Unbalanced Forces Part #1

Horizontal Problems Remember that net force is the sum of all forces acting on an object Several forces may be acting on an object at once Forces are divided into two categories: Applied Forces Resistive Forces

Applied and Resistive Forces Applied Force: External force acting on the object Examples: tension (a.k.a. apparent weight) and gravity Resistive Force: Any forces trying to stop the object from moving Examples: friction and gravity

Free Body Diagrams A rough sketch to help analyze a question – these can be useful when determining the types of forces acting on an object

Calculating Net Force To calculate net force, the following equation is used DIRECTION IS ALREADY ACCOUNTED FOR IN THE EQUATION!!! Resistive forces ALWAYS act in the opposite direction on applied forces

Sample Problem A wooden block at rest on a table top experiences a maximum frictional force of 5.0 N. Describe what will happen to the body when it is pulled with a horizontal force of: a) 1.0 N b) 3.0 N c) 4.9 N d) 5.1 N e) 6.0 N

Sample Problem A scuba diver pushes herself through the water with a net force of 19.0 N. If her applied force is 26.0 N, Find the frictional force of the water on her.

Sample Problem If the coefficient of static friction between a 40.0 kg box and a horizontal floor is 0.20, what is the maximum horizontal force that could be applied to the box without causing motion?

Sample Problem A 2.5 kg block of ice slides at a steady speed across a horizontal wooden floor where the coefficient of friction is 0.25. Estimate the frictional force on the ice.

Homework p. 79 #2, 4, 5, 8-14

Vertical Problems

On the vertical axis, depending on the problem, resistive force may become applied forces and applied forces may become resistive forces. Consider: An elevator going upwards 2. An elevator going down Vertical Problems

Elevators and Scales When an elevator accelerates upwards, the applied force must be greater than the resistive force (or, tension on the cable is greater than the force of gravity)

Elevators and Scales When an elevator accelerates downwards, the applied force must be greater than the resistive force (or, the gravitational is greater than the tension on the capable)

Summary If you go in an elevator with a scale, you will “weigh” more going up than going down. Meaning your apparent weight is greater when the elevator is travelling in the upwards direction.

Sample Problem An elevator with a mass of 9.00x102 kg is accelerating downward at a rate of 1.30 m/s2. What is the tension in the cable? (Fg is the applied force on the elevator, T is the resistive force)

Sample Problem A 3.0 kg object is thrown vertically upward with a force of 55 N. What is the acceleration of the object while the force is applied to it?

Sample Problem A 75.0 kg student stands on a bathroom scale on an elevator that is accelerating upward at a rate of 1.00m/s2. What is the reading in Newtons on the scale? (the scale reading is the applied force)

Sample Problem What will the reading on the above scale be if the elevator accelerates down at 1.00 m/s2?

Homework p. 79 #1, 3, 6 & 7