Newton’s 3^rd Law

Unit 2: Dynamics

Newton’s Third Law

• Action-Reaction Principle

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• When two objects interact, the forces on the objects from each other are ALWAYS equal in magnitude and opposite in direction.

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• Two objects, ONE force between them.
• Force-pairs (name is deceiving, it’s one force)

Labeling Forces �(Agent/Object Notation)

Agent (“Dealer”)

Object (“Feeler”)

Type of Force

In person-wall system, identify a force-pair.

The earth pulls down on the apple, what is the reaction force?

In the horse cart system, identify a force-pair.

How does the horse-cart system ever move if the cart pulls on the horse with equal force as the horse pulls the cart.

Atwood’s Machine

• One block has mass m₁ = 1.30 kg; the other has mass m₂ = 2.80 kg. The rope is mass-less and the pulley is frictionless.
• What are:
• (a) the magnitude of the blocks’ acceleration
• (b) the tension in the cord?

Modified Atwood’s Machine

A box of mass m₂ = 1.0 kg on a frictionless plane inclined at angle θ = 30°. It is connected by a cord of negligible mass to a box of mass m₁ = 3.0 kg on a horizontal frictionless surface. The pulley is frictionless and massless.

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If F is 2.3N, what is the tension of the

string?

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What is the largest F can be without the string becoming slack?

(a) Construct free-body diagrams for each block.

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(b) Identify the horizontal force that causes the block of mass m to accelerate.

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(c) Assume that the upper block does not slip on the lower block, and find the acceleration of each block in terms of m and F and F_f.

A block of mass 3m is placed on a frictionless horizontal surface, and a second block of mass m is placed on top of the first block. The surfaces of the blocks are rough. A constant force of magnitude F is applied to the first block.

Two objects with masses of 3.00 kg and 5.00 kg are connected by a light string that passes over a frictionless pulley, as in the Figure.

(a) Determine the tension in the string,

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(b) the acceleration of each object, and

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(c) the distance each object will move in the first second of motion if both objects start from rest.

An object with mass m₁ = 5.00 kg rests on a frictionless horizontal table and is connected to a cable that passes over a pulley and is then fastened to a hanging object with mass m₂= 10.0 kg, as shown in the Figure.

(a) Find the acceleration of each object and

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(b) the tension in the cable.

Two packing crates of masses 10.0 kg and 5.00 kg are connected by a light string that passes over a frictionless pulley as in the Figure. The 5.00-kg crate lies on a smooth frictionless incline of angle 40.0°.

(a) Find the acceleration of the 5.00kg crate

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(b) Find the tension in the string.

Solutions