Newton’s Laws of Motion

What is a Force???

Forces: The strong force: (holds quarks and gluons together to form protons and nuetrons, 1038 times stronger than the gravitational force) The weak force: (affecting leptons, quarks, and nuetrinos – 1011 times weaker than the electromagnetic force and 1013 times weaker than the strong force) Electromagnetic force: (holds e- and protons together in atoms, and holds atoms together in molecules) gravitational force: (gravitational attraction b/w any two bodies that have mass 1. What is a Force? 2. What causes one object to exert a force on another? 3. Give as many examples of force as you can: 4. What are the SI units for force? 5. Is force a vector or a scaler? A force causes an acceleration (meaning a change in an objects velocity or direction) Through contact: a push or a pull At a distance: field forces (examples gravitational or electromagnetic) Newton (N=(kg*m) /s2 Vector – Force has a magnitude and a direction

Forces can change motion Start movement, stop movement, or change the direction of movement Cause an object in motion to speed up or slow down

Forces Diagrams and Free Body Diagrams: Forces are vectors (magnitude and direction). Force diagram (a) Shows all forces acting during an interaction On the car and on the wall Free-body diagram (b) Shows only forces acting on the object of interest On the car

Net Force - the Sum of the Forces This car is moving with a constant velocity Fforward = road pushing the tires Fresistance = force caused by friction and air Forces are balanced Velocity is constant because the net force (Fnet) is zero

Equilibrium The state in which the net force is zero All forces are balanced Object is at rest or travels with constant velocity In the diagram, the bob on the fishing line is in equilibrium The forces cancel each other If either force changes, acceleration will occur

Newton’s First Law: Experimentation led Galileo to the idea that objects maintain their state of motion or rest. Newton developed the idea further, in what is now known as Newton’s first law of motion: An object at rest remains at rest and an object in motion continues in motion with constant velocity unless acted upon by an outside force

Newton’s First Law: This is also called the Law of Inertia Inertia Tendency of an object to not accelerate Mass is a measure of inertia More mass produces more resistance to a change in velocity Which object in each pair has more inertia? A baseball at rest or a tennis ball at rest Answer: the baseball A tennis ball moving at 125 mi/h or a baseball at rest

Newton’s 1st Law: (inertia)

Newton’s Second Law ΣF=ma The acceleration of an object is directly proportional to the force acting on the object and inversely proportional to the mass ΣF=ma

Newton’s Second Law Increasing the force will increase the acceleration Which produces a greater acceleration on a 3-kg model airplane, a force of 5 N or a force of 7 N? Answer: the 7 N force Increasing the mass will decrease the acceleration A force of 5 N is exerted on two model airplanes, one with a mass of 3 kg and one with a mass of 4 kg. Which has a greater acceleration? Answer: the 3 kg airplane

Newton’s 2nd Law: F=ma

Practice Problem Space-shuttle astronauts experience accelerations of about 35 m/s2 during takeoff. What force does a 75 kg astronaut experience during an acceleration of this magnitude? Answer: 2600 kg•m/s2 or 2600 N

What do you think? Two football players, Zach and Jake, collide head-on. They have the same mass and the same speed before the collision. How does the force on Alex compare to the force on Jason? Why do you think so? Two football players, Jordan and Jake, collide head-on. Suppose Jake has twice the mass of Jordan. How would the forces compare? Suppose Jake has twice the mass and Jordan is at rest. How would the forces compare? When asking students to express their ideas, you might try one of the following methods. (1) You could ask them to write their answers in their notebook and then discuss them. (2) You could ask them to first write their ideas and then share them with a small group of 3 or 4 students. At that time you can have each group present their consensus idea. This can be facilitated with the use of whiteboards for the groups. The most important aspect of eliciting student’s ideas is the acceptance of all ideas as valid. Do not correct or judge them. You might want to ask questions to help clarify their answers. You do not want to discourage students from thinking about these questions and just waiting for the correct answer from the teacher. Thank them for sharing their ideas. Misconceptions are common and can be dealt with if they are first expressed in writing and orally. This question will likely produce a wide variety of responses. Some students may believe that the forces are always equal. Many will believe they are equal for the first example but not so for the second and third examples (next slide).

Newton’s Third Law Forces always exist in pairs You push down on the chair, the chair pushes up on you Called the action force and reaction force Occur simultaneously so either force is the action force

Newton’s Third Law For every action force there is an equal and opposite reaction force The forces act on different objects Therefore, they do not balance or cancel each other The motion of each object depends on the net force on that object