Newton’s Laws Governing Motion Forces Newton’s Laws Governing Motion

Force Types Forces most easily experienced as pushes or pulls on an object, known as contact forces They vary in magnitude and can be applied in many directions, so they are vector quantities. There are also forces that act at a distance without contacting an object, like gravitational attraction.

Fundamental forces To explain how at a distance forces work, Faraday conceptualized a FIELD around objects that produce at a distance forces. These forces are also known as field forces. The Earth, based on its mass, projects a field out into space and when any mass enters that field, it will be attracted toward Earth.

Earth is attracted toward the Sun by being within the gravitational Field projected by the sun, but they never contact each other.

Newton’s Laws First Law- Law of inertia Inertia - a property of an object to RESIST changes in motion. Thought of as mass. It is not a measurable quantity Stated various ways- A body will remain in a uniform state of rest or linear motion unless acted upon by an outside force.

Examples Discarding friction, a bowling ball will continue to roll in a linear path at constant velocity unless a force acts on it. A large box of books will sit on a table forever until a force pushes it over the edge. If a moving body does not maintain constant speed in a straight line, then it must be experiencing some external force.

Second Law An unbalanced force must cause an object to accelerate (or change momentum). This acceleration is directly proportional to the net force acting on the object and inversely proportional to the mass of the object. a = Σ F m Most commonly expressed Fnet = ma

Sum of Forces Σ Fx = max Σ Fy = may Generally the two above expressions will be important to remember as we break forces down into two separate dimensions as we did with projectiles. Free body diagrams- a succinct method to show all forces acting on a body at one time. Isolate the system, anywhere the universe touches the system, there will be a force, Gravity and any other at a distance forces must be remembered.

One we know the sum of all the forces we can use F = (m)(a) to predict the motion of an object. A prospector pushes a 2030 kg mine cart with a force of 700. N for 5.0 s. If the cart starts at rest, how far will it go during the 5.0 s that the force is applied? F = ma a = 0.34 m/s2 X = X0 + V0t + ½ at2 X = ½ (0.34)(5.0)2 X = 4.3 m

The gravitational force exerted on an object by Earth is given a special name, Weight. Fw = (m)(g) (g = GME /(RE)2) or 9.8 N/kg If a car has a weight of 2200N, what is its mass. Fw = (m)(g) 2200 N = m = 224 kg 9.8 N/kg

Third Law and Weight Forces Between objects occur in pairs of equal and opposite magnitude F = GME mo (RE) Fw = (g) mo We calculate GME (RE)2 To get a values for the effect of Earth on all masses around it we call this value g “little g” and the force is weight. The force that Earth puts on an object is the called the object’s weight. The object will put an equal force of attraction onto Earth and this force does not have a special name.

Objects on Objects The force that supports the object, the force of the ground on the object, (written FGO) will counteract the force that the object puts on the ground, (FOG) these two forces are an example of Newton’s third law. The supporting force from a surface onto an object is called the Normal Force (FN) When the object is on a horizontal surface, the FN may have a value equal to the weight of the object, but FN and Fw are never a Newton’s third law pair.

Also commonly stated- Forces occur in action-reaction pairs. The hand applies a force on the balloon, which applies a force on the hand, and the balloon applies a force on the door which applies a force on the balloon.

This opposing force is called the Normal force and it will act perpendicular to the surface on which the object rests.

When pushing or pulling, we must often resolve applied forces into components to find the part of the force that is actually being applied to an object to cause acceleration.

Inclined planes We know that Fw is down and parallel to a. FN is perpendicular to hypotenuse c. The object will accelerate down the plane and any friction will act up the plane.