FORCES

Isaac NewtonLived 1642-1727

Mathematician, Physicist, AstronomerLAWS of Motion

Law of Universal GRAVITATIONDevelopment of Calculus

Newton’s 1st Law • An object at rest stays at rest and an object in

motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

• Key idea: INERTIA

Inertia

• Tendency of an object to resist a change in motion• As mass INCREASES inertia increases

mass = inertia• As mass DECREASES inertia decreases

mass = inertia

• Ex: A big truck has more inertia than a small car. More inertia makes it more difficult for the big truck to start moving, stop moving, or CHANGE DIRECTION

Friction

• A force that works against motion• Makes objects slow down, stop or stay still

Example: While riding a skateboard (or wagon or bicycle), you fly forward off the board when

hitting a curb, a rock or another object which abruptly halts the motion of the skateboard

• Do not let this be you! Wear your seat belt!

• Because of inertia, objects (including you) resist changes in their motion. When the car going 80 mph is stopped by the brick wall, your body keeps moving at 80 mph.

If no force acted on a body it remains at rest OR continues motion along a straight line with constant speed

Newtons 2nd Law

• The acceleration of an object depends on the mass of the object and the amount of force applied

• Describes the motion of an object when an unbalanced force acts on the object

Expressing 2nd Law Mathematically

• The relationship of acceleration (a) to mass (m) and force (F) can be expressed mathematically:

• F = m x a • m = F / a • a = F / m

Acceleration Depends on Mass

The acceleration decreases as its mass increases

The acceleration increases as its mass decreases

mass & acceleration are indirectly proportional

Mass Depends on Force

• An object’s mass increases as the force on the object increases.

• An object’s mass decreases as the force on the object decreases.

• Mass and force are directly proportional. Double the mass you double to force to move the object.

Force and Acceleration

• The direction of the acceleration of an object is always in the direction of the force (push or pull)

Example• What is the acceleration of a 3 kg

mass if a force of 15 N is used to move the mass?

• (Note: 1 N is equal to 1 kg x m/s2)

Example• Suppose you are pushing an empty cart. You

have to exert only a small force on the cart to accelerate it. But, the same amount of force will not accelerate the full cart as much as the empty cart.

Example• Suppose you give the cart a hard push.

The cart will start moving faster than if you gave it only a soft push. The cart will move forward because the push was in the forward direction.

Newton’s 3rd Law

• For every action there is an equal and opposite reaction.

• These forces can be balanced or unbalanced. • Unbalanced forces will result in movement.

Balanced forces are all around us!

• Anything that has mass (everything) will exhibit a force on anything touching it.

• If an object is at rest, then the force it has on other objects is equal to the forces they have on it.

What does this mean?

• The giant boulder exerts a force of 900 N on the ground underneath it.

• Why doesn’t the boulder fall through the Earth?

• The ground exerts a counter force of 900 N back onto the boulder, in the opposite direction.

Even things in motion follow Newton’s 3rd Law:

• A bird flies by use of its wings.• The wings of a bird push air downwards. • The air must also be pushing the bird upwards.• The size of the force on the air equals the size of the

force on the bird• The direction of the force on the air (downwards) is

opposite the direction of the force on the bird (upwards).

• Action-reaction force pairs make it possible for birds to fly.

EXIT TICKET:

• Summarize each law in 1-2 sentences.• Use your OWN WORDS. • You should have 3-6 sentences total for your

summary. • TURN IN YOUR NOTES W/ SUMMARY – I will return these to you tomorrow.