LR247.1.1: Newton's Laws

There can be many different forces acting on an object at the same time. These forces may be working in the same direction or different directions. Some forces may be strong and others might be weak. The combination of all these forces is the net force.

Net force determines what an object will do. An object could:

•start moving

•stop moving

•change direction

To calculate net force, you add the forces that are being exerted in the same direction, and you subtract forces that are exerted in opposite directions. Select the buttons in numerical order to see how to calculate the net force below.

When forces are working in opposite directions, the smaller force should be subtracted from the larger force. The object always moves in the direction of the larger force. Select the buttons in numerical order to see how to calculate the net force.

In this picture, the box is in equilibrium. Select the buttons in numerical order to see how to calculate the net force.

It should take a net force of 25 N to move a box. Tina tries to push the heavy box with 10 N of force to the right. It does not budge. She gets tired and takes a break. Then, Carla pushes it with 15 N of force to the right, and still cannot move the box.

Neither girl provides enough force (25 N) to move the box. A net force measures all of the forces on an object happening at one time. The girls need to push the box at the same time and in the same direction to move the box.

Check for Understanding:

1. Net forces are all the forces acting on an object at one time.

2. The net force determines what direction an object will move, if it moves at all.

3. Net forces are unbalanced when force in one direction is greater than force in the opposite direction.

4. Net forces are balanced, or in equilibrium, when equal forces act in opposite directions.

Sir Isaac Newton lived from 1643-1727. He is known as one of the most influential scientists of his time. He is well known for using math to study nature and describing the three laws of motion.

The first law of motion Newton described was that an object in motion will remain in motion unless acted upon by an unbalanced force. Things like air and friction are examples of unbalanced forces. He also noted that an object at rest will remain at rest unless acted upon by an unbalanced force.

Check for Understanding:

Newton's first law of motion states:

1. an object in motion will remain in motion unless acted upon by an unbalanced force.

2. an object at rest will remain at rest unless acted upon by an unbalanced force.

Newton also noticed that when an object had more mass, it resisted moving more than an object with less mass. The tendency of an object to resist any change in motion is known as inertia.

Newton's second law teaches us:

1. force = mass × acceleration.

2. acceleration is the change in velocity over time.

3. when a greater force acts on an object, it has a greater acceleration.

4. when an object has less mass, it is easier to accelerate.

Newton's third law shows us that for every action there is an equal and opposite reaction. For example, when the diver pushes down on the diving board, the diving board will push up on the diver with an equal force.

1. The combination of all the forces acting on an object is net force.

2. Newton's first law teaches us that inertia is the tendency of an object to resist any change in motion.

• An object at rest stays at rest.

• An object in motion stays in motion.

3. Newton's second law describes the relationship between acceleration, mass, and net force.

4. To calculate force, multiply the mass times the acceleration.

5. Newton's third law states that for every action there is an equal and opposite reaction.