Newton’s First Law of Motion Newton’s first law is often called the law of

inertia• Newton’s First Law of Motion states—An object

at rest will remain at rest, or an object in motion will remain in motion in a straight line at constant speed, unless an external force is applied to it and changes its state motion

The Friction Force Ff

• Friction is often minimized in solving force and motion problems, but in the real world, friction is everywhere

– You need friction to both move and stop a bike or a car• Two types of Friction Force• Static Friction —the force exerted between the two objects in contact with one

another that opposes the movement of the objects; there is no relative motion between the objects

Think about friction as you push a heavy crate across the floor – You give the crate a push, but it doesn’t move

• Newton’s laws tell you it should move unless there is a second horizontal force on the crate, opposite in direction to your force, and equal in size

– That force is called the static friction force.• You can push harder and harder, but if the crate still doesn’t move, the friction

force also must be getting larger– The static friction force acts in response to other forces

• Finally, when your push gets hard enough, the crate begins to move– the static friction force can grow only so large.

• Kinetic Friction—the force exerted between objects in contact with each other when there is relative motion between the objects

• The magnitude of the friction force depends on the surfaces in contact and the mass of the object, but not on the area of the surfaces in contact nor the speed of their relative motion

• The magnitude of the friction force is proportional to the magnitude of the force pushing the two objects together, one surface against the other.– That force, perpendicular to the surface, is the normal force FN

• The forces, Ff and FN, are at right angles to each other

FN

Ff Fp FpFf

FN

• Although typical coefficients of friction can be less than 1, this doesn’t mean that the coefficient of friction must be less than 1

• Coefficients as large as 5.0 are experienced in drag racing.

• Forces are needed to overcome any friction that may be present and to set objects in motion initially

• Once the object is moving in a force-free environment, it will move in a straight line indefinitely– The motion of the object becomes constant– Forces are needed to accelerate objects, but not to maintain

motion if there is no friction.

• If you slide a hockey puck along the surface of a city street, the puck comes to rest almost instantly– Slide it along ice, it slides for a longer distance. This is because

the friction force is very small– Slide it along an air table where friction is practically absent, it

slides with no apparent loss in speed

• In the absence of forces, a moving object tends to move in a straight line at constant speed indefinitely

• Toss an object from a space station located in the vacuum of outer space, and the object will move forever

Mass—A Measure of Inertia• Kick an empty can and it moves• Kick a can filled with sand and it doesn’t move as much• Kick a can filled with steel nails and you’ll hurt your foot

– The nail-filled can has more inertia than the sand-filled can, which in turn has more inertia than the empty can.

• The amount of inertia an object has depends on its mass • The more mass an object has, the greater its inertia and

the more force it takes to change its state of motion.

Mass Is Not Weight

• Which has more mass, a feather pillow or a common automobile battery?

• Clearly an automobile battery is more difficult to set into motion– The battery greater inertia and hence it has

greater mass– A pillow may be bigger, but it has less mass

Mass is often confused with weight. • Mass–measure of the amount of material in an object and depends only on the

number of and kind of atoms that compose it– The amount of material in a particular object is the same whether the object is located

on the earth, on the moon, or in outer space– The object’s mass is the same in all of these locations– The same force would be required to shake the object with the same rhythm whether

the object was on the earth, on the moon, or in a force-free region of outer space– The object’s inertia, or mass, is solely a property of the object and not its location.– SI units for mass is the kilogram (kg)

• Weight–a measure of the gravitational force acting on an object– Weight depends on an object’s location– The weight of an object would be very different on the earth and on the moon, and

still different in outer space– On the moon, the object would have only one-sixth the weight it has on the earth.

This is because the force of gravity on the moon is only one-sixth as strong as it is on the earth

– If the object were in a gravity-free region of space, its weight would be zero. Its mass, on the other hand, would not be zero

– Standard scales give a measure of an object’s mass not weight– SI units for weight is the Newton (N)

• While mass and weight are not the same thing, they are proportional to each other in a given place

– Objects with great mass have great weight; – objects with little mass have little weight. In the same location, twice the mass weighs

twice as much. Mass and weight are proportional to each other, but they are not equal to each other, mass has to do with the amount of matter in the object, while weight has to do with how strongly that matter is attracted by gravity.

One Kilogram Weighs 9.8 Newtons• Weight = (mass) x (acceleration due to gravity)

w = mg Where w = weight (N); m = mass (kg); g = acceleration due to the force of gravity (m/s2)

• If you know the mass of something in kilograms and want its weight in Newtons, multiply the number of kilograms by 9.8

• If you know the weight in Newtons, divide by 9.8 and you’ll have the mass in kilograms

• On earth the acceleration due to gravity is 9.8 m/s2

– That is g = 9.8 m/s2

– One kilogram of nails weighs 9.8 Newtons, which is equal to 2.2 pounds– Away from the earth’s surface, where the force of gravity is less, the bag

of nails weighs less

• If a woman has a mass of 50 kg, what is her weight in newtons?

w = mg

w = (50 kg) x (9.8 m/s2)

w = 490 kg m/s2

recall that 1 kg m/s2 = 1 Newton w = 490 N

• Calculate the weight in Newtons of a 2000 kg elephant.

w = mg

w = (2000 kg) x (9.8 m/s2)

w = 19600 kg m/s2

recall that 1 kg m/s2 = 1 Newton w = 19600 N

• On earth an apple weighs 1 N. What is is mass in kilograms?

w = mg m = w/g m = (1 N) / (9.8 m/s2) m = 0.102 N/(m/s2) recall that 1 kg m/s2 = 1 Newton m = 0.102 (kg m/s2)/(m/s2) m = 0.102 kg

• Susie Small finds she weighs 300 N on earth. Calculate her mass

w = mg m = w/g m = (300 N) / (9.8 m/s2) m = 30.61 N/(m/s2) recall that 1 kg m/s2 = 1 Newton m = 30.61 (kg m/s2)/(m/s2) m = 30.61 kg