2Gravity at Rest The force of gravity acts on all objects all the time. If an object is at rest, the law of inertia says that the net force is zero.There must be a force opposite to gravity that cancels it out.F = -Fgrav = mgFgrav = -mg
3Normal ForceThe force that opposes gravity for objects on the ground is called the normal force.It is perpendicular (normal) to the plane of the ground.The force is a result of the law of reaction.FN = mgFsb = -Fbs = mg
4Normal Force and Weight The normal force pushing up against gravity can be measured.We measure weight with a scale that measures normal force.Weight is a force, not a mass.Pounds measure weight, so force (not mass) can be measured in pounds.
5Forces on a Slope Draw the forces acting on the block. The force of gravity points down with magnitude Fg=mg.The normal force FN points away from the surface of the inclined plane.An applied force Fapp holds the block in place.mq
6Net Force The block stays in place on the plane. No net force along the surfaceNo net force away from the surfaceFind the components of gravity in those directions.Find the unknown forcesmqThe diagram is the same for constant velocity!
7Mechanical AdvantageThe inclined plane has been used as a machine for centuries.Pushing a block up an incline is easier than lifting.The ratio of the force to lift directly compared to the force using the incline is the mechanical advantage.mq
8Tension Force A taut rope has a force exerted on it. If the rope is lightweight and flexible the force is uniform over the entire length.This force is called tension and points along the rope.Tension acts on both ends.FTforces on the ropeFT = -Fg = mg-FTforces on the ballFg = -mg
9Tension or Normal Force Tension and normal forces are different.A pull on an object - tensionA push from a surface - normal forceEither one or both may be present.Normal forceTension forceFNFTFfrmFg
10Equilibrium in One Dimension Two weights are hung supported by strings.On the lower block the two forces balance: FT2 = m2gOn the upper block there are three: FT1 = m1g + FT2FT1 = (m1 + m2)gThe upper string has more tension than the lower string.3 forces on the upper blockFT1m1m1gFT2FT2m2m2g2 forces on the lower block
11Pulley A pulley uses tension to transfer a force to another direction. FTforces on the ropem1FTm2forces on block 1forces on block 2FfrFropem1m1Fropem2m2Fg
12Mechanical Advantage next With more than one pulley the force needed to lift an object can be reduced.The pulley is a simple machine.The mechanical advantage to the left is 2.next