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SPH4C: Physics, Grade 12, College Preparation

# Unit 1: Motion and Its Applications

## Activity 4: Force, Free-Body Diagrams, and Net Force

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 Pulling this sled would be morechallenging without the snow! Net Force The thing that determines the motion of an object is not individual forces and their magnitudes and directions, but rather the net (or total) effect of all these forces acting together. For example, the reason that children can be pulled easily on a sled on snow is not just because of the force applied by the person pulling the sled, but also because the force of friction is very low. In summer, this sled would not move nearly as easily!

Net force is the sum of all the individual forces acting on an object.

Net force may also be referred to as resultant force or unbalanced force.

Note: Forces that are acting in the same direction can be simply added together. If they act in different directions, the rule is to choose one direction to be the positive direction, call the other direction negative, and then add the two forces together. This is illustrated for you in the example below.

#### Example

Assume that the force of gravity on the sled holding the two children in the above picture is 525 N [down]. Because the sled is not moving in the vertical direction, we know the normal force is equal, but opposite in direction, to the force of gravity and therefore equal to 525 N [up]. The vertical forces, then, add to give a net force of zero (we will talk more about this in the next activity). If the man pulls the sled with a force of 15 N [forward] and the force of friction on the sled is 6 N [backward], the free-body diagram of the sled (containing children) would be:

Letting [forward] be positive, the net force on the sled is calculated as follows:

Therefore, the net force is 9 N [forward]. This net force will change the motion of the sled; it will accelerate in the same direction of the net force.

1. A van is stopped at a red light. The total force of gravity on the van with the driver and passengers in it is 25 000 N. Draw a FBD of this van and determine the net force.