# Newton’s Laws of Motion

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Newton’s Laws of Motion
Taken from - Chapter 10, sec. 3 and 4

February 3, 2011 Your word is Inertia chapter 10 , sec. 3

The First Law of Motion Sir Isaac Newton proposed the three basic laws of motion in the late 1600s. His laws were based on Galileo’s suggestions about motion. ‘Galileo suggested that, once an object is in motion, no force is needed to keep it in motion. Force is needed only to change the motion of the object.’ Prentice Hall Science Explorer, Grade 8, p. 349

The First Law of Motion Newton’s first law is a restatement of Galileo’s theory. The First Law states that an object at rest will remain at rest, and an object in motion will remain in motion, unless acted upon by an unbalanced force.

The First Law of Motion This means that you have to apply some sort of force (a push or pull) to make an object move if it is still. This also means that you have to apply a force to make an object stop moving. Otherwise the object in motion will continue moving forever. The force that we know of that helps stop objects that are in motion is friction. On Earth both gravity and friction are unbalanced forces that can change an objects motion.

Inertia Galileo introduced the idea of ‘inertia’.
‘Inertia is the tendency of an object to resist a change in motion’. Prentice Hall Science Explorer, Grade 8, p. 350 Newton’s first law is also called the law of inertia. Inertia explains why you move forward after coming to a sudden stop on a carnival ride. In this instance, a force is required to keep you from continuing to move forward, such as a seat belt or a bar. Inertia depends on mass – the greater the mass of an object, the greater its inertia!

The Second Law of Motion
Newton’s second law states that acceleration depends on the object’s mass and on the net force acting on the object. An object with a larger mass requires more force to cause a change in its motion. An object with a smaller mass requires less force to cause a change in its motion. Acceleration = Net force Mass – if you rearrange this formula to solve for Net force you get mass x acceleration = Net force which gives us Kg-m/s2 = newton

Calculating Net Force You can calculate the net force even if you don’t know what the forces are that are acting on an object as long as you know the mass and the object’s acceleration rate. Remember if an object is not falling, it is not accelerating at 9.8 m/s2 What is the net force on a 1,000 kg object accelerating at 3 m/s2 ? Fnet= 1,000 kg x 3 m/s2 = 3,000 N

Exit Slip What are two ways to increase the acceleration of an object?
You can decrease the mass of the object. You can increase the force applied to the object.

February 7, 2011 Your word is Momentum. Chapter 10, sec. 4

Newton’s Third Law of Motion
‘Newton’s third law state that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object.’ Prentice Hall Science Explorer Grade 8, p. 353 Basically, this means that for every action there is an equal but opposite reaction.

Newton’s Third Law of Motion
Action-Reaction Pairs The third law explains action-reaction pairs – when you jump, you exert a downward force on the floor and then the floor exerts an equal force back up to you. See p. 354 for examples of action-reaction pairs. Name a few action-reaction pairs that you have observed!

Newton’s Third Law of Motion
Can we always detect motion in an action-reaction pair? No – sometimes the reaction to the action is so small that it is undetectable. Do Action- Reaction forces ever cancel out? No – because they are acting on different objects Unlike balanced forces acting on the same object, an action-reaction pair has forces acting on two different objects so they cannot cancel each other out. If you hit a volleyball with your arms, the volleyball exerts an equal force back to you, not back to the ball.

Momentum Momentum is a characteristic of a moving object that is related to the mass and velocity of that object. Momentum is calculated by multiplying the mass times the velocity. Momentum = Mass x Velocity Momentum = kg x m/s (NOTE: this is not equal to a newton – remember a newton = 1kg-m/s2) The unit for momentum is simply kg-m/s

Calculate Momentum What is the momentum of a bird with a mass of kg flying at 15m/s? Answer = 0.27 kg-m/s Momentum involves direction of motion so it is calculated using velocity, not speed. The more momentum and object has, the harder it is to stop.

Conservation of Momentum
The Law of Conservation of Momentum states that, without outside forces, the total momentum of objects that interact does not change. This means that momentum is conserved (or saved) – momentum is neither lost or gained when two objects collide – it is simply transferred from one object to the other. The total momentum of any group of objects remains the same unless outside forces act on the group.

Conservation of Momentum
Collisions with 2 moving objects – the momentum of one object decreases while the momentum of the other object increases but total momentum stays the same. Collisions with 1 moving object – the momentum from the moving object is transferred to the stationary object which causes the moving object to stop and the stationary object to start moving, but the total momentum stays the same. Collisions with connected objects – when two objects collide and become connected as a result of that collision, ½ of the momentum of the moving object is transferred to the nonmoving object when they connect making the total momentum stay the same.

Centripetal Force This is taken from chap. 10, sec. 5.
Centripetal force is what makes objects move in a circle. Centripetal means center-seeking. Something traveling in a circle is still accelerating because it is constantly changing direction. Satellites (any object that orbits another object in space) travel in a circle around the Earth because of the Earth’s gravity pulling the satellites toward the center of the Earth.

Exit Slip Since an object’s momentum depends on its mass and velocity, if two dogs are running at the same velocity, but one dog is large and one dog is small, which dog will have the greatest momentum? Answer – the dog with the greater mass will have the greater momentum.