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

Dawson High School Physics Newton’s First Law Dawson High School Physics

Observation #1 An object at rest remains at rest, unless something makes it move. Put piece of dry ice on table. Ask why it doesn’t move. Answers might be because it doesn’t have any “force”. Then push it and make it move.

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. Focus- Show dry ice again. Push slowly. As it moves ask if I have to keep pushing it. Will its motion change? When? So, it will stay moving in one direction until made to do something else. So, what is the key idea here? Is something necessary for an object to move, or change its motion? So, something is needed to cause something to speed up or slow down, but not to keep moving. (1) Now push a cart against another cart with magnets out. Ask what is happening when it hits the barrier? Did it change it’s velocity? How. (2)

Combining Observations 1 & 2 An object left alone will not change it’s 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. GP- p. 128, #1, Three examples each of forces causing objects to start moving, stop moving, change direction.

Observation #3 An object will not change its velocity unless a net external force acts on it. Then ask about tug of war. Ask if force was being applied to the rope. Ask why it didn't move. Elucidate it’s unbalanced or net forces that matter. (1)

Newton’s 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.

Force SI unit of force is the Newton (N). A force is a vector. 1 N = 0.225 lb 1 lb. = 4.448 N A force is a vector. It has a magnitude, measure in N or lbs. It acts in a particular direction. (1) Say that forces are measured in Newtons. Do demonstration 2 with a long and short piece of rope. Ask why more force had to be applied in one situation. So clearly, direction matters.

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

Common Forces The normal force (FN) is the support force from a surface. It is called “normal” because it is always perpendicular to the surface. The tension (FT) is the force in a rope or string. The tension is the same in every part of a rope. Ask why does the book not fall through the table. Are their forces acting on it? Elucidate that both gravity and the “force of the table” act on it. Do demo- Normal force. (1) (2)

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. Assign Worksheet 1 for homework

Working With Newton’s 1st Law

Newton’s 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.

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. Demonstration 3 with physics books and paper.

Inertial reference frame Newton’s 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. Imagine your in an airplane. A bowling ball is on the aisle next to you. Suddenly the bowling ball starts to move backwards without anything pushing on it. Is Newton’s First Law violated? Work toward idea that plane is accelerating, so can’t use Newton’s First law in that reference frame. (1) Why do hurricanes get their motion? (2)

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 don’t have to be. Ask what I had to do to get dry ice moving. (Touch it.) Is a touch necessary for a force to act? (Yes, no, whatever.) Then drop ball and ask what caused ball to accelerate. (Gravity.) Did earth touch the ball? Did it still accelerate? (1), (2)

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 (1) Do weight vs. mass lab as demo.

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

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 30o from the horizontal. What is the tension in the ropes?

Equilibrium Example T1 T1 T2 θ FG