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Contact Force. Gravity 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.

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Presentation on theme: "Contact Force. Gravity 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."— Presentation transcript:

1 Contact Force

2 Gravity 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 grav =  mg F =  F grav = mg

3 Normal Force  The 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. F N = mg F sb =  F bs = mg

4 Normal 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.

5 Forces on a Slope  Draw the forces acting on the block. The force of gravity points down with magnitude F g =mg. The normal force F N points away from the surface of the inclined plane. An applied force F app holds the block in place. m 

6 Net Force  The block stays in place on the plane. No net force along the surface No net force away from the surface  Find the components of gravity in those directions.  Find the unknown forces m  The diagram is the same for constant velocity !

7 Mechanical Advantage  The 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. m 

8 Tension 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. F g =  mg F T =  F g = mg FTFT -F T forces on the rope forces on the ball

9 Tension or Normal Force  Tension and normal forces are different. A pull on an object  tensionA pull on an object  tension A push from a surface  normal forceA push from a surface  normal force  Either one or both may be present. m Normal force FNFN F fr FgFg Tension force FTFT

10 Equilibrium in One Dimension  Two weights are hung supported by strings. On the lower block the two forces balance: F T2 = m 2 gOn the lower block the two forces balance: F T2 = m 2 g On the upper block there are three: F T1 = m 1 g + F T2On the upper block there are three: F T1 = m 1 g + F T2 F T1 = (m 1 + m 2 )gF T1 = (m 1 + m 2 )g  The upper string has more tension than the lower string. m1m1 F T1 m1gm1g 3 forces on the upper block F T2 m2m2 m2gm2g 2 forces on the lower block

11 Pulley  A pulley uses tension to transfer a force to another direction. m1m1 m2m2 m1m1 m1m1 m2m2 m2m2 F rope F fr F rope FgFg FTFT FTFT forces on block 1forces on block 2 forces on the rope

12 Mechanical Advantage  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


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