Free Body Diagrams & Net Force Today, you will learn: The four main forces working on a moving object: The force of gravity The force of friction The normal force The applied force How to combine the above four forces to determine the net force. How to draw the above four forces as a free body diagram. How to draw a net force diagram. How to solve mathematical problems involving the above four forces.

There are numerous forces working on moving objects There are numerous forces working on moving objects. The main forces are: The force of gravity (Fg = 9.81 m/s2 [down]) The force of friction (Ff – air resistance is a type of friction force) The normal force (FN – not natural force) The applied force (Fapp – the push or pull on an object) The above forces work in different directions and when they are combined, this results in the net force (Fnet). The net force determines in which direction an object will accelerate based on its mass (Fnet = ma). In order to determine the net force on an object we will need to be able to draw a free body diagram, which shows all of the forces acting upon a moving object.

Consider an object resting on a surface Consider an object resting on a surface. The force of gravity (Fg) acts to pull the object downwards. Obviously, if gravity were the only force acting upon this object, then it would fall at a rate of 9.81 m/s2 [down]. However, the object is at rest and not accelerating downwards. This occurs because the surface is producing an equal and opposite force (according to Newton’s Third Law of Motion). The opposite force being applied to the object (by the surface) is known as the NORMAL FORCE. The normal force is always perpendicular (at right angles) to the surface. If the surface is flat, then the normal force is in the opposite direction of gravity. According to Newton’s Third Law of Motion, the normal force must be the same strength as the force of gravity. These forces cancel out and there is no net force, so the object does not accelerate.

Now imagine that you pushed or pulled on an object, i. e Now imagine that you pushed or pulled on an object, i.e. you applied a force to the object. In this case, you would need to account for the applied force, as well as the force of friction. These two forces oppose each other. FNET = Fapp – Ff If the applied force does not overcome the force of friction, then the object will not move (i.e. Fapp < Ff, then FNET is a negative number). At this point, you need to be able to draw the four types of forces on an object. Remember that gravity must always be assumed. PROBLEM: A block is pushed with 25 N of force. The force of friction is 10 N. Draw the free body diagram.

Afterwards, you need to draw the net force diagram Afterwards, you need to draw the net force diagram. The net force diagram should only have one arrow, in the direction in which the object will accelerate. A stationary object will have no net force and will not accelerate. PROBLEM: If the object has a mass of 5 kg, what will be its acceleration?

PROBLEM: A 264 N force is applied to a 39. 5 kg object PROBLEM: A 264 N force is applied to a 39.5 kg object. If it accelerates at a rate of 2.60 m/s2, what is the force of friction?

PRACTICE: A block is pushed, experiencing a force of friction of 33 PRACTICE: A block is pushed, experiencing a force of friction of 33.2 N. If the block is 14.3 kg and accelerates at a rate of 2.87 m/s2, what is the applied force?

H. W. Try the following problems H.W. Try the following problems. Draw the free body diagram as well as the net force diagram. A 23 kg object is pushed with 54.2 N. If it experiences 23.1 N of friction, at what rate will it accelerate? An object is pushed with 31.9 kN, experiencing 11.7 kN of friction. If it accelerates at a rate of 19.4 m/s2, what is its mass? A 46.5 N force is applied to a 21.6 kg object. If it accelerates at a rate of 4.6 m/s2, what is the force of friction? A block is pushed, experiencing a force of friction of 34.9 N. If the block is 12.0 kg and accelerates at a rate of 4.23 m/s2, what is the applied force? ANSWERS: a = 1.4 m/s2, m = 1040 kg, Ff = 52.9 N, Fapp = 85.7 N