Physics Chapter 6 Concepts

Distinguish between a force and an impulse.

An impulse is force x time

In terms of impulse and momentum, why do padded dashboards make automobiles safer?

Padded dashboards increase the time of impact while reducing the force of impact; impulse reduces momentum

Why would it be a dangerous mistake for a bungee jumper to use a steel cable instead of an elastic cord?

The lack of stretch in the steel cable minimizes time and maximizes force

How much impulse stops a 50-kg carton that is sliding at 4/ms when it meets a rough surface? (I = △mv)

200 N s

What is the momentum of a 50-kg carton that slides at 4 m/s across an icy surface? (p = mv)

200 kg m/s

What impulse occurs when an average force of 10 N is exerted on a cart for 2.5 s? (impulse = Ft)

25 N s

What does it mean to say that momentum is conserved?

A quantity is conserved if its magnitude before the event is the same as after the event

Which exerts the greater impulse on a steel plate: machine gun bullets bouncing from the plate or the same bullets squashing and sticking to the plate?

Bouncing bullets

Railroad car A rolls at a certain speed and makes a perfectly elastic collision with car B of the same mass. After the collision, car A is observed to be at rest. How does the speed of car B compare with the initial speed of car A?

Car B and car A have the same speed before the collision

In a movie, the hero jumps straight down from a bridge onto a small boat that continues to move with no change in velocity. What physics is being violated here?

Conservation of momentum (the boat would need to slow down due to friction)

Two automobiles, each of mass 1000 kg, are moving at the same speed, 20 m/s, when they collide and stick together. In what direction and at what speed does the wreckage move (a) if one car was driving north and one south and (b) if one car was driving north and one east?

Conservation of momentum. (a) Momentum is zero after collision, no direction or speed. (b)

Many years ago, automobiles were manufactured to be as rigid as possible, whereas today's autos are designed to crumple upon impact. Why?

Crumpling increase the time to reduce the momentum of a car, resulting in less impact on the people inside

If you toss a ball upward, is the momentum of the moving ball conserved? Is the momentum of the system consisting of the ball + Earth conserved?

For just the ball, momentum changes and is not conserved; for the larger system momentum is conserved due to the forces acting being only internal

A car carrying a 75-kg test dummy crashes into a wall at 25 m/s and is brought to rest in 0.1 s. Show that the average force exerted by the seat belt on the dummy is 18,750 N.

Ft =△mv F =△mv/t

Bronco dives from a hovering helicopter and finds his momentum increasing. Does this violate the conservation of momentum?

If the system is just Bronco then the external forces of the world act to change momentum

Why are the impulses that colliding objects exert on each other equal and opposite?

Impulse is force x time and newton's third law states that they are equal and opposite forces on each other

What are the two ways to increase impulse?

Increasing force or increasing time

To impart the greatest momentum to an object, should you exert the larges force possible, extend that force for as long a time as possible, or both?

Largest force and longest time

Why can you throw an egg at a sheet without it breaking, but at the same speed into a wall it breaks?

Longer impact time on the sheet allows for less force on the egg, and less chance of breakage

A 5-kg fish swimming at 1 m/s swallows an absentminded 1-kg fish swimming toward it at a speed that brings both fish to a halt immediately after lunch. Show that the speed of the approaching smaller fish before lunch must have been 5 m/s.

Momentum(before) = Momentum(after)

Is a head-on collision between two cars more damaging to the occupants if the cars stick together or if the cars rebound upon impact?

More damage occurs when they bounce, due to a greater impulse

Can the internal forces within a soccer ball produce an impulse on the soccer ball that will change its momentum?

No they cannot, it must be an external force

Can you produce a net impulse on an automobile if you sit inside and push on the dashboard?

Only external forces produce changes in momentum, so no you cannot as it is an internal force.

Suppose a ball of putty moving horizontally with 1kg m/s of momentum collides with and sticks to an identical ball of putty moving vertically with 1 kg m/s of momentum. What is the magnitude of their combined momentum?

Since they are same-magnitude vectors at right angles to each other, the combined momentum is √2 kg m/s

How can a fully dressed person at rest in the middle of a pond on perfectly frictionless ice get to shore?

The absence of friction means that the person would have to throw keys or some other object, which will exert an equal and opposite force on the person and push them across

If only an external force can change the velocity of a body, how can the internal force of the brakes bring a moving car to rest?

The force of the road on the tire is what stops the car, not the brake

If a tennis ball and a bowling ball collide in midair, does each undergo the same amount of momentum change?

Yes, due to newton's third law