1. Newton’s Third Law Reaction forces Multiple bodies problems
Chapter 4 – part B
Newton’s Third Law
Reaction forces
Multiple bodies problems

2. Newton’s Third Law
If an object experiences a force then there is another object that experiences the opposite force

3. Reaction forces
The reaction forces appear when an object has some prohibited positions, The object is confined in a particular part of the space. Examples:
A book cannot sink beneath the table
A door cannot translate, it rotates maintaining the hinges fixed
Two objects linked by a rope cannot move apart by more than the rope length (unless it snaps…)

4. Solving a problem of dynamics

5. Exercise 4.18
18. A bag of cement of weight 325 N hangs in equilibrium from three wires as suggested in Figure P Two of the wires make angles θ1 = 60.0° and θ2 = 25.0° with the horizontal. Find the tensions T1, T2, and T3 in the wires.

6. Exercise 4.26
26. Draw a free-body diagram of a block that slides down a frictionless plane having an inclination of θ = 15.0° (Fig. P4.26). Assuming that the block starts from rest at the top and that the length of the incline is 2.00 m, find (a) the acceleration of the block and (b) its speed when it reaches the bottom of the incline.

7. Exercise 4.30
30. Two objects are connected by a light string that passes over a frictionless pulley as shown in Figure P4.30. Draw free-body diagrams of both objects. The incline is frictionless, and m1 = 2.00 kg, m2 = 6.00 kg, and θ = 55.0°. Study the motion

8. Exercise 4.33
33. In Figure P4.33, the man and the platform together weigh 950 N. The pulley can be modeled as frictionless. Determine how hard the man has to pull on the rope to lift himself steadily upward above the ground.