physics exam 2

A uga running back is running toward you at a constant velocity. From a physics perspective why is it so hard to stop him

Because the running back has a large mass and high velocity

A block rests on a smooth frictionless ramp that is tilted at an angle greater than zero. How does the normal force on the block compare to its weight

The normal force is less than the weight

You push a book horizontally against a wall and hold it there. The book doesn’t slide down. What describes the normal force exerted by the wall on the book

It equals the horizontal force you apply

What is tension

The pulling force transmitted through a string rope cable or similar object when it is pulled tight by forces acting on both ends

Tension acts along the length of the rope and pulls

Equally on objects at both ends

Tension is always directed

Away from the object it’s attached to

Tension always acts in the direction of the

Connector

Pulleys

Change the direction of the force exerted by a string

Ideal pulleys are assumed to be

Massless and frictionless

If two objects move together whether in contact with one another or connected by a string the

Accelerations have the same magnitude

When treating multiple objects as one the free body diagrams and in the 2nd law setup we include only

Those forces which are external to the system

Friction

Comes from surfaces that aren’t completely smooth

Kinetic and static

Both depend on how hard the surfaces are pressed together

Static friction

Surfaces are at rest with respect to eachother

Kinetic friction

Surfaces are moving relative to one another

This type of friction resists the onset of sliding of one surface against another

Static

Static friction provides an opposing force that

Exactly cancels the component of the pushing force parallel to the floor

If you push with a force greater than the max you overcome

Static friction

This type of friction resists the in progress sliding of one surface against another

Kinetic

Kinetic friction provides a force that acts to

Oppose the sliding

Kinetic friction is independent of the

Applied force

A box sits on a flat board. You lift one end of the box making an angle with the floor. As you increase the angle the box will begin to slide why

The component of the gravity force parallel to the plane increased

The normal force exerted by the board decreased

An object is help in place by friction on an inclined surface. The angle of inclination is increased until the object tarts moving. If the surface is kept at this angle the object

Speeds up

You are trying to move your desk by pushing as hard as you can but the desk doesn’t move. What happens to the force you apply

It is balanced by the static friction force so the net force on the desk is zero

A box rests on the rough surface of a movable cart when the cart is pulled to the right with a constant force what is the direction of the friction force acting on the box

Right

A spring has a

Natural length

Not stretched or compressed

Elasticity

when spring is pushed or pulled a force acts to bring it back to its original unstretched length

For an ideal spring the force it exerts is

Proportional to the distance it is stretched or compressed

A block hangs from a vertical spring and stretched it by 10cm under its weight. You then pull the block down an additional 5cm and release it. As the block passes the point where the spring is stretched only by the blocks weight what is the spring force

Upward and equal to the blocks weight

A spring is stretched by a certain amount and then released to return to its original length. Then it is stretched twice the initial amount. How much force does the second case need with respect to the force needed during the first stretching

Twice as much

Nearly everything that stretches compresses bends or twists can be called

Elastic

A change in length is produced when a force is applied to an object

Parallel to its length

Change in length depends on

Original length

Applied force

Cross section

Material property

A sideways deformation is produced when a

Shear force applied to an object perpendicular to its length

Uniform circular motion

An object moving along a circular path at constant speed

Work is formed by a force acting on the system through a

Displacement

Work is

Scalar

Can be positive negative or zero

The dot product is one way of

Multiplying vectors

Yields a scalar result

Work is blank if the force has a component in the direction of motion

Positive

Work is blank if the force is perpendicular to the direction of motion

Zero

Work is blank if the force has a component opposite to the direction of motion

Negative

Energy

Ability to do work

Never created or destroyed

Kinetic energy

The capacity of the object to do work based on its motion

Two students ride on the same rotating merry go round. Alex sits near the center while Jordan sits near the outer edge. Who has the greater angular speed

They have the same

Alex and Jordan sit on the same merry go round with Alex near the center and Jordan near the outer edge. Who has the greater linear speed

Jordan

Two marbles one with twice the mass of the other are lifted from the ground to the roof of the same building. Which marble requires more energy to lift and by what factor

The heavier marble it requires twice as much energy

A car starts from rest and accelerates to 30 mph. Later it gets on the highway and accelerates to 60mph. Which takes more energy 0-30 or 30-60

30-60

Negative work means energy is removed from

The objects kinetic energy

The velocity vector is always

Tangent to the circle

The centripetal force pints

Radially inward

A block is attached to a horizontal spring on a frictionless surface. The block is initially at rest at the springs equilibrium length. You now pull the block to the right stretching the spring. While you are pulling the block to stretch the spring what is the work done by the spring on the block

Negative work because the spring force opposes the displacement

You are trying to push a heavy box across the floor. What describes the work done by friction in each case

Static friction does zero work when the box doesn’t move

A satellite is moving in a perfectly stable circular orbit around earth at a constant speed. The gravitational force provides the necessary centripetal force to keep it in orbit. Over one complete orbit how much work does gravity do on the satellite

Zero work because gravity only changes the direction of velocity not its magnitude

Conservative force

Force which does zero net work on any closed path

The total work done by the force depends only on the initial and final positions and doesn’t depend on the path taken

Ex gravity

Nonconservative force

A force that doesn’t satisfy conditions of conservative force. Work done depends on path taken. Work done on a closed path isn’t zero

Ex friction

Potential energy

Stored energy

Two types of potential energy

Gravitational

Spring

Mechanical energy

Sum of kinetic energy and potential energy

What about forces work and potential energy is correct

Only conservative forces can be associated with a potential energy

Power

Rate at which work is done

Work per time

Momentum is

Conserved under certain conditions

Vector quantity

A system of particles is known to have a total kinetic energy of zero. What can you say about the total momentum of the system

Momentum of the system is zero

A system of particles is known to have a total momentum of zero. The gag can you say about the total kinetic energy of the system

Not enough info to determine the systems total KE

Two objects are known to have the same momentum do these two objects necessarily have the same KE

No

Law of conservation of momentum

The total momentum ptot of an isolated system is constant

Interactions within the system don’t change the systems total momentum

The total momentum of an isolated system is

Conserved

Perfectly elastic collisions

KE of system is conserved

Inelastic collision

KE is not conserved

Perfectly inelastic collision

Objects have same final velocity

Max amount of KE lost