Newton S 2Nd Law Problems: Free Body Diagrams

Name: ______Date: ______Period: _____

Newton’s 2nd Law Problems: Free Body Diagrams

Part I: Features of free-body diagrams The main features of free-body diagrams • Free-body diagrams are vector diagrams used to show the magnitude and direction of forces acting on an object in a given situation. • The size of the arrow in a free-body diagram represents the magnitude of a force. The arrow’s direction shows the direction that the force acts. (The sizes of the arrows are not always drawn to scale but should be suggestive of relative sizes.) • Each force arrow is labeled to indicate the type of force. • The object that forces are acting upon is represented by a box. Force arrows are drawn from the center of the box outward in the direction in which each force acts.

Part II: Typical Forces to Account for in Mechanics Problems

Fapp: Applied force. This is a force applied to an object, for example, a push or pull.

Fg: Force due to gravity; also known as weight. All objects on Earth experience this force, which is directed to Earth’s center.

Fn: Normal force. This is the support force exerted on an object in contact with another stable object.

Ff: Friction force. This force is exerted by a surface on an object and opposes the motion of the object.

Fair: Air resistance. Friction force applied by the air on objects traveling through air.

Ft: Tension. Tension is a force transmitted through supports such as strings or wires. It is directed along the strings.

1 Name: ______Date: ______Period: _____

Part III: Free-Body Diagrams and Force Equilibrium Draw a free-body diagram to represent each of these equilibrium situations. What is the acceleration in each case?

1. A pencil rests on a desk.

2. You push a book across your desk from right to left.

3. A girl sits motionless on a tree swing which has two ropes and hangs from a tree branch.

4. A skydiver coasts down to the ground with constant velocity.

Part IV: Free-Body Diagrams and Objects Experiencing a Non-zero Net Force Free-body diagrams helps one visualize all the forces acting on an object so the net force can be determined. Solve the following problem, draw a free-body diagram to help you visualize the forces.

Problem: Suppose a 45.0 N block is being pushed along a floor, where the force due to friction is 15.0 N. If a force of 25.0 N is applied, at what rate will the block accelerate? The mass of the block is 46.0 kg.

1 Name: ______Date: ______Period: _____

Exercises Use a free-body diagram to help find each solution. 1. A model rocket has a mass of 0.12 kg. It accelerates vertically from rest to 60.0 m/s in 1.2 s. (a) What is the average acceleration? (b) What is the net force on the rocket? (Ignore air resistance.) (c) If the force of gravity on the rocket is 1.20 N, what is the total thrust of the engine?

2. A fully loaded military rocket has a mass of 3.0 x 106 kg, and the force of gravity on it at ground level is 2.9 x 107 N. What is the rocket’s acceleration at lift-off if the engines provide a thrust of 3.3 x 107 N?