Net Force Physics 11

Comprehension Check 1. An object has a weight of 12.2N; what is its mass? 2. If you were to take the weight of a 1.0kg object on the moon, what would it be? 3. A 5.00kg wooden crate is slid across a wooden floor; what frictional force is experienced by the crate? 4. If a 150N force is applied to a 25kg object, what would its acceleration be?

Comprehension Check kg N N m/s 2

Putting it All Together Now that we have considered Newton’s Second Law, you can use that to analyze kinematics problems with less information than we have used previously Now that we have considered Newton’s Second Law, you can use that to analyze kinematics problems with less information than we have used previously We can either use dynamics information to then apply to a kinematic situation or vice versa We can either use dynamics information to then apply to a kinematic situation or vice versa

Problems Page 168 Page 168 Questions 4-8Questions 4-8

Free Body Diagrams A free body diagram will be used in most dynamics problems in order to simplify the situation A free body diagram will be used in most dynamics problems in order to simplify the situation In a FBD, the object is reduced to a point and forces are drawn starting from the point In a FBD, the object is reduced to a point and forces are drawn starting from the point FgFg FNFN FaFa FfFf

The Net Force In most situations, there is more than one force acting on an object at any given time In most situations, there is more than one force acting on an object at any given time When we draw the FBD we should label all forces that are acting on an object and also determine which would cancel each other out When we draw the FBD we should label all forces that are acting on an object and also determine which would cancel each other out Ones that do not completely cancel out will be used to determine the net force Ones that do not completely cancel out will be used to determine the net force

The Net Force The net force is a vector sum which means that both the magnitude and direction of the forces must be considered The net force is a vector sum which means that both the magnitude and direction of the forces must be considered In most situations we consider if Physics 11, the forces we consider will be parallel or anti-parallel In most situations we consider if Physics 11, the forces we consider will be parallel or anti-parallel

An Example A 25kg crate is slid from rest across a floor with an applied force 72N applied force. If the coefficient of kinetic friction is.27, determine: A 25kg crate is slid from rest across a floor with an applied force 72N applied force. If the coefficient of kinetic friction is.27, determine: The acceleration of the crate?The acceleration of the crate? The time it would take to slide the crate 5.0m across the floor.The time it would take to slide the crate 5.0m across the floor.

FBD F g =-250N F N =250N F a =72N F f =?

Use the frictional force equation to determine the magnitude of the frictional force

The net force is the sum of the forces (acting parallel or anti- parallel)

Use Newton’s Second Law to solve for the acceleration

Use kinematics to solve for the time taken to cross the floor

Practice Problems Page 170 Page 170 9, 10 and 139, 10 and 13 Physics 20 Physics 20 Page 104Page Page 108Page