Forces

What is a Force? A force is any push or pull on an object A force does NOT always require contact –Gravity –Electrostatic –Magnetism

Force: A vector quantity The direction of the force is the direction of the push or the pull. –More specifically, the direction of a particular force is the direction in which the object would accelerate if the force was the only one acting on the object. The magnitude, or strength, of the force is measured in the Newton. 1 Newton = 1

The Newton A 1 Newton force has the strength to cause a 1 kg object to accelerate at 1m/s 2 for as long as the force is applied. A 2 Newton force can cause a 1 kg object to accelerate at a rate of 2 m/s 2, or could cause a 2 kg object to accelerate at a rate of 1 m/s 2 1 Newton = 1

The sum of all Forces When multiple forces are acting on an object at the same time, the sum of the forces is called the net force, ΣF Example: FGFG FNFN ΣF = F N + F G

Newton’s First Law of Motion: Inertia Every object in a state of uniform motion tends to remain in that state of motion unless a NET external force is applied to it. FGFG FNFN ΣF = F N + F G ΣF = 0 N Since the cup is in a uniform state of motion (rest), there must not be a net force applied to the cup. F N + F G = 0 N F N = - F G The table must be pushing up on the cup with the same strength that gravity is pulling down on the cup.

Newton’s Second Law of Motion: II. A net force, ΣF, will cause a mass, m, to accelerate, a. This relationship can be quantified by the equation: ΣF = ma

Try this: What is the magnitude (strength) of the net force required to accelerate a 10 kg object at a rate of 6 m/s 2 ? Givens: m = 10 kg a = 6 m/s 2 ΣF = ? ΣF = ma ΣF = 10 kg * 6 m/s 2 ΣF = 60 = 60 N ΣF = ma

Try this: If a total force of 450 N is applied to a 30 kg object, what will be the object’s acceleration? Givens: ΣF = 450 N m = 30 kg a = ? ΣF = ma

Try this: A net force of 200 N is acting upon an box of things. If the box accelerates at a rate of 4 m/s 2, what is the combined mass of the box and the things? Givens: ΣF = 200 N a = 4 m/s 2 m = ? ΣF = ma

Try this: A 1500 kg car accelerates from rest to 60.0 km/h in 6.0 seconds. What net force was required to do this? Given: ΣF = ? v = 60.0 km/h = 16.7m/s v o = 0 km/h m = 1500kg t = 6.0 s a = ? ΣF = 4200N ΣF = ma

A few forces to get us started Force of Gravity Normal Force Force of Friction Applied forces There are MANY other forces, these are just a few to get us started.

The Force of Gravity: a.k.a: an object’s weight A non-contact force F G or F w The force that gravity exerts on an object is proportional to the object’s mass. Specifically, F G =mg, –g is gravitational acceleration –g = -9.8 m/s 2 on earth The direction of the force of gravity is towards the center of the earth.

The Normal Force, F N Normal force is the force exerted by a rigid surface when an object comes in contact with it. The Normal force is usually equal and opposite the force exerted by the object on the surface. Normal means perpendicular to a plane It is called the “Normal” force because it is always exerted perpendicular to the surface of contact.

Force of Friction A retarding, contact force Friction acts to oppose motion between two surfaces in contact F F

Applied Forces An applied force is a name given to a variety of forces that might act on an object –Man pushing on a box –Child pulling a rope attached to a sled –Mother pushing a swing These forces are almost always specifically stated in a problem.

Free-Body Diagrams A free-body diagram is a picture where all forces acting on an object are drawn from the center of the object. FGFG FNFN F FGFG FNFN FAFA

Free Body Diagrams A girl is suspended motionless from the ceiling by two ropes. A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance. A car is coasting to the right and slowing down.

Subtleties of Forces The direction of the net force, ΣF, always indicates the direction of the acceleration, but not necessarily the direction of motion. –Example: A projectile FGFG FGFG FGFG FGFG FGFG FGFG

Subtleties of Forces The direction of the net force, ΣF, always indicates the direction of the acceleration, but not necessarily the direction of motion. A force that acts in two dimensions is typically separated into its components F x = │F│cos F y = │F│sin F Thrust F Thrust-x F Thrust-y

Subtleties of Forces The direction of the net force, ΣF, always indicates the direction of the acceleration, but not necessarily the direction of motion. A force that acts in two dimensions is typically separated into it’s components. Newton’s second law is most often applied in each dimension separately. ΣF x = ma x ΣF y = ma y

Subtleties of Forces The direction of the net force, ΣF, always indicates the direction of the acceleration, but not necessarily the direction of motion. A force that acts in two dimensions is typically separated into it’s components. Newton’s second law is most often applied in each dimension separately. Mass and weight are NOT the same thing. –Mass is a scalar measurement of how much matter an object is made of. An object’s mass does not change because it’s location has changed. –Weight is a vector measurement of the force gravity exerts on an object. If you take an object from the surface of the earth to the surface of the moon, it’s weight will change, but it’s mass will not.