AP Physics work&energy

How much work is done by a 15 N force applied to a 5kg box on a frictionless surface which

moves 3 m?

W=Fd W=45J

A block of mass 3kg slides along a horizontal surface that has negligible friction except for

one section, as shown below. The block arrives at the rough section with a speed of 5 m/s and

leaves it 0.5 s later with a speed of 3 m/s. What is the magnitude of the work done by the

frictional force exerted on the block by the rough section of the surface?

1/2mv^2-1/2mv^2 =24J

A stuntwoman (mass = 60kg) scales a 40 meter tall rock face. If she jumps off the cliff,

what would be her final speed as she landed on a large, air filled cushion lying on the ground?

1/2mv^2=mgh v=28m/s

A 2 kg block is placed at the top of an incline and released from rest near Earth's surface and

unknown distance H above the ground. The angle between the ground and the incline is also

unknown. Frictional forces between the block and the incline are considered to be negligible.

The block eventually slides to the bottom of the incline after 0.75 seconds. The block's velocity

v as a function of time t is shown in the graph starting form the instant it is released. How could

a student use the graph to determine the total energy of the block-Earth system?

Use ½mv f 2 with m v f equal to the final velocity of the object that can be found from

the graph

A rubber ball with mass 0.20 kg is dropped vertically from a height of 1.5 m above a

floor. The ball bounces off of the floor, and during the bounce 0.6 J of energy is dissipated.

What is the maximum height of the ball after the bounce?

mgH-0.6=mgh h=1.2m

An amusement park ride has two identical carriages that revolve around the center of the

ride's axel, as shown in the figure. Both carts travel at a constant tangential speed at all points

along the circular path. At what position is the total mechanical energy of the cart-cart-Earth

system at its maximum value?

The total energy of the Earth-Ferris wheel ride is constant

An automobile traveling on a straight, level road has an initial speed v when the

brakes are applied. In coming to rest with a constant acceleration, it travels a distance x.

How far would the automobile travel in coming to a rest if it had the same acceleration but

an initial speed of 2v?

4x

A cart is moving on a level track in the positive x-direction. A force acting parallel to

the x-axis is exerted on the cart. The graph above shows the net force exerted on the cart as

a function of displacement. As the cart travels from x = 0 m to x = 4 m, what is the net

change in the kinetic energy of the cart?

Increase of 10J

A force F is exerted on a 5kg block to move it across a rough surface, as shown

above. The magnitude of the force is initially 5 n, and the block moves at a constant

velocity. While the block is moving, the force is instantaneously increased to 12 n. How

much kinetic energy does the block now gain as it moves a distance of 2m?

A skydiver jumps from a hovering helicopter that's 3,000 meters above the ground.

If air resistance can be ignored, how fast will he be falling when his altitude is 2,000 m?

1/2mv^2=mgH-mgh v=140m/s

A spring with a force constant of 5.2 N/m has a relaxed length of 2.45 m. When a mass is

attached to the end of the spring and allowed to come to rest, the vertical length of the spring

is 3.57 m. Calculate the elastic potential energy stored in the spring.

P.Ee=1/2kx^2 =3.26N

k is the spring constant. Which of the following is true.

The stiffer the spring the larger the k value

For the system that only includes the object, which of the following claims best

describes what happens to the mechanical energy of the system while the object is moving

form the bottom of the ramp to the top of the ramp?

The mechanical energy of the object is constant because the kinetic energy of the object

is constant.

Blocks X and Y are connected by a string that passes over a pulley. Block Y has more

mass than Block X. The string and pulley have negligible mass and the pulley rotates with

negligible friction. After the blocks are released from rest what happens to the mechanical

energy of the system consisting of the two blocks and Earth?

E mech remains constant

A mover pushes a large crate (mass = 75 kg) from the inside of the truck to the back end

(distance = 6 m), exerting a steady push of 300 N. If he moves the crate in 20 s, what is his

power output during this time?

P=Fd/t =90W

A force of 200 N is required to keep an object sliding at a constant speed of 2 m/s across a

rough floor. How much power is being expended to maintain this motion?

P=Fv =400W

A 40 N box is pulled 5 m up along a 37 o plane. What is the work done by the weight

(gravitational force) of the box?

W=Fd= 5(40)cos(53)=-120W

Can work be done on a system if there is no motion?

No, because of the way work is defined

If you push twice as hard against a stationary brick wall, the amount of work you do:

remains constant at zero

You throw a ball straight up. Compare the sign of the work done by gravity while the ball

goes up with the sign of the work done by gravity while it goes down.

Work is - on the way up and + on the way down

Calculate the work required to compress an initially uncompressed spring with a spring

constant of 20 N/m by 15 cm.

P.Ee=1/2kx^2 =0.225J

You slam on the brakes of your car in a panic, and skid a certain distance on a straight,

level road. If you had been traveling twice as fast, what distance would the car have skidded,

under the same conditions?

It would have skidded 4 times farther

A horizontal force of 200 N is applied to move a 55kg cart (initially at rest) across a 10 m

level surface. What is the final speed of the cart?

W=Fd=1/2mv^2 =8.5m/s