3rd Law Newton’s third law (lecture 7) • If object A exerts a force on object B, then object B exerts an equal force on object A For every action there is an in the opposite direction. equal and opposite reaction.

B discuss collisions, impulse, A momentum and how airbags work

B Æ A A Æ B

Example The bouncing ball • What keeps the box on the table if gravity • Why does the ball is pulling it down? bounce? • The table exerts an • It exerts a downward equal and opposite force on ground force upward that • the ground exerts an balances the weight upward force on it of the box that makes it bounce • If the table was flimsy or the box really heavy, it would fall!

Action/reaction forces always You can move the earth! act on different objects • The earth exerts a force on you • you exert an equal force on the earth • The resulting accelerations are • A man tries to get the donkey to pull the cart but not the same the donkey has the following argument: •F = - F on earth on you • Why should I even try? No matter how hard I •MEaE = myou ayou pull on the cart, the cart always pulls back with an equal force, so I can never move it.

1 Friction is essential to movement You can’t walk without friction

The tires push back on the road and the road pushes the tires forward. If the road is icy, the friction force You push on backward on the ground and the between the tires and road is reduced. ground pushes you forward.

Demonstrations Impulse • When two objects • Bouncy and non- collide they exert forces bounce ball on each other that last • Dropping the beakers only a short time • We call these short • Stunt man jumping off lasting but usually of a building strong forces IMPULSIVE forces. • For example when I hit a nail with a hammer, I give it an impulse.

What is impulse Momentum • The term momentum is used quite often in • If a force F acts for a time t, then the everyday conversation about many things. impulse is the Force × time = F × t • For example, you may hear that one team • Since force is measured in Newtons and has the momentum, or that a team has time in seconds, impulse will be measured lost its momentum. in Newton-seconds. • Momentum is a physics term that has a • IMPULSE = F × t very definite meaning. If an object has a mass m and moves with a velocity v, then its momentum is mass × velocity = m × v

2 Momentum = m × v Knock the block over

• In physics, if something has momentum, it The bouncy side knocks doesn’t loose it easily and if it doesn’t have the block over but not the it, it doesn’t get it easily – something has non-bouncy side to happen to an object to change its momentum • Impulse can change momentum, in fact Impulse = change in momentum • If an object gets an impulse, F × t, then its momentum changes by exactly this amount

Elastic and inelastic Collisions Elastic and inelastic Collisions (bouncy) (non-bouncy) (bouncy) (non-bouncy)

Which ball experiences the largest upward force when it hits the ground? Force on Force on The ball The ball Force on The ball

Bouncing ball Physics explains it!

• The force that the ball exerts on the • Beakers dropped from ground is equal to and in the opposite same height so then direction as the force of the ground on the have the same velocity ball. (and momentum) when they get to the bottom. • The ball that bounces back not only must be stopped, but must also be projected • One falls on a hard surface back up. • The other falls on a • The ground exerts more force on the ball cushion. that bounces than the ball that stops. hard soft

3 • Why prevents the beaker that falls on the • The beaker that shatters comes to rest cushion from breaking? more quickly than the one that gently slows • First, what causes anything to break? down on the cushion Å this is the key point! • If an object experiences a large enough • According to the impulse-momentum FORCE then it might break. relation: • Why does the beaker that falls on the Impulse = Force × time (F × t) cushion experience a smaller force? = change in momentum • Both beakers have the SAME change in •F × t is the same for both. Since the one on their momentum – they both hit the bottom the cushion takes longer to slow down the with the same speed and both end up with force on it is less, t is biggerÆ F smaller zero velocity.

Air bags Read more about it

• The same thing is true for airbags http://www.physicsclassroom.com/Class/momentum/U4L1c.html • They protect you by allowing you to come to rest more slowly, then if you hit the http://www.geocities.com/thesciencefiles/homerun/homerun.html http://www.k12.nf.ca/gc/Science/Physics3204/Projects2003/SlotA/P steering wheel or the dash board. rojectA2/index1.htm • Since you come to rest more slowly, the force on you is less. • You will hear that “airbags slow down the force.” this is not entirely accurate but it is one way of thinking about it.

Momentum and Collisions Conservation of Momentum

• The concept of momentum is very useful • One consequence of Newton’s 3rd law is when discussing how 2 objects interact. that if we add the momentum of both • Suppose two objects are on a collision objects before the collision it MUST be the course. AÎÍB same as the momentum of the two objects after the collision. • We know their masses and speeds before they hit • This is what we mean by conservation: when something happens (like a collision) • The momentum concept helps us to see something doesn’t change – that is very what can happen after they hit. useful to know because collisions can be very complicated!

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