Biology – Premed Windsor University School of

Medicine and Health Sciences

J.C. RoweCourse Instructor.

Pre Med – Physics Chapter 2

Forces and Laws of Motion

There is more to lectures than the power point

slides!

Engage your mind

LAW OF MOTIONLAW OF MOTION

“If I have ever made any valuable discoveries, it has been owing more to patient attention, than to any other talent”

Isaac Newton

What is the law of motion ?

• Newton's laws of motionNewton's laws of motion are three physical laws which provide relationships between the forcesforces acting on a body and the motionmotion of that body.

Newton’s Laws of MotionNewton’s Laws of Motion

• 11stst Law Law – An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

• 22ndnd Law Law – Force equals mass times acceleration.

• 33rdrd Law Law – For every action there is an equal and opposite reaction.

Newton’s first law

An object at rest tends to stay at rest and object in motion tends to stay in motion unless acted upon by an unbalanced force.

What does this mean ?!

Basically, an object will keep doing what it was doing, unless acted on by an unbalanced force.

If the object was sitting still, it will remain stationary. If it was moving at a constant velocity, it will keep moving.

It takes force to change the motion of an object.

What is meant by unbalanced force?

SO

If the forces on an object are equal and opposite, they are said to be balanced, and the object experiences no change in motion. If they are not equal and opposite, then the forces are unbalanced and the motion of the object changes.

Balanced vs Un-Balanced

• A balanced force occurs if two opposite but equal forces act an object.

• An unbalanced force occurs on an object when two forces:

a. of different sizes act in the opposite direction b. are acting in the same direction

• An object moves if there is an unbalanced force acting on it.

Newton’s First Law is also called the Law of Inertia

Inertia: the tendency of an object to resist changes in its state of motion

Inertia: the tendency of an object to resist changes in its state of motion

The First Law states that all objects have inertia. The more mass an object has, the more inertia it has (and the harder it is to change its motion).

More Examples from Real LifeMore Examples from Real Life

A powerful locomotive begins to pull a long line of boxcars that were sitting at rest. Since the boxcars are so massive, they have a great deal of inertia and it takes a large force to change their motion. Once they are moving, it takes a large force to stop them.

If objects in motion tend If objects in motion tend to stay in motion, why to stay in motion, why dondon’’t moving objects t moving objects keep moving forever?keep moving forever?

Things don’t keep moving forever because there’s almost always an unbalanced force acting upon it.

A book sliding across a table slows down and stops because of the force of friction.

If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

Once airborne, unless acted on by an unbalanced force (gravity and air – fluid friction), it would never stop!

If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

Objects on earth, unlike the frictionless space the moon travels through, are under the influence of friction.

• There are four main types of friction:– Sliding friction: ice skating– Rolling friction: bowling– Fluid friction (air or liquid): air or water resistance– Static friction: initial friction when moving an

object

What is this unbalanced force that acts on an What is this unbalanced force that acts on an object in motion?object in motion?

Newtons’s 1st Law and You

Please don’t let this be you. Wear seat belts.Please don’t let this be you. Wear seat belts.

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

NewtonNewton’’s Second Laws Second Law

Force equals mass times acceleration.

F = ma

AccelerationAcceleration: a measurement of how quickly an object is changing speed.

What does What does F = maF = ma mean? mean?Force is directly proportional to mass and acceleration.

Imagine a ball of a certain mass moving at a certain acceleration. This ball has a certain force.

Now imagine we make the ball twice as big (double the mass) but keep the acceleration constant. F = ma says that this new ball has twice the force of the old ball.

Now imagine the original ball moving at twice the original acceleration. F = ma says that the ball will again have twice the force of the ball at the original acceleration.

More about More about F = maF = maIf you double the mass, you double the force.

If you double the acceleration, you double the force.

What if you double the mass and the acceleration?

(2m)(2a) = 4F

Doubling the mass and the acceleration quadruples the force.

So . . . what if you decrease the mass by half? How much force would the object have now?

What does What does F = maF = ma say? say?

F = maF = ma basically means that the force of an object comes from its mass and its

acceleration.

Something very massive (high mass) that’s changing speed very slowly (low acceleration),

like a glacier, can still have great force.

Something very small (low mass) that’s changing speed very quickly (high acceleration), like a bullet, can still have a great force. Something very small changing speed very slowly will have a very weak force.

What does What does F = maF = ma say? say?

Can A Coin Kill ?Can A Coin Kill ?

2nd Law (F = m x a)

• How much force is needed to accelerate a 1400 kilogram car 2 meters per second/ second?

• Write the formula• F = m x a• Fill in given numbers and units• F = 1400 kg x 2 meters per second/second• Solve for the unknown• 2800 kg-meters/second/second or 2800 N

If mass remains constant, doubling the acceleration, doubles the force. If force remains constant, doubling the mass, halves the acceleration.

Newton’s 2nd Law proves that different masses accelerate to the earth at the same rate, but with different forces.

• We know that objects with different masses accelerate to the ground at the same rate.

• However, because of the 2nd Law we know that they don’t hit the ground with the same force.

F = maF = ma

98 N = 10 kg x 9.8 m/s/s98 N = 10 kg x 9.8 m/s/s

F = maF = ma

9.8 N = 1 kg x 9.8 m/s/s9.8 N = 1 kg x 9.8 m/s/s

Check Your Understanding

1. What acceleration will result when a 12 N net force applied to a 3 kg object? A 6 kg object?

2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass.

3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?

4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?

Check Your Understanding

1. What acceleration will result when a 12 N net force applied to a 3 kg object?

12 N = 3 kg x 4 m/s/s 2. A net force of 16 N causes a mass to accelerate at a rate of 5

m/s2. Determine the mass. 16 N = 3.2 kg x 5 m/s/s

3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?

66 kg-m/sec/sec or 66 N

4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?

9800 kg-m/sec/sec or 9800 N

NewtonNewton’’s Third Laws Third Law

For every action there is an equal and

opposite reaction

3rd LawFlying gracefully Flying gracefully through the air, through the air, birds depend on birds depend on Newton’s third Newton’s third law of motion. As law of motion. As the birds push the birds push down on the air down on the air with their wings, with their wings, the air pushes the air pushes their wings up their wings up and gives them and gives them lift.lift.

What does this mean?What does this mean?

For every force acting on an object, there is an equal force acting in the opposite direction. Right now,

gravity is pulling you down in your seat, but Newton’s Third Law says your seat is pushing up against you with equal force. This is why you are not moving.

There is a balanced force acting on you– gravity pulling down, your seat pushing up.

3rd Law

• Consider the motion of a car on the way to school. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards.

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).

Think about it . . .