Isaac Newton England mid-1600s

Newton’s 3 Laws of Motion

Newton's First Law"An object at rest remains at rest, and an object in motion continues in motion with constant velocity unless it experiences a net external force."

This is really Galileo’s Law of Inertia

Here the net external force is zero.

That doesn’t mean that no forces are acting….they could justadd up to zero.

Here, the normal reaction force and weight cancel

Does the normal force of the floor always cancel your weight? Only if the floor is horizontal !

Newton's Second Law• "The acceleration of an object is directly proportional to the net external force acting on the object and inversely proportional to the mass of the object.“

a=F/mor as commonly known

F= ma

Newton's Second Law

• What would increase the rate at which this guy accelerates backwards?

Newton's Third LawFor every action there is an equal and opposite reaction.

Newton's Third Law

The amount of friction between two surfaces depends on 3 factors:

A man pushes a 50 kg load on a cart with 90 Newtons of force . Find the cart’s acceleration.

Wto men each push a 50 kg load on a cart with 90 Newtons of force each. Find the cart’s acceleration.

A man pushes a 100 kg load on a cart with 90 Newtons of force . Find the cart’s acceleration.

Free body diagram: a drawing of all the forces acting on a body. It is helpful to isolate the body first by drawing a circle around it. All objects that cut the circle provide a contact force. Invisible forces that may be present are weight and friction.

The force of gravity I also called….. weightFgravity W

W = mgWhere g = 9.81 m/s2

and is called“the acceleration of gravity”

The General Strategy

for solving problems involving force

1. Draw FBD 2. Say F = ma in one direction at a a time.

3. Solve for desired variable

A Static Problem: where F = 0

A 300 N flower basket is held up by two vertical ropes. Find the tension in each rope.

1. Draw FBD

2. Say F = ma in one direction at a a time.

3. Solve for desired variable

As you pull an object, friction grows to stop you, until the rough interlocking ridges between surfaces finally gives. This maximum value is called “static friction”

After your moving, the rough ridges don’t lock as tightly. You have less friction…..

Called “kinetic friction”

A 4 kg block is pushed up with 50 N of force. Find its acceleration.

Sally pushes an old fashioned lawnmower at a constant speed of 3 m/s. She pushes with 100 N at an angle of 65°.

Draw a FBD and find the vertical and horizontal components of her push.

How much friction must there be?

Find the horizontal force on the box.

NO FRICTION

Draw FBD

Find the vertical component of the pull of the rope.

If the box has a mass of 8 kg

Find its acceleration

Find its weight Find the normal force that the ground pushes up.