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ForcesNewtons First and Second LawsLecture 11

2 Describes the relationship between the motion of objects in our everyday world and the forces acting on themClassical (Newtonian) MechanicsConditions when Classical Mechanics does not applyvery small objects (< atomic sizes)objects moving with very large speed (close to the speed of light) 3

Forces Usually think of a force as a push or pull Force is a vector quantity

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Forces May be contact (pushing and pulling) or long range (field) force

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Units for force SI unit of force is a Newton (N)6

More about forces What if there are more than one forces acting on the object? Different forces can be added vectorially The net force is the vector sum of all the forces acting on the object:

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Example: Find the net force? Each grid has 2 N side.ABCF8Newtons First LawAn object moves with a velocity that is constant in magnitude and direction unless a non-zero net force acts on it.If an object is at rest then it will remain at rest or if it is moving along a straight line with uniform speed then it will continue to keep moving unless an external force is applied on it to change its existing stateWhen there are no external forces acting on an object, the acceleration of the object is zero.9

Newtons First Law

10Newtons First Law: An example

11Because of inertia, objects resist changes in their motion

Inertia12Inertia is the tendency of an object to continue in its original motionMass is a measure of the inertia, i.e resistance of an object to changes in its motion due to a forceRecall: mass is a scalar quantityInertia and Mass

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Newtons Second LawThe acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.

Since both the force and acceleration are vector quantities, the acceleration can also be caused by change of the direction of velocity14

Newtons Second Law

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Newtons Second Law

Note: represents the vector sum of all external forces acting on the object.Since Newtons second law is a vector equation, we can always write it in terms of components

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Units for force

An inertial reference frameFrame AFrame B

18A car rounds a curve while maintaining a constant speed. Is there a net force on the car as it rounds the curve?1. Noits speed is constant.2. Yes.3. It depends on the sharpness of the curve and the speed of the car.4. It depends on the driving experience of the driver.

Concept test19Summary: Newtons First LawAn object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

20Inertia is the tendency of an object to resist changes in its velocity: whether in motion or motionless.Summary: Inertia

Inertia is proportional to the mass of the object.

21 Force equals mass times accelerationSummary: Newtons Second Law

Acceleration22Summary: Newtons Second Law

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