Phys 1st Law Balance Forces

Law 1 - Inertia Balanced and Unbalanced Forces A force is a push or a pull on an object. Forces are at work all around you all the time. More than one force can-and usually does- act on an object at the same time. Sometimes two forces act in the same direction. An example is when two people work together to push a heavy object. Sometimes the forces act in different directions. Imagine a tug-of-war between you and one friend. If you are stronger, you apply more force to the rope. You pull your friend across the line, and you are the winner! If your friend is stronger, he might pull you across the line. Sometimes the forces are equal. Neither you nor your friend moves across the line. The two forces are balanced. We say that the net force on an object is the combination of all the forces acting on it. To find the net force of forces that are acting in the same direction, add them together. For example, if you pull on a box with a force of 25 newtons (N) while your friend pushes the box (in the same direction you are pulling ) with a force of 30 N, the net force applied to the box in that direction is 55 newtons. To find the net force of forces that are acting in opposite directions, subtract the smaller force from the larger one. If you are pulling on a tug-of-war rope with a force of 40 N, and your friend is pulling with a force of 35 Nin the opposite direction, the net force on the rope is 5 newtons in your direction. You win! When the net force on an object is zero, the two forces are balanced. Balanced forces don't cause any change in the motion of an object. Balanced forces are equal and in opposite directions. If the object is not moving and two forces are applied to it that equal zero when combined, then the object will not move. If the object is already moving and two balanced forces are applied to it, the object will continue moving at the same speed and in the same direction that it was before the forces were applied. That doesn't mean that balanced forces have no effect on an object, however. Think about what would happen to an empty soda can if you pushed against it in one direction, and a friend pushed against it in the opposite direction Definition 1: If two opposing forces act on an object, the difference between the strength of the two forces is the new force, • If Force 1---+ and Force 2 <-- are equal, the net force is zero, and the cart won't move. • If Force 1---+ is 100 N and Force 2<-- is 90 N, there is a new force of 10 N to the right, so the cart will start to move to the right. Definition 2: The sum of the forces acting on an object is the net force. If the net force is positive the object moves to the right. If the new force is negative, the object moves to the left. If the new force is zero, no change of motion results. 9. Continue with the cart. • Gently push cart, but not enough to move it. Ask: I'm pushing the cart with 3 0 N offorce but it is not moving. Why? ( A 3 0 N friction force is pushing on the cart in the negative direction. The net force is zero.) • Resist the cart moving while a student is pushing it in the opposite direction. Ask: Ann was pushing with aforce of500 N in the negative direction. Why didn 't the cart move? (You were pushing with a force of 500 N in the positive direction; net force was zero.) • Reduce your force so the cart rolls toward you. Ask: Why did the cart move? (You were pushing with a force of less than +SOON. There was a net force acting in the negative direction.) • Have a student push the cart against the wall. Ask: How do you explain that {f;e cart is not moving even though a lot of force is being used? ( The wall is exerting an equal force in the d,irec;tion opposing the force the student i� exerting. The net force 1s ON, so the car.'s motion does not change:, ·r: " • . •• •'. it !J, 't . - - - - - -