Contact Force Gravity at Rest The force of

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Contact Force

Contact Force

Gravity at Rest ] The force of gravity acts on all objects all the

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 = -Fgrav = mg Fgrav = -mg

Normal Force Fsb = -Fbs = mg ] The force that opposes gravity for

Normal Force Fsb = -Fbs = mg ] 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. FN = mg

Normal Force and Weight ] The normal force pushing up against gravity can be

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.

Forces on a Slope ] m q Draw the forces acting on the block.

Forces on a Slope ] m q 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.

Net Force ] The block stays in place on the plane. • No net

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

Mechanical Advantage ] The inclined plane has been used as a machine for centuries.

Mechanical Advantage ] The inclined plane has been used as a machine for centuries. ] Pushing a block up an incline is easier than lifting. m q ] The ratio of the force to lift directly compared to the force using the incline is the mechanical advantage.

Tension Force ] ] A taut rope has a force exerted on it. If

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. FT forces on the rope FT = -Fg = mg -FT forces on the ball Fg = -mg

Tension or Normal Force ] Tension and normal forces are different. • A pull

Tension or Normal Force ] Tension and normal forces are different. • A pull on an object - tension • A push from a surface - normal force ] Either one or both may be present. Normal force Tension force FN FT Ffr m Fg

Equilibrium in One Dimension ] Two weights are hung supported by strings. • On

Equilibrium in One Dimension ] Two weights are hung supported by strings. • On the lower block the two forces balance: FT 2 = m 2 g • On the upper block there are three: FT 1 = m 1 g + FT 2 • F T 1 = ( m 1 + m 2 )g ] The upper string has more tension than the lower string. 3 forces on the upper block FT 1 m 1 g FT 2 m 2 g 2 forces on the lower block

Pulley ] A pulley uses tension to transfer a force to another direction. FT

Pulley ] A pulley uses tension to transfer a force to another direction. FT forces on the rope m 1 FT m 2 Ffr Frope forces on block 1 m 1 forces on block 2 m 1 m 2 Frope m 2 Fg

Mechanical Advantage ] With more than one pulley the force needed to lift an

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