Forces

1. Forces

2. What is a force? • A force is a push or a pull on an object. • Only an unbalancedforce causes a change in the state of motion of an object—that is, it causes an acceleration. • Forces are measured in SI units of Newtons (N). • Forces are vectors! They have direction.

3. Fundamental Forces in Nature • There are four fundamental forces (listed from strongest to weakest): 1) strong nuclear force, 2) electromagnetic force, 3) weak nuclear force, and finally 4) gravitational force (or gravity). • The two nuclear forces have a limited range –basically on the scale of an atom. The other two forces extend their influence to the edge of the universe.

4. Terms: Net Force & External Force • Often more than one force acts on an object. The combination or sum of those forces is called the netforce. That is expressed mathematically as: Fnet = ∑F= F1 + F2 + …. • When only one force acts on an object, as in free fall, that is the net force. • An externalforce is any force whose source is outside of the object itself.

5. Mechanical Equilibrium • Mechanical equilibrium describes the situation in which the net force on an object equals zero: Fnet = ∑F= 0 That is, all the forces on it balance out. • In this case, the object might be at rest (no motion). Static equilibrium • …or the object might be moving with constant velocity. Dynamic equilibrium

6. Newton’s First Law of Motion • An object maintains its natural state of motion unless acted upon by a nonzero net external force (or by an unbalanced force). • This is also called the Law of Inertia. • Natural states of motion requiring no net force to maintain that condition are: 1) being at rest (v = 0) or 2) moving in a straight line with a constant speed (v≠ 0).

7. Inertia • Inertia is a property of matter that describes matter’s resistance to changes in its motion. • Inertia is directly related to mass ( how much matter is in object). • More mass meansmore inertia which means more resistance to changes in motion.

8. Implications of the 2nd Law: a = F/m • A larger mass requires more net force to achieve the same acceleration as a smaller mass with less force on it. • Doubling the net force on an object doubles its acceleration, tripling the force triples the acceleration, and so on. • Equal forces applied to different size masses results in a greater acceleration of the smaller mass.

9. Free Body Diagrams (FBDs) • Since forces are vectors, we can represent any object by a simple box with arrows pointing outward to indicate the external forces acting on it. The length of the arrows indicate the relative magnitude of each force. • Equilibrium is indicated when all force arrows balance or cancel each other.

10. Free Body Diagram: example Fn = 90 N Fnet = 50N , right a Ff = 70 N Fapp = 120 N Fg = 90 N Acceleration points in the same direction as the net force.

11. Newton’s Second Law of Motion • The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass (inertia). • a ~ Fnetand a ~ 1/m • Equation: a = F/m where F = Fnet • It’s often re-written as F = ma • SI units of force: 1 Newton = 1 kg·m/s2

12. Newton’s Third Law of Motion • When one object exerts a force on a second object, the second object exerts an equal force back on the first. • Forces always interact simultaneously in pairs: action force and reaction force. • 3rd Law forces are equal in magnitude but opposite in direction. F12 = −F21 • 3rd Law paired forces act on two different objects. They do not cancel each other!