NEWTON’S LAWS OF MOTION

The 2nd and 3rd Law

REVIEW

• NEWTON’S FIRST LAW OF MOTION: Every object continues in its state of rest, or uniform velocity in a straight line, as long as no net force acts on it.

• MASS: measure of the inertia of an object

• FORCE: measure of the magnitude and direction of the interactions

Newton’s Second Law of Motion

• Why objects change velocity?• What happens if force acts:

-in direction of motion

-opposite the direction of motion

-sideways• What is the relation between acceleration and

force?• What is the relation between mass and force?

Statement of the 2nd Law

• The acceleration of an object is directly proportional to the net force acting on it, and is inversely proportional to its mass. The direction of the acceleration is in the direction of the net force acting on an object

m

Fa

F

-vector sum of all forces acting on the object (net force)

• If the mass is constant we can write

amF

EQUATION OF MOTION

Force vector can be written in component form in rectangular coordinates as

xx maF yy maF zz maF

Find the acceleration...

m = 5kg

F1=10NF2=5N

…and some more

m = 5kg

F1=10N

Newton’s Third Law of Motion

• Whenever an object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first.

2112 FF

Force on an object and by an object

• How is a car accelerating forward?

• Which force makes us walk?

• Why you feel hurt when you kick the wall?

• How do birds fly?

IMPORTANT: only force exerted on an object influences the motion, not the force exerted by an object itself

Force exertedby runneron the ground

Force exertedby the groundon the runner

Direction of motion

Direction of the tyre rotation

Force by tyre on the ground

Force by ground on the tyre

Force that person exerts on the wall

Force that wall exerts on the person

Air pushes the wing up

Wing pushes the air down

Questions

• 1. While driving, Anna observed a bug striking the windshield of her car. Obviously, a case of Newton's third law of motion. The bug hit the windshield and the windshield hit the bug. Which of the two forces is greater: the force on the bug or the force on the windshield?

Rockets are unable to accelerate in space because ...

A-there is no air in space for the rockets to push off of.

B-there is no gravity is in space.

C-there is no air resistance in space.

D-... nonsense! Rockets do accelerate in space.

• . A gun recoils when it is fired. The recoil is the result of action-reaction force pairs. As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards. The acceleration of the recoiling gun 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.