Motion & ForcesMotion & Forces

Defining ForceDefining Force ForceForce Newton’s First LawNewton’s First Law FrictionFriction

Newton’s First LawNewton’s First Law

Newton’s First Law of MotionNewton’s First Law of Motion An object at rest will remain at rest An object at rest will remain at rest

and an object in motion will continue and an object in motion will continue moving at a constant velocity unless moving at a constant velocity unless acted upon by a net force.acted upon by a net force.

ForceForce

ForceForce a a push or pull that one body push or pull that one body exerts on anotherexerts on another What forces are being What forces are being

exerted on the football?exerted on the football?

Fkic

k

Fgrav

ForceForce

The force exerted by the players foot is greater The force exerted by the players foot is greater than the forces keeping the ball at rest.than the forces keeping the ball at rest.

Therefore, the resultant force is in the direction Therefore, the resultant force is in the direction of the greater force, the kick.of the greater force, the kick.

Fkic

k

Fgrav

ForceForce

BalancedBalanced ForcesForces forces acting forces acting

on an object on an object that are that are opposite in opposite in direction and direction and equal in sizeequal in size

no change in no change in velocityvelocity

ForceForce

Sometimes it is obvious that a force Sometimes it is obvious that a force has been applied. has been applied.

But other forces aren't as noticeable.But other forces aren't as noticeable. A force can cause the motion of an A force can cause the motion of an

object to change.object to change.

ForceForce Net ForceNet Force

unbalancedunbalanced forces that are not forces that are not opposite and equalopposite and equal

velocity changes (object velocity changes (object accelerates)accelerates)

Ffriction

W

Fpull

Fnet

NN

Balanced ForcesBalanced Forces

When two or more forces act on an When two or more forces act on an object at the same time, the forces object at the same time, the forces combine to form the net forcecombine to form the net force

The net force on the box is zero The net force on the box is zero because the two forces cancel each because the two forces cancel each other. other.

Forces on an object that are equal in Forces on an object that are equal in size and opposite in direction are size and opposite in direction are called balanced forces. called balanced forces.

Unbalanced ForcesUnbalanced Forces

When two students are pushing with When two students are pushing with unequal forces in opposite directions, a unequal forces in opposite directions, a net force occurs in the direction of the net force occurs in the direction of the larger force.larger force.

The net force that moves the box will be The net force that moves the box will be the difference between the two forces the difference between the two forces because they are in opposite directions. because they are in opposite directions.

They are considered to be unbalanced They are considered to be unbalanced forces.forces.

Unbalanced ForcesUnbalanced Forces

The net force The net force that acts on that acts on this box is this box is found by found by adding the two adding the two forces forces together.together.

Newton’s First LawNewton’s First Law Newton’s First Law of MotionNewton’s First Law of Motion

““Law of Inertia”Law of Inertia”

InertiaInertia tendency of an object to resist any tendency of an object to resist any

change in its motionchange in its motion increases as mass increasesincreases as mass increases

Inertia and MassInertia and Mass

InertiaInertia is the tendency of an object is the tendency of an object to resist any change in its motionto resist any change in its motion

If an object is moving, it will have If an object is moving, it will have uniform motion.uniform motion.

It will keep moving at the same It will keep moving at the same speed and in the same direction speed and in the same direction unless an unbalanced force acts unless an unbalanced force acts on iton it

What happens in a What happens in a crash?crash?

The law of inertia can The law of inertia can explain what happens explain what happens in a car crash.in a car crash.

When a car traveling When a car traveling about 50 km/h collides about 50 km/h collides head-on with head-on with something solid, the something solid, the car crumples, slows car crumples, slows down, and stops within down, and stops within approximately 0.1 s.approximately 0.1 s.

What happens in a What happens in a crash?crash?

Any passenger not wearing a safety belt Any passenger not wearing a safety belt continues to move forward at the same continues to move forward at the same speed the car was traveling.speed the car was traveling.

Within about 0.02 s (1/50 of a second) Within about 0.02 s (1/50 of a second) after the car stops, unbelted passengers after the car stops, unbelted passengers slam into the dashboard, steering wheel, slam into the dashboard, steering wheel, windshield, or the backs of the front seats.windshield, or the backs of the front seats.

The force needed to slow a person from 50 The force needed to slow a person from 50 km/h to zero in 0.1 s is equal to 14 times km/h to zero in 0.1 s is equal to 14 times the force that gravity exerts on the the force that gravity exerts on the person. person.

What happens in a What happens in a crash?crash?

The belt loosens a little as it restrains The belt loosens a little as it restrains the person, increasing the time it the person, increasing the time it takes to slow the person down.takes to slow the person down.

This reduces the force exerted on the This reduces the force exerted on the person.person.

The safety belt also prevents the The safety belt also prevents the person from being thrown out of the person from being thrown out of the car.car.

Safety BeltsSafety Belts Air bags also reduce injuries in car Air bags also reduce injuries in car

crashes by providing a cushion that crashes by providing a cushion that reduces the force on the car's occupants.reduces the force on the car's occupants.

When impact occurs, a chemical reaction When impact occurs, a chemical reaction occurs in the air bag that produces occurs in the air bag that produces nitrogen gas.nitrogen gas.

The air bag expands rapidly and then The air bag expands rapidly and then deflates just as quickly as the nitrogen deflates just as quickly as the nitrogen gas escapes out of tiny holes in the bag.gas escapes out of tiny holes in the bag.

Concept Test 1Concept Test 1

TRUE or FALSE?TRUE or FALSE?

The object shown in the diagram must be at The object shown in the diagram must be at rest since there is no net force acting on it.rest since there is no net force acting on it.

FALSE! A net force does not FALSE! A net force does not cause motion. A net force cause motion. A net force causes a causes a changechange in motion, or in motion, or acceleration.acceleration.

Taken from “The Physics Classroom” © Tom Henderson, 1996-2001.

Concept Test 2Concept Test 2You are a passenger in a car and not wearing You are a passenger in a car and not wearing your seat belt. your seat belt.

Without increasing or decreasing its speed, the Without increasing or decreasing its speed, the car makes a sharp left turn, and you find yourself car makes a sharp left turn, and you find yourself colliding with the right-hand door. colliding with the right-hand door.

Which is the correct analysis of the situation? ...Which is the correct analysis of the situation? ...

ConcepTest 2ConcepTest 21. 1. Before and after the collision, there is a Before and after the collision, there is a

rightward force pushing you into the door. rightward force pushing you into the door.

2. Starting at the time of collision, the door 2. Starting at the time of collision, the door exerts a leftward force on you. exerts a leftward force on you.

3. both of the above 3. both of the above

4. neither of the above4. neither of the above

2. Starting at the time of collision, 2. Starting at the time of collision, the door exerts a leftward force the door exerts a leftward force on you. on you.