Newton’s Laws of Motion

Dynamic dynamics and unexpected returnsChapter 3

Sir Isaac Newton

• Determined many physical laws based on mathematics.

• His book, Principia, revolutionized the fields of both science and mathematics

Newton’s First Law of Motion

• Inertia• An object at rest will remain at rest, and

an object in motion will remain in motion unless acted upon by an outside force– Dependent on mass

Newton’s 1st Law of Motion

Which has more inertia?

1. An empty dump truck2. A full dump truck

Which has more inertia?

1. A quarter2. A dime

Which has more inertia?

1. A freight train2. A full dump truck

Which has more inertia?

1. A freight train2. A cargo plane

Which has more inertia?

1. An empty dump truck going 170 km/h

2. A cargo plane sitting on the runway

Newton’s Second Law of Motion

• Force equals mass multiplied by acceleration

• F = m•a– Force is any push or pull that can affect

motion– Mass in kg, acceleration in m/s2

– Newtons: 1N = 1kg•m/s2

Newton’s 2nd Law of Motion

What is the force applied by a mass of 2 kg with an acceleration of 6 m/s2?1. 6 N2. 3 kg●m/s3. 12 kg●m/s2

What is the acceleration of a mass of 6 kg applying a force of 24 N?1. 4 m/s2

2. 144 m/s2

3. 0.25 m/s2

What is the force applied by the acceleration of a 3.5 kg mass to 7 m/s2?1. 2 N2. 24.5 kg●m/s2

3. 0.5 kg ●m/s2

What mass is required to apply a force of 28 N after an acceleration of 7 m/s2?1. 196 kg2. 4 kg3. 0.25 kg

What acceleration would cause a 12 kg mass to produce a force of 90 kg●m/s2?1. 7.5 N2. 7.5 m/s3. 7.5 m/s2

Balanced v. Unbalanced Forces• Balanced forces are equal and

opposite on the same object– Balanced forces result in NO

acceleration• Equilibrium

– Unbalanced forces are added geometrically• The Resulting or Net Force always results

in an acceleration

Showing “Equal and Opposite”

• The observer determines the direction of motion

• Directions opposite that motion can be considered NEGATIVE

• Because the direction is considered negative, the forces can be considered negative

What would the net force be if you pushed against the wall with 122,616N?1. 122,616 N2. -122,616 N3. 0 N

What would the net force be if two players kick a soccer ball from opposite directions according to the diagram?

1. 60 N, to the left2. 60 N, to the right3. 0 N4. 180 N, upwards

What would the net force be if you pushed a hockey puck with 200 N?1. 0 N2. 200 N against the push3. 200 N in the direction of the push

Newton’s Third Law of Motion

• For every action, there is an equal and opposite reaction– These forces act on two different

objects, so they are not balanced forces

Newton’s 3rd Law of Motion

Good Examples

• Rocket engines• Firing a gun• Jumping off of a johnboat

Forces and Vectors

Gravity

• Gravity is a force of attraction between all objects of mass.

• Depends on:– the distance between the masses

• “inverse square” law– the size of the masses

• Often as a percentage of the whole

Weight

• Weight is the force of attraction caused by gravity acting on a mass.

Fw = m•g

• The g means local gravity– on Earth, g = 9.8m/s2

Gravity

Weight v. Mass

• We use weight and mass interchangeably because the only comparison we have is the Earth’s gravity.

• Weight will change based on local gravity; NASA has to take this into effect

How much does a 25 kg object weigh in Newtons?

1. 245 N2. 2.6 N3. 0.392 N

If an object weighs 397 N, what is its mass?

1. 0.02 kg2. 3890.6 kg3. 40.5 kg

If an object weighs 1100 N but has a mass of 125 kg, which planet is it on?1. Venus, g = 8.8 m/s2

2. Mars, g = 3.7 m/s2

3. Jupiter, g = 24.8 m/s2

Friction

• Friction is the force that opposes all motion.

• A moving object will always lose energy to friction

Types of Friction

• Sliding Friction– When two surfaces slide along each

other– Most resistive friction– Can lead to buildup of heat

Types of Friction

• Rolling Friction– When an object or surface rolls along

another– Less resistive than sliding, more

resistive than fluid

Types of Friction

• Fluid Friction– When a surface slides along a fluid

(gas or liquid)– Least resistive friction

• Lubrication

– Air resistance

What type of friction is exhibited by your shoes on the ground?1. Sliding2. Rolling3. Fluid4. Air Resistance5. None of the

above

What type of friction is exhibited by a lubricant?1. Sliding2. Rolling3. Fluid4. Air Resistance5. None of the

above

What type of friction is exhibited by the wheels on a cart?1. Sliding2. Rolling3. Fluid4. Air Resistance5. None of the

above

What type of friction is exhibited by your fingerprints?1. Sliding2. Rolling3. Fluid4. Air Resistance5. None of the

above

The Big “Mo”

• Momentum is the product of an object’s mass multiplied by its velocity.

• p = m • v• kg•m/s = kg • m/s

What is the momentum of a 210 kg hog running at 12 m/s?

1. 17.5 kg●m/s2. 2520 kg●m/s3. 0.057 kg●m/s

Law of Conservation of Momentum

• Momentum can not be created or destroyed under normal circumstances

• Momentum can be changed from one form to another– Total momentum before a change must

equal total momentum after a change• Ex. truck of gravel coming to a stop,

pieces of a dropped light bulb

Law of Conservation of Momentum

• Formulam1v1+m2v1 = m1v2+m2v2

Conservation of Momentum and Newton’s Laws

• An untethered astronaut is stranded away from his spaceship while working on a satellite in space. The only equipment he has is all the tools he was using to repair the satellite.

• Write a short narrative of how you think he can get back to his spaceship.