends on the surfaces in contact Coefficient of friction – ratio of the force of friction to the normal force acting between 2 objects For static friction   S = F s (max) /F n For kinetic friction   k = F k /F n