Mass vs. Weight Newton’s Laws of Motion 8 th Grade Science
-Mass is defined as the amount of matter an object has. -does NOT change no matter where you are in universe! -The weight of an object on earth depends on the force of attraction (gravity) between the object and earth -DOES change depending on where you are 2
3 Formula to calculate weight: Weight = Mass × Gravity W = mg Mass: measured in Kg Gravity on Earth: 9.8 m/s/s
Practice -Calculate the weight of a car with a mass of 1500 kg. -14700N -Calculate the weight of an object with a mass of 550 kg. -5390N 4
5 1 st Law of Motion (Law of Inertia) An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.
6 1 st Law Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.
7 Newton’s First Law is also called the Law of Inertia Inertia: the tendency of an object to resist changes in its state of motion The First Law states that all objects have inertia. The more mass an object has, the more inertia it has (and the harder it is to change its motion).
8 Examples from Real Life A powerful locomotive begins to pull a long line of boxcars that were sitting at rest. Since the boxcars are so massive, they have a great deal of inertia and it takes a large force to change their motion. Once they are moving, it takes a large force to stop them. On your way to school, a bug flies into your windshield. Since the bug is so small, it has very little inertia and exerts a very small force on your car (so small that you don’t even feel it).
9 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. A book sliding across a table slows down and stops because of the force of friction. If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.
10 Newtons’s 1 st Law and You Don’t let this be you. Wear seat belts. Because of inertia, objects (including you) resist changes in their motion. When the car going 80 miles/hour is stopped by the brick wall, your body keeps moving at 80 miles/hour.
12 What does F = m x a mean? F = m x a basically means that the force of an object comes from its mass and its acceleration. Something very small (low mass) that’s changing speed very quickly (high acceleration), like a bullet, can still have a great force. Something very small changing speed very slowly will have a very weak force. Something very massive (high mass) that’s changing speed very slowly (low acceleration), like a glacier, can still have great force.
13 2 nd Law (F = m x a) How much force is needed to accelerate a 1400 kilogram car 2 meters per second/per second? Write the formula F = m x a Fill in given numbers and units F = 1400 kg x 2 meters per second/second Solve for the unknown 2800 kg-meters/second/second or 2800 N
14 3 rd Law For every action, there is an equal and opposite reaction. The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).