1. A force is It is the cause of an acceleration, or the change in an object's velocity. A force can cause an object to: -- Forces -- Changes in Motion tart moving stop moving change direction acceleration start moving stop moving change direction a push or a pull. slow down speed up

2. You exert a force on a ball when you throw, hit or kick it. You exert a force on a chair when you sit in it. The brakes on your car exert a force when they stop it. Examples:

3. The SI unit of Force is the Newton (N). A Newton is the amount of force that, when acting on a 1 kg object, produces an acceleration of 1 m/s2. Units 1 kg1 m/s2 1 N =1 kg x 1 m/s2

4. The weight of an object is a measure of the magnitude of the gravitationa l force acting on the object. Many of the units you use every day to talk about weight are really units of force and can be converted to Newtons. Did you know? 1 lb = 4.45 N 1 N = 0.23 lb How much do you weigh in Newtons? weight gravitational force lbs N ? lbs a ¼ lb stick of margarine is about 1 N of force.

5. Forces can act through contact or at a distance. Contact forces: force that arises from the physical contact of two objects. Examples: you pull a wagon, the wagon moves a football is caught, it stops you pull on a spring, the spring stretches

6. Field forces: force that can exist between objects, even in the absence of physical contact between the objects. Examples: force of gravity attraction or repulsion between electrical charges

7. Force is a vector - the effect of a force depends on its magnitude and direction. Diagramming Forces Turn to a neighbor and describe the differences between a force diagram and a free-body diagram in 30 seconds. A force diagram shows the object's involved in a situation and the forces exerted on the objects. Wall Force diagram Free-body diagram A free-body diagram shows an isolated object and the forces acting on that object only. vector magnitude direction

8. This photo shows a car being pulled by a tow truck. There are many forces acting on the car. What are they? tow truck exerts a force on the car in the direction of the cable the road exerts forces on the car car is acted upon by gravitational forces (1) (3) (2)

9. (1) draw a simple sketch of object (2) draw in forces acting on the car using arrows (direction) (3) label each vector with magnitude (if it is given or calculated) Let's draw a free-body diagram of the car... (1) Fapp Fg FN Ff Fapp = force of cable on car = 5800 N Fg = gravitational force (weight) = 14,700 N directed toward the Earth's center FN = (Normal force) = force of road on the car = 13,690 N directed upward Ff = frictional force = 775 N due to interaction of road with car's tires directed backward

10. Now, let's practice drawing free-body diagrams... 1. A book is at rest on a table top. Diagram the forces acting on the book.

11. 2. A girl is suspended motionless from the ceiling by a rope. Diagram the forces acting on the girl as she holds onto the rope. Ftens = tension force

12. 3. An egg is free-falling from a nest in a tree. Neglect air resistance. Diagram the forces acting on the egg as it falls.

13. 4. An egg is falling (not freely, do not neglect air resistance) from a nest in a tree. Diagram the forces acting on the egg as it falls.

15. Newton's Laws of Motion

16. Newton’s First Law in other words... (a) An object at rest tends to stay at rest and (b) an object in motion tends to stay in constant motion (c) until acted upon by an outside unbalanced force. an object will maintain a constant state of motion until acted upon by an unbalanced force.

17. Also called the Law of Inertia basically, an object will “keep doing what it was doing” until something makes it change this is a property called inertia

18. Newton’s Second Law ∑F = m x a This law is for situations when the forces ARE NOT BALANCED. Net force equals mass times acceleration Acceleration A net force will cause an object to accelerate in the direction of that force.

19. What's the Force? An 1800 kg car has an acceleration of 3.8 m/s2. What is the force acting on the car? Givens: Equation: Unknown: Substitute: Solve: m = 1800 kg a = 3.8 m/s2 F = ? F = ma F = (1800 kg) (3.8 m/s2) F = 6,800 N a = 3.8 m/s2

20. Notice that acceleration on an object depends on 2 variables. 1) The NET forces acting on an object. 2) The mass of an object. a =a = m Fnet Acceleration is directly proportional to the NET force and travels in the same direction as the force. Acceleration is inversely proportional to the mass of the object. ∑F = m x a rearranging to solve for acceleration... F = a m = a

21. What is “Net” force? It is defined as the sum of all of the forces acting on the object. Example: if there are 2 forces, acting in equal magnitude and opposite directions they will cancel out. But what happens when there are 2 forces with different magnitudes and with opposite directions? 5N 15N 5N

22. Mass and Weight Weight is a force! It is caused by gravity’s pull on your mass, so it is dependent on gravity F = ma since the Force is your weight and the acceleration is g... W = mg Mass is the amount of matter in an object... It is independent of gravity.

23. Mass and Weight This astronaut has a mass of 80 kg on Earth. a) What is his mass on the moon? b) What is his weight on Earth? c) What is his weight on the moon? (moon's gravity is ~ 1/6 of Earth's)

25. Newton’s Third Law Action/Reaction For every action force there is an equal and opposite reaction force.

26. What does this mean? Action – Reaction F or any 2 objects: Forces are always produced in pairs with equal magnitudes & opposite directions Force on box by floor Force on floor by box RULE: If the action force is A acting on B, then the reaction force is B acting on A.

27. You hold an apple weighing 1 N at rest on the palm of your hand. What forces act on the apple? a) A downward gravitational force of 1 N is exerted on the apple by _______. b) FN is called ____________ and has a magnitude of ______. c) Since the apple is at rest, the net force on the apple is ___. Fg = W = 1 N FN = 1 N gravity normal force 1 N 0

28. Since W and FN are equal and opposite, are they considered an action-reaction pair? Because action and reaction forces always act on ___________ object. Here we don't see that... Fg and FN both acting on the __________. The action force is _______________________________________ and the reaction force is ____________________________________________. yes or no Fg = W = 1 N FN = 1 N apple the force of gravity (weight) pulling down on the apple the apple pulling back up on the Earth different