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From Looney Tunes Movie Collection, Warner Bros. Entertainment, 2005.

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Presentation on theme: "From Looney Tunes Movie Collection, Warner Bros. Entertainment, 2005."— Presentation transcript:

1 From Looney Tunes Movie Collection, Warner Bros. Entertainment, 2005

2 Newtons First Law Dawson High School Physics

3 Observation #1 An object at rest remains at rest, unless something makes it move.

4 Observation #2 A object in motion continues in motion with constant velocity, unless something makes it change its velocity. Constant velocity means constant speed in the same direction.

5 Combining Observations 1 & 2 An object left alone will not change its velocity. Something must cause a change in velocity. A force is something that causes an acceleration or change in velocity Change in speed Change in direction.

6 Observation #3 An object will not change its velocity unless a net external force acts on it.

7 Newtons First Law An object at rest remains at rest, and an object in motion continues in motion with constant velocity, unless the object experiences a net external force. A net external force is required to change velocity.

8 Force SI unit of force is the Newton (N). 1 N = lb 1 lb. = N A force is a vector. It has a magnitude, measure in N or lbs. It acts in a particular direction.

9 Common Forces The force of gravity (F g ) pulls straight down. The force of friction (F f ) occurs between two objects that can slide against each other. It opposes motion.

10 Common Forces The normal force (F N ) is the support force from a surface. It is called normal because it is always perpendicular to the surface. The tension (F T ) is the force in a rope or string. The tension is the same in every part of a rope.

11 Free-body diagram Free-body diagrams consider just one object and the forces that act on it. To draw a free body diagram Draw a dot to represent the object. Draw and label vector arrows representing all the forces acting on the object. All the vectors should be shown as acting at a single point.

12 WORKING WITH NEWTONS 1 ST LAW

13 Newtons First Law An object at rest remains at rest, and an object in motion continues in motion with constant velocity, unless the object experiences a net external force. A net external force is required to change velocity.

14 Inertia Another way to say the First Law is to say that objects have inertia. Inertia is the tendency of objects to resist changes in motion. The amount of inertia an object has is determined by its mass.

15 Inertial reference frame Newtons Laws require measurements to be made in a reference frame that is not accelerating. This excludes situations where the rotation of the Earth is noticeable. Large air and ocean currents. Long range missiles.

16 Types of forces Contact forces- result from physical contact between two objects. Field forces- force that can exist between two objects even in the absence of physical contact. E.g. gravity, electric forces Objects may be in contact, they just dont have to be.

17 Determining the force of gravity. The magnitude of the force of gravity on something is called the weight. The weight of an object is related to its mass. Weight = mass x g g = 9.81 N/kg

18 Equilibrium model Objects that are at rest or moving with constant velocity are in equilibrium. According to Newtons First Law, objects in equilibrium have a net external force that equals 0. Δv = 0, F = 0 F = 0, Δv = 0

19 Equilibrium Example A rock-climber who weighs 800 N is held in place by two ropes. One pulls horizontally to the right, and the other pulls up and to the left at an angle of 30 o from the horizontal. What is the tension in the ropes?

20 T1T1 T2T2 FGFG θ T1T1 Equilibrium Example


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