Physics Section 4.4 Describe various types of forces Weight is a measure of the gravitational force exerted on an object. It depends upon the objects mass and its location in a gravitational field. Example: Calculate the weight of an object on the Earth’s surface, whose mass is 15 kg. To calculate the weight of an object F g = mg F g = objects weight (N) m = objects mass (kg) g= acceleration due to gravity

A normal force is a force that acts perpendicular to a surface. The normal force can be calculated by: F n = mg cos Θ The normal force,F n, is perpendicular to the surface of the incline plane but not opposite to the weight F g. The normal is opposite to the component of the weight that is perpendicular to the contact surface.

example A box with a mass of 25 kg sits on an inclined plane at a 35 degree angle to the horizontal. Find the normal force.

Friction forces Static friction, F s, is a force that resists the initiation of sliding motion between two surfaces that are in contact and at rest. As long as the box does not move, the static friction is equal to and opposite in direction to a force applied parallel to the surface. F s = - F applied When the applied force exceeds the maximum static friction force F s,max, the box begins to slide.

Kinetic friction, F k, is the force that opposes the movement of two surfaces that are in contact and are sliding over each other. Properties of friction. 1. If F a > F k,the box accelerates. 2. If F a = F k,the box moves at a constant velocity. 3. Both F a and F k are parallel to the surfaces in contact. 4.The force of friction (both F k and F s ) are proportional to the normal force. 5. F k is less than or equal to F s.

The coefficient of friction is the ratio of the magnitude of the force of friction between two objects in contact to the magnitude of the normal force with which the objects press against each other. Coefficient of kinetic friction µ k = F k F n Coefficient of static friction µ s = F s,max F n

example A 32 kg box initially at rest on a horizontal floor requires a 65 N horizontal force to set it in motion. Find the coefficient of static friction. After it starts to move, a horizontal force of 45 N will keep it moving at a constant velocity. Find the coefficient of kinetic friction.

example A student pulls a 30 kg box with a force of 85 N with a rope that makes a 25 degree angle with the ground. If the coefficient of kinetic friction is.22, find the acceleration of the box.

Fluid friction is the friction exerted on an object as it moves through a fluid. (gas or liquid) As a free falling object accelerates, its velocity increases as does the air resistance force, until the force due to gravity is equal to the fluid resistance force. At this point the object no longer accelerates and has reached terminal velocity. At relatively low velocities, the magnitude of the fluid force is proportional to the magnitude of the velocity

Four Fundamental Forces 1.Electromagnetic – friction, magnetic, electrical 2.Gravitational 3.Strong Nuclear 4.Weak Nuclear

Assignment Page 143 Problems 1,2,3,4,5