Newton’s 3 Laws and Free Body Diagrams

J. PulickeelSPH4U1

February 2010

Newton’s Three Laws1. An object in motion tends to stay in motion

and an object at rest tends to stay at rest, unless acted upon by an external force

2.

3. For every action, there is an equal and opposite reaction. Equal in magnitude and opposite in direction.

Explain what’s Happening using Newton’s Laws….

In the picture below, Kent Budgett is pulling upon a rope which is attached to a wall. In the bottom picture, the Kent is pulling upon a rope which is attached to an elephant. In each case, the force scale reads 500 N. Kent is pulling ...

a. with more force when the rope is attached to the wall.

b. with more force when the rope is attached to the elephant.

c. the same force in each case.

While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus?

3. Many people are familiar with the fact that a rifle recoils when fired. This recoil is the result of action-reaction force pairs. A gunpowder explosion creates hot gases which expand outward allowing the rifle to push forward on the bullet. Consistent with Newton's third law of motion, the bullet pushes backwards upon the rifle. The acceleration of the recoiling rifle is ...

a. greater than the acceleration of the bullet. b. smaller than the acceleration of the bullet.c. the same size as the acceleration of the bullet.

Free Body DiagramsList all the forces acting on the

hockey puck The Normal Force: The force acting perpendicular to the surface of contact

NSs FF

NKk FF

The Force of Gravity: The force pulling an object towards the centre of the earthForce of Friction: the force resisting the motion of solid surfaces.

- Static Friction: friction between two solid objects that are not moving

relative to each other

- Kinetic Friction: occurs when two objects are moving relative to each other and rub together.

The Applied Force: The force applied.

Calculating the Forces... The Normal ForceIN THIS CASE, the normal force has the same magnitude as the Force of Gravity.

NSs FF

NKk FF

The Coefficient of Friction (µ)The coefficient of Friction a dimensionless scalar value which describes the ratio of the force of friction between two bodies and the force pressing them together.

The coefficient of friction is an empirical measurement – it has to be measured experimentally, and cannot be found through calculations

This value will always be given to you!

FBD: Level Planes vs. Ramps

The block is not moving forward because there is no horizontal applied force.

What would the FBD look like if a force was applied, but the block was not moving?

What would the FBD look like if a force was applied, but the block was moving?

FBD: Tension

What would the FBD for the following setup look like?

FA FB

FC

9.8N

Is the object in motion or at rest?

+x

+y

FBD: Tension

The sign below hangs outside the physics classroom, advertising the most important truth to be found inside. The sign is supported by a diagonal cable and a rigid horizontal bar. If the sign has a mass of 50 kg, then determine the tension in the diagonal cable which supports its weight.

Since the mass is 50 kg, the weight is 490 N. Since there is only one "upward-pulling" cable, it must supply all the upward force. This cable pulls upwards with approximately 490 N of force.

Thus, sin(30°) = (490 N ) / (FT).

The tension is 980 N.

FT

Fg

FBD: Level Planes vs. Ramps

An object placed on a inclined surface will often slide down the surface. The rate at which the object slides down the surface is dependent upon how inclined the surface is; the greater the incline, the faster the rate at which the object will slide down it.

You will note that in both cases, the box not only fell down, but it also travelled horizontally. That means there is an x and y Force acting on the box!

FBD: Level Planes vs. Ramps

In this situation, no external force is being applied, but the block slid down the ramp.

This is because some of the force of gravity is pulling the block into the plank of wood (opposite to the Normal Force)

Some of the Force of Gravity is overcoming friction to cause the block to move forward .

This means that Fg is the sum of FN and Fx

+y

+x

PROBLEM 1: A board is inclined at 35°. The forces acting on the block are illustrated. Find the net external force on the block and determine whether it will move.

+y

+x

35°

18 N

22 N

11 N

11 N 18 N

22 N

y

x

35°

35°Fg,y

Fg,x

PROBLEM 1: A board is inclined at 35°. The forces acting on the block are illustrated. Find the net external force on the block and determine whether it will move.

+y

+x

11 N 18 N

22 N

35°

y

x

35°18 N

13 N

13 N

Therefore the net force is 2 N in the positive x direction.