Forces

Forces

1What is a Force?A force is any push or pull on an object

A force does NOT always require contact GravityElectrostaticMagnetism

2Force: A vector quantityThe 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 = 13The NewtonA 1 Newton force has the strength to cause a 1 kg object to accelerate at 1m/s2 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/s2, or could cause a 2 kg object to accelerate at a rate of 1 m/s2

1 Newton = 14The sum of all ForcesWhen multiple forces are acting on an object at the same time, the sum of the forces is called the net force, FExample:

FGFNF = FN + FG5Newtons First Law of Motion: InertiaEvery object in a state of uniform motion tends to remain in that state of motion unless a NET external force is applied to it.

FGFNF = FN + FGF = 0 NSince the cup is in a uniform state of motion (rest), there must not be a net force applied to the cup.FN + FG = 0 NFN = - FGThe table must be pushing up on the cup with the same strength that gravity is pulling down on the cup. 6Newtons Second Law of Motion:II. A net force, F, will cause a mass, m, to accelerate, a. This relationship can be quantified by the equation:

F = ma7Try this:What is the magnitude (strength) of the net force required to accelerate a 10 kg object at a rate of 6 m/s2?

Givens:m = 10 kga = 6 m/s2F = ?F = maF = 10 kg * 6 m/s2F = 60 = 60 N

F = ma8Try this:If a total force of 450 N is applied to a 30 kg object, what will be the objects acceleration?

Givens:F = 450 Nm = 30 kga = ?F = ma

F = ma9Try this:A net force of 200 N is acting upon an box of things. If the box accelerates at a rate of 4 m/s2, what is the combined mass of the box and the things?

Givens:F = 200 Na = 4 m/s2m = ?F = ma

F = ma10Try 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:F = ?v = 60.0 km/h = 16.7m/svo = 0 km/hm = 1500kgt = 6.0 sa = ?

F = 4200NF = maA few forces to get us startedForce of GravityNormal ForceForce of FrictionApplied forces

There are MANY other forces, these are just a few to get us started.

12The Force of Gravity:a.k.a: an objects weightA non-contact forceFG or FwThe force that gravity exerts on an object is proportional to the objects mass.Specifically, FG=mg, g is gravitational acceleration g = -9.8 m/s2 on earthThe direction of the force of gravity is towards the center of the earth.13The Normal Force, FNNormal 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 planeIt is called the Normal force because it is always exerted perpendicular to the surface of contact.

14Force of FrictionA retarding, contact forceFriction acts to oppose motion between two surfaces in contactFF

FF15Applied ForcesAn applied force is a name given to a variety of forces that might act on an objectMan pushing on a boxChild pulling a rope attached to a sledMother pushing a swingThese forces are almost always specifically stated in a problem.

16Free-Body DiagramsA free-body diagram is a picture where all forces acting on an object are drawn from the center of the object.

FGFN

FFFGFNFA17Free Body DiagramsA 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 ForcesThe direction of the net force, F, always indicates the direction of the acceleration, but not necessarily the direction of motion.Example: A projectile

FGFGFGFGFGFG19Subtleties of ForcesThe direction of the net force, F, 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 componentsFx = Fcos

Fy = Fsin

FThrustFThrust-xFThrust-y20Subtleties of ForcesThe direction of the net force, F, 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.Newtons second law is most often applied in each dimension separately.Fx = maxFy = may21Subtleties of ForcesThe direction of the net force, F, 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.Newtons 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 objects mass does not change because its 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, its weight will change, but its mass will not.

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