Newton’s 1st Law of motion Law of inertia

Newton’s first law of motion An object at rest will stay at rest and an object in motion will continue in motion with the same speed and direction unless acted on by a force. aka – Law of Inertia

What does this mean? Basically, an object will “keep doing what it was doing” unless acted on by an unbalanced force. If the object was sitting still, it will remain stationary. If it was moving at a constant velocity, it will keep moving. It takes force to change the motion of an object.

Real-life examples 1. Explain three real-life examples of Newton’s First Law of Motion.

Some Examples from Real Life Two teams are playing tug of war. They are both exerting equal force on the rope in opposite directions. This balanced force results in no change of motion. A soccer ball is sitting at rest. It takes an unbalanced force of a kick to change its motion.

Force Force A push or pull Any action that has the ability to change motion. Units = newtons (N) There can be no change in motion without the presence of a force.

Inertia Inertia: the tendency of an object to resist changes in its state of motion Objects with more mass have more inertia = more resistant to changes in motion

Learning Check What is a force? What are the units for force? 4. What has more inertia a bowling ball or a golf ball? Why?

Mass vs. weight Mass: The amount of matter contained in an object Units: kilograms (kg) Weight: The gravitational force of an object Units = newtons (N) The weight of an object is proportional to its mass!

What is a newton? One Newton = amount of force needed to cause a mass of one kilogram to speed up by 1.0 m/s/s. 1.0 pound = 4.448 N One pound is force exerted by gravity on a mass of 0.454 kg.

g = acceleration due to gravity = 9.8m/s2 Calculating weight W = m·g W = weight (N) m = mass (kg) g = acceleration due to gravity = 9.8m/s2

Examples of weight problems 5. What is the weight of each of the following objects? 0.113 kg hockey puck 108 kg football player 871 kg automobile Answers: 1.11 N 1060 N 8540 N

6. Find the mass, in kilograms, of each of these weights. More Examples 6. Find the mass, in kilograms, of each of these weights. 95 N 82 N 0.98 N Answers: 9.7 kg 8.4 kg 0.10 kg

If objects in motion tend to stay in motion, why don’t moving objects keep moving forever? Things don’t keep moving forever because there’s almost always an unbalanced force acting upon it. 7. A book sliding across a table slows down and stops because of the force of ___________. 8. If you throw a ball upwards it will eventually slow down and fall because of the force of _____________.

In outer space, away from gravity and any sources of friction, a rocket ship launched with a certain speed and direction would keep going in that same direction and at that same speed forever.

What is Friction? The force that results from the relative motion between objects that are in contact. Example: The tire and the road Ice skate and ice surface Air resistance Friction between sled and snow surface

Learning Check 9. Explain the difference between mass and weight. 10. Give three examples of friction affecting motion.

Net Force Net Force = total of all forces acting on an object. Dependent on the amount of forces and the direction of the forces. Balanced Forces: Net force equals zero (forces are equal but opposite). No change in motion of object Unbalanced Forces: Net force does not equal zero. Results in change on motion on object.

Examples

Drawing Net Force Diagrams What is the net force on the truck? 100 N – 60 N = 40 N right Use arrows to display the direction and magnitude of the force being applied!