Chapter 4 Force Ewen et al (2005) 4.4 Total Forces Objective: Analyze forces in one and two dimensions.

What’s wrong with this picture? Answer: The normal force is suppose to be perpendicular surface of contact.

Adding Forces Gets You Net Force In many cases, more than one force acts on an object. To find the net force on an object we must total, or sum (), the forces. Forces are vector quantities, having magnitude and direction, so direction needs to be taken into account when adding forces.

Example 1 Two workers push in the same direction (to the right) on a crate. The force exerted by one worker is 150. lb. The force exerted by the other is 175 lb. Find the net force. Ignore firction.

Example 2 The same two workers push the crate to the right, and the motion is opposed by a frictional force of 3.00 E2 lb. Find the net force.

Newton’s first and second motion laws apply When the net force on an object is zero, the object stays at rest or maintains its velocity. When the net force is other than zero, then the object will accelerate in the direction of the net force.

Example 3 The crate in example 2 has a mass of 5.00 slugs. What is the acceleration when the workers are pushing against the frictional force?

Example 4 Three ropes are tied to a central ring. Rope 1 has a force of 10.0 N, north. Rope 2 has a pulling force of 20.0 N, east. And the third rope has a force of 60.0 N, 20.0o south of west. (a) What is the net force acting on the central ring? (b) What will the acceleration of the ring be if the three ropes and ring have a combined mass of 2.75 kg?