Force. The cause of an acceleration or change in an object s motion. Any kind of a push or pull on an object.

Transcription

1 Force The cause of an acceleration or change in an object s motion. Any kind of a push or pull on an object. Forces do not always give rise to motion. Forces can be equal and opposite. Force is a vector It has direction and magnitude. SI unit: Newton (N) 1 Newton = 1 kg m/s 2

2 Common Types of Forces Contact forces Results from physical contact between 2 objects. Field forces The force exerted by such fields as gravity, magnetism, and electricity. No actual contact between 2 objects The result of an attraction or repulsion in a force field.

3 Force Diagrams A diagram of the objects involved in a situation and the forces exerted on the objects. Vectors The tail of the arrow is attached to the object on which the force is acting. Free-body diagram shows only the object and it s forces acting on it.

4 Balanced vs. Unbalanced Balanced Forces: Forces Combined forces can either act against one another or together. When forces are equal and opposite, they are balanced and don t cause motion. Unbalanced Forces A force that is not opposed equally by another force on the same object. Results in motion.

5 Newton s 1st Law of Motion The Law of Inertia An object at rest remains at rest; An object in motion continues in motion with constant velocity; Unless an external force acts upon it.

6 Newton s First Law of Motion (cont.) Inertia The tendency of a body to maintain its state of motion. Net external force The vector sum of all the forces acting on a body. Called the resultant force.

7 Solving Net External Force Draw a free-body diagram. Apply a coordinate system to the freebody diagram. Calculate all x-axis components. Calculate all y-axis components. Add all x-axis components. Add all y-axis components. Calculate the resultant vector and angle.

8 Objects Inclined at an Angle Apply a coordinate system to the freebody diagram. X axis incline surface Y axis perpendicular to incline surface

9 Inclined Plane (cont.) The F g vector is drawn by dropping a perpendicular line from the inclined plane to the earth s surface. Resolve the force of gravity vector into its x-axis and y-axis components. Calculate the resultant F g vector. The F n vector is along the y-axis. The F f and F A vector are along the x-axis.

10 Inclined Plane (cont.) Total the x-axis and y-axis components for all force vectors. Using Pythagorean Theorem, calculate the resultant vector of the above x-axis and y-axis force vectors. Use trig functions to calculate the angle of the net external force.

11 Mass The measure of the inertia of a body. Mass is the quantity of matter. It is directly proportional to inertia. The greater the mass, the greater the inertia. (It takes more force to move or stop more mass.)

12 Equilibrium Exists when the net external force acting on a body is equal to zero. All forces are equal and opposite.

13 Newton s 2nd Law of Motion The acceleration of an object is directly proportional to the net force acting on it. The acceleration is inversely proportional to its mass. F = ma

14 Newton s 2nd Law of Motion (cont.) The direction of the acceleration is in the direction of the applied net force. The greater the force exerted on an object, the faster it will move. The heavier the object, the more force it takes to get it move it at the same speed as a lighter object.

15 Newton s 3rd Law - Interacting Forces When 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 an equal and opposite reaction.

16 Interacting Forces (cont.) Action-reaction pair the two forces that exist in pairs acting in equal but opposite directions. Each force in an action-reaction pair acts on 2 different objects.

17 Newton s 3rd Law (cont.) Field forces Exist in pairs. Gravity exerts force on a falling body but the falling body also exerts force on the earth.

18 Newton s 3rd Law (cont.) The end result of the action and reaction forces depends on the masses of the objects involved. If masses are equal, then the end result is equilibrium. If they are unequal, then motion occurs.

19 Weight Everyday Forces The magnitude of the force of gravity on a quantity of mass. Scalar quantity (magnitude only) F g = m g g = 9.81 m/s 2 at sea level Weight decreases as you move higher above sea level where gravitational pull is less.

20 Everyday Forces (cont.) Normal Force a contact force exerted by one object on another in a direction perpendicular to the surface of contact.

21 Everyday Forces (cont.) Friction Force The force that opposes an applied force. Friction comes from molecular interaction between the two surfaces at contact points.

22 Friction Force Static friction the force exerted on a motionless body by its environment to resist an external force. F s = F F s reaches its maximum value at the point just before the object is able to move. F s,max

23 Friction Force (cont.) Kinetic Friction The force exerted on a moving object (F k ). Lower value than F s. Once the object starts moving, the force of friction changes from static to kinetic friction. Net external force = applied force force of friction

24 Coefficient of Friction The ratio of the force of friction to the normal force acting between two objects. This force is different for different surfaces. Coefficient of friction is represented by (μ)

25 Coefficient of Friction (cont.) Coefficient for static friction: μ s = F s, max F n Coefficient for kinetic friction: μ k = F k F n See p.144 table 4-2 (Table of Coefficients of friction)

26 Air Resistance Air is considered a fluid medium which provides an opposing force on any object moving through it. F R acts in the opposite direction and is proportional to the object s speed. Air resistance increases as speed increases.

27 Air Resistance (cont.) When F R is equal to the force acting in the opposite direction, acceleration is zero and the object moves at a constant speed. Terminal speed when the air resistance on a falling object equals the downward force of gravity, the net force is zero and the object s acceleration is zero.

28 Sample Problem 1 Draw a force diagram of a crash-test dummy in a car at the moment of collision. Assume that the forces acting on the car are 19,600 N downward, 17,800 N forward, and 25,000 N backward. The forces acting on the dummy are 585 N downward, 175 N backward, and 585 N upward.

29 Sample Problem 2 A man is pulling on his dog with a force of 70 N directed at an angle of 30 to the horizontal. Find the x component of this force. Find the y component of this force.

30 Sample Problem 3 A 5.5 kg watermelon is pushed across a table. If the acceleration of the watermelon is 4.2 m/s 2 to the right, find the net external force exerted on the watermelon.

31 Sample Problem 4 Identify the action-reaction pairs in the following situations: a person takes a step a snowball hits someone in the back a baseball player catches a ball a gust of wind strikes a window

32 Sample Problem 5 A 24 kg crate initially at rest on a horizontal surface requires a 75 N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor.