Newton's 1st Law of Motion

					Newton’s Laws of Motion

Newton’s st Law of Motion 1

(law of inertia)

Newton’s 1st Law of Motion
An object in motion tends to stay in motion and an object at rest tends to stay at rest, unless the object is acted upon by an outside force. The Law of Inertia-due to an object’s mass

We feel the effects of Newton's First Law every day, but usually don't notice them because other forces interfere. In space, the First Law is much more obvious. Objects will follow their natural path until they are stopped by an outside force. On Earth, the atmosphere will eventually slow down all moving objects, but in a vacuum (basically an empty space with no air or atmosphere), like space, it will be more obvious that objects obey Newton's Laws.

Some Examples
• The person is standing on the floor. • The only forces acting on the person are the force due to gravity pulling down & the normal force pushing up. The net force is zero and the person remains still.

Another Example
One of the most common places people feel the First Law is in a fast moving vehicle, such as a car or a bus, that comes to a stop. An outside force stops the vehicle, but the passengers, who have been moving at a high speed, are not stopped and continue to move at the same speed.

Balanced vs. Unbalanced Forces

Some Problems
An astronaut in outer space away from gravitational or frictional forces throws a rock. The rock will…..

And now the answer
…continue to move in a straight line at a constant speed. The rock’s tendency to do this is called INERTIA.

A rock is being whirled at the end of a string in a clockwise direction. If the string breaks, the path of the rock is

It will follow an “inertial path” so it will follow path “C”. Once the rock leaves the string, there are no unbalanced forces to affect its motion.

A Weighty Problem
I spend most Sunday afternoons at rest on the sofa, watching football and consuming large quantities of food. What effect (if any) does this practice have upon my inertia? Explain. My inertia will most definitely increase. My mass will increase because of this practice and if mass increases, then inertia increases.

How much force?
An 2-kg object is moving horizontally with a speed of 4 m/s. An object in motion will How much net force maintain its state of is required to keep motion. The presence the object moving of an unbalanced force with the same speed will change the and in the same velocity of an object. direction?


Is a force required?
If you were in a weightless environment in space, would it require a force to set an object in motion?
Yes, because even in outer space, an object has mass. If an object has mass then the object is going to resist changes in its motion. A force must be applied to set the object in motion. Newton’s Laws rule—Everywhere!

A Final Problem
Why isn't the girl hurt when the nail is driven into the block of wood?

And the answer is….
• Due to the large mass of the books, the force of the hammer is sufficiently resisted (inertia). • This is demonstrated by the fact that the blow of the hammer is not felt by the girl.

Newton’s Second Law of Motion







F m m m






  aF



1 a m

  F  ma

Newtons’ Second Law
• F=ma • The acceleration of an object is directly proportional to the net force acting on the object… • …and inversely proportional to the mass of the object.

Example Questions
• How much acceleration does a 747 jumbo jet of mass 30,000kg experience in takeoff when the thrust of all of the engines is 120,000N? • F= ma  a = F/m • a = 120000 N/30000 kg

•a= 4

2 m/s

Example Questions

• The same net force is applied to two blocks. • If the blue one has a smaller mass than the yellow one, which one will have the LARGER acceleration?

If the NET FORCE is parallel to the velocity, then the speed of the object increases.

If the NET FORCE is anti-parallel (or opposite) to the velocity, then the speed of the object decreases.

If the net force is perpendicular to the velocity, the direction of the velocity changes.

• Force and acceleration are vector quantities. • If v is parallel to F, speed increases. • If v is antiparallel to F, speed decreases. • If v perpendicular to F, direction of v changes.

When Acceleration Is Zero...
• …we say the object is in Mechanical Equilibrium. • …the net force is zero. • For Static Equilibrium the velocity is zero. • For Dynamic Equilibrium the velocity is constant.

When Acceleration Is Zero - Equilibrium

Static Equilibrium
Velocity is zero Examples:
Computer setting on a table

Scales pushing up Normal up

Weight down

Weighing yourself on a set of scales
Hanging from a tree Car parked on an incline
Friction Tree pulling up Weight down


Weight down

Weight down

Dynamic Equilibrium
Velocity is nonzero and constant Examples: Driving at constant velocity
Normal up Air resistance Force from road Friction Weight down

Terminal velocity in parachuting

Weight down

Newton’s 3rd Law
• For every action there is an equal and opposite reaction.

Book to earth

Table to book

Think about it . . .
What happens if you are standing on a skateboard or a slippery floor and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force. Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts).

Newton’s Third Law
• A bug with a mass of 5 grams flies into the windshield of a moving 1000kg bus. • Which will have the most force? • The bug on the bus • The bus on the bug

Newton’s Third Law
• The force would be the same.
• Force (bug)= m x A • Force (bus)= M x a

Think I look bad? You should see the other guy!

Action and Reaction on Different Masses Consider you and the earth

Action: earth pulls on you

Reaction: you pull on earth

Reaction: road pushes on tire

Action: tire pushes on road

Reaction: gases push on rocket Action: rocket pushes on gases

Consider hitting a baseball with a bat. If we call the force applied to the ball by the bat the action force, identify the reaction force.

(a) the force applied to the bat by the hands (b) the force applied to the bat by the ball (c) the force the ball carries with it in flight (d) the centrifugal force in the swing


rd 3


• Suppose you are taking a space walk near the space shuttle, and your safety line breaks. How would you get back to the shuttle?


rd 3


• The thing to do would be to take one of the tools from your tool belt and throw it is hard as you can directly away from the shuttle. Then, with the help of Newton's second and third laws, you will accelerate back towards the shuttle. As you throw the tool, you push against it, causing it to accelerate. At the same time, by Newton's third law, the tool is pushing back against you in the opposite direction, which causes you to accelerate back towards the shuttle, as desired.

What Laws are represent

Newton’s First Law: Objects in motion tend to stay in motion and objects at rest tend to stay at rest unless acted upon by an unbalanced force. Newton’s Second Law:

Force equals mass times acceleration (F = ma).
Newton’s Third Law:

For every action there is an equal and opposite reaction.

1stlaw: Homer is large and has much mass, therefore he has much inertia. Friction and gravity oppose his motion. 2nd law: Homer’s mass x 9.8 m/s/s equals his weight, which is a force.

3rd law: Homer pushes against the ground and it pushes back.