ICP “Forces and the Laws of Motion”

Students who demonstrate understanding can: HS-ETS1- 1. Analyze a major global challenge to specify qualitative and quantitative criteria and constraints for solutions that account for societal needs and wants. HS-ETS1- 2. Design a solution to a complex real-world problem by breaking it down into smaller, more manageable problems that can be solved through engineering. HS-ETS1- 3. Evaluate a solution to a complex real-world problem based on prioritized criteria and trade-offs that account for a range of constraints, including cost, safety, reliability, and aesthetics as well as possible social, cultural, and environmental impacts. HS-ETS1- 4. Use a computer simulation to model the impact of proposed solutions to a complex real-world problem with numerous criteria and constraints on interactions within and between systems relevant to the problem.

What is a force? u A force will change the state of motion of an object, a push or pull. u Newton’s second law defines a force as the product of a body’s mass and acceleration. u F = ma

Types of Forces u 1. Electric Forces -- forces caused by the interaction of electrons. Most forces we deal with are electric. For example, when you push an object you are creating electric forces between the electrons in your hand and the electrons in the object. Mechanical and frictional forces are electrical forces. u 2. Gravitational forces -- The force of attraction between masses. Gravitational forces are the weakest of all forces. But gravitational forces act over very long distances. u 3. Magnetic forces -- forces produced by moving electric charges. Magnetic forces are closely related to electric forces but the relationship is not completely understood at present. u 4. Nuclear forces -- forces within the nucleus which hold particles together. Nuclear forces are the strongest of the known forces but they act over the shortest distances. u 5. Weak Interaction forces -- the forces which is believed to cause atoms to break apart.

Changes in Motion u 1. A net force will change the state of motion of an object. u 2. Forces can be exerted through long distances. u 3. Forces always occur in pairs. u 4. In each pair of forces, one acts opposite the other.

Units System Mass Acceleration Force SI kg m/s 2 N- kg m/s 2 cgs g cm/s 2 dyne-g cm/s 2 Avoirdupois slug ft/s 2 lb-slug ft/s 2 Forces are vectors, the product of a vector and a scalar.

Newton's Laws of Motion u Newton's First Law of Motion ( Law of Inertia ) u An object at rest or in motion will remain so until acted upon by an outside (net) force. u inertia - the resistance of a body to change its state of motion.

Newton's Second Law of Motion u When an unbalanced force acts on an object, the object will be accelerated. The acceleration will vary directly with the applied force and inversely with the mass of the object. u Equation: F = ma

Sample Problem u A 5.0kg watermelon is pushed across a table. If the acceleration of the watermelon is 4.0m/s 2 to the right, find the net external force exerted on the watermelon.

Newton's Third Law of Motion u For every action there is an equal and opposite reaction. u This is the law of interaction. u Page 103; 1-7

Everyday Forces Everyday Forces u Mass depends on the amount of matter a body possesses. u Weight depends on the distance an object is from the center of the earth. u A question for thought; If an object fell down a very deep hole, is there a point at which the object would become weightless? u F = maF w = mgF w = weight, g = 9.8 m/s 2, etc.

Sample Problems u Find the weight of a 2 kg object. u Find the mass of an object which weighs 49N.

Friction u Friction is the force which opposes the motion of two surfaces in contact. u Static friction is the force which opposes the start of motion. u Sliding friction is the force which opposes motion in progress. Sliding friction is much less than static friction.

Open Response u A boat crosses a stream. u A. Identify two forces that may act on the boat. u B. Using a vector diagram, describe the effect of these forces acting on the boat.

Experiment u My Earthly Attraction u 1. Find your weight, or the weight of an 80Kg person, in newtons, N. F = mg, g = 98.m/s/s. u 2. Find the Gravitational Force between you and a person sitting one meter from you. u m 1 m 2 u F= G u r 2 u 3. Find the Gravitational Force between you and the earth.

Experiment: Paper Tower u You will be given the following material: 1 sheet of 8 X 10 paper 50cm of tape one ruler one pencil one pair scissors u Purpose: Construct a tower of paper and tape of maximum height.