Newton’s Laws of Motion

Newton’s Laws of Motion

Newton’s First Law of Motion • An object at rest will remain at rest and an object in motion will remain in motion at constant speed in the same direction unless an unbalanced force acts on it • In other words: • If something is just sitting there, it is not going to move on its own without a new force being introduced • Likewise, if an object is moving, it will not stop moving without a new force being introduced

Balanced or unbalanced? • According to Newton’s first law, will an object’s motion change if an unbalanced force is not introduced? • No! • So, what is the net force on the object’s described by the first law? • Zero Newtons!

Resting? Are they asleep? • Why are a chair on the floor and a golf ball sitting on a tee said to be “at rest”? • They are not moving • When will they no longer be at rest? • When an unbalanced force is applied to them (golf club hits the ball or hand pushes the chair) and they begin to move

It keeps going and going and going • According to Newton’s First law, an object will continue to move forever with the same velocity unless an unbalanced force acts on it – what does this mean? • Any object will continue to move in the same direction and at the same speed forever if left untouched by other forces • What happens to a ball when it is left to roll on a field? • It will eventually stop because friction slows it down • What do you think would happen to a ball if left to “roll” in outer space? • It will continue to “roll” because there is no friction in space to sow it down

Bump in the night (or in the car) • What happens to you when the bumper car you are riding in hits another bumper car? • You keep moving forward even though the bumper car stops • Hitting the other bumper car does not affect your motion, only the motion of your car • That is why there are seat belts in bumper cars!!

What about Friction? • According to Newton’s first law, when you push your desk, it should continue to move forever, but it doesn’t – it stops quickly. Why? • Friction between the desk and floor works against the motion of the desk • Friction causes motion to slow down and eventually stop

Inertia • Newton’s First Law is also known as the Law of Inertia • Inertia is the tendency of all objects to resist any change in their motion • All objects either want to remain at rest, or keep moving • This is why you are pushed to the door of the car when it turns • Your body wants to continue to move forward, but the car begins to turn

Increasing Inertia • How are mass and inertia related? • Mass is a measure of inertia • The more mass an object has, the more inertia • Similarly, the less mass, the less inertia • This is why its easier to pitch a softball than a bowling ball • It is easier to change the motion (throw) the softball because it has less mass – you can get it moving easier than the bowling ball

Newton’s Second Law of Motion • The acceleration of an object depends on the mass of the object and the amount of force applied to the object • If you apply the same force when kicking a wall and kicking a soccer ball, which will move faster/farther? • The soccer ball because it has less mass • The wall will accelerate, but because it is so massive, you will not be able to see it

Mass and Acceleration • In other words: • If you increase mass, but use the same force, acceleration will decrease • If you decrease mass, but use the same force, acceleration will increase • Ex: a small force applied to an empty shopping cart vs a full shopping cart will cause the empty cart to move farther away • Acceleration increases as the force on an object increases • Acceleration decreases as the force on an object decreases

Newton and Math • To find acceleration, divide the force applied by the mass of the object: • A = F M • To find the force applied, multiply the mass of the object times its acceleration: • F = M x A

Hit the floor • If you were to drop a watermelon and an apple at the same time, from the same height, they would hit the floor at the same time • HOW???? • It takes more force to accelerate the watermelon because of its greater mass • The larger force needed, slows down the watermelon’s acceleration • The apple, which has a smaller mass, requires a smaller force by gravity, so its acceleration is faster

Affects of changing force or mass • You are pushing a 52 kg object with a force of 54N – what is the object’s acceleration? • A=F/M  A=54N/52Kg  A= 1.04 m/s2 • What happens to the acceleration if the force you apply doubles to 108N, but the mass of the object stays the same? (will it increase or decrease?) • Acceleration will increase • What happens to acceleration if the force you apply stays at 54N, but the mass of the object is reduced by half to 26kg? (will it increase or decrease?) • Acceleration will increase • What kind of force are you applying if the mass stays the same, but acceleration decreases to 1 m/s2? (larger or smaller force?) • Smaller force • What happens to the mass of the object if its acceleration stays the same, but force has been increased to 208N? (mass increases or decreases?) • Mass would have increased too

Newton’s Third Law of Motion • When an object exerts a force on a second object, the second object exerts an equal and opposite force on the first object • Forces act in pairs • If a force is exerted, another force occurs that is equal in size and opposite in direction • **For every action, there is an equal and opposite reaction • EX: your action is pushing down on the chair, the reaction is the chair pushing back up on you • You do not move because the force pushing up is equal to your weight • The net force is 0N

Keep it moving • Action and reaction forces do not work on the same objects • If they did, nothing would ever move because net force would always be 0! • Think about swimming • When you push your hands through the water, you are exerting a force on the water – the action force • The reaction is the water pushing back on your hand • You move forward in the water because your action force is stronger than the water’s reaction force

You can’t see me • Its not always easy to see the reaction force • Take for example a falling ball: • The action force is gravity pulling the ball down • Every object exerts a gravitational force (creates gravity) • That means that the ball is also creating a gravitational force on the Earth • Why can’t you see the effect of the ball’s gravity on Earth? • Earth is so massive, that you can not see its acceleration • Thus, it is hard to detect the reaction force of the ball

Newton’s Laws of Motion