Working with Newtons Laws

Working With Newtons Laws

The Strategy

1. Make a sketch of the problem.

2. Draw a FBD (Free Body Diagram) for each object in the system (label ALL the forces).

3. Resolve forces into x and y components

4. Solve for the ax and ay if possible.

4. Use ∑Fx = 0 and ∑Fy = 0 OR (if object is accelerating) ∑Fx = max and ∑Fy = 0y

5. Decide on a coordinate system (where is your 0,0) so you can keep track of force direction and determine the sign (+ or -) of the force.

6. Develop equations SEPERATELY for the x and y directions.

7. Solve the equations.

Important types of forces to know:

Drag/Resistance/Friction –

Normal – applied perpendicular to the surface the object is setting on.

Tension – a force that exists throughout the length of a stretched string/cord and is exerted at both ends of the cord.

Weight – force due to gravity. Directed toward the center of the Earth (straight down).

Free Body Diagrams:

Diagram used to show the magnitude and direction of all forces acting on an object.

1) 
A crate rests on very low friction wheels. The crate and the wheels and stuff have a weight of 785 N. You pull horizontally on a rope attached to the crate with a force of 135 N. (a) What is the acceleration of the system? (b) How far will it move in 2.00 s?

2) A 85.0 kg traffic light is supported as shown. Find the tension in each cable.

3) Two masses, 4.00 kg and 5.25 kg are connected by a light string to a frictionless pulley as shown. Find the tension in the string, and the acceleration on the system.

·  4) 2 blocks hang in an elevator as shown. The elevator accelerates upward at 3.00 m/s2. Find the tension in each rope.

5) A 20.0 kg cart with very low friction wheels sits on a table. A light string is attached to it and runs over a low friction pulley to a 0.0150 kg mass. What is the acceleration experienced by the cart?

6) You pull a 65.0 kg crate of mass m across the floor with a force F at an angle of 34.0°. The coefficient of friction between the surfaces is 0.235. If the crate moves at a constant speed, what is the applied force?