Physics- Chapter 9 and 10

 A man accelerates a crate across a rough, level floor with an efficiency of 50%. Which is true?

• Energy would still not be conserved if the man were to push the crate at an angle.

• Part of the man's work goes into the kinetic energy of the crate, while the other part of his work is done against friction.

• If the floor were smooth, the man's efficiency would decrease.

• The work done by the crate equals the work done on the crate.

• Energy is not conserved, because the efficiency is less than 100%

Answer explanation

If efficiency is 50%, then that means that only half of the work the man does actually adds to the kinetic energy of the crate. The other half is lost to heat by friction.

A horizontal spring is compressed 0.17 m and has a spring constant of 1000 N/m. If this spring is used to launch a 7.0 kg ball, what is the velocity of the ball just as it leaves the spring?

• 0.35 m/s

• 0.100 m/s

• 2.0 m/s

• 1.0 m/s

• 8.1 m/s

Answer explanation

PE=KE

Ryan's toboggan is sliding across a level, icy field at a speed of 8.0 m/s. If the coefficient of kinetic friction between the toboggan and ground is 0.20, how far does Ryan travel before coming to a stop?

• 33 m

• 3.3 m

• 16 m

• 160 m

• 2.0 m

Answer explanation

W=ΔKE

Fd=μmgd=1/2mv²

The mass m = 2.8 kg swings as shown on this illustration. If the maximum height h = 17 cm, what is the maximum velocity of the ball?

• 7.4 m/s

• 3.3 m/s

• 1.8 m/s

• 4.7 m/s

• 18 m/s

Answer explanation

PE=KE

A small engine is rated at 5.0 horsepower (1 hp = 746 W). How much mechanical energy can the engine produce in 50 s?

• 0.10 J

• 10 J

• 250 J

• 3700 J

• 1.9×10⁵ J

Answer explanation

E = P×t = (3730 W)(50.0 s) = 1.87×10⁵ J

How much time is required for a machine to do 3000 J of work if it has a power output of 80 W?

• 55 s

• 38 s

• 160 s

• 2.4×10⁵ s

• 80 s

Answer explanation

t = E/P = (3000 J) ÷ (80.0 W) = 37.5 s

A 8.0 kg ball rolls off of a 2.0 m high cliff traveling at 20.0 m/s. If it lands on a spring with a spring constant of 30000 N/m, how far can it compress the spring? (Assume the spring is pointed in the direction of the ball.)

• 0.12 m

• 0.17 m

• 0.10 m

• 0.34 m

• 0.33 m

Answer explanation

Ep+Ek=Ep

mgh+1/2mv²=1/2kx²

George does 36 joules of work to lift a 8.0 kg box at a constant speed. If he drops it, how fast will the box be going when it hits the ground?

• 2.1 m/s

• 4.5 m/s

• 9.0 m/s

• 8.5 m/s

• 3.0 m/s

Answer explanation

In lifting the box, George provides 36 J of gravitational potential energy (GPE) to it. When he drops it, this is converted to kinetic energy (KE). At the point when the box hits the ground, all of the GPE will be converted to KE. Using energy conservation, we have E_p=E_k

What is the minimum force required to increase the energy of a car by 140 J over a distance of 41 m? Assume the force is constant.

• 5700 N

• 0.29 N

• 140 N

• 180 N

• 3.4 N

Answer explanation

We can rearrange W = Fd to F = ^W⁄_d . Work done is equal to the change in the car's energy, so W = 140 J and F = ^{140 J}⁄_{41 m} = 3.41 N.

The driver of a 1400 kg car traveling at 25 m/s suddenly sees a stop sign 80 m away. How much force must be applied using the brakes for the car to stop just in time?

• 4.4×10⁵ N

• 5500 N

• 11000 N

• 3.9 N

• 220 N

Answer explanation

W=ΔEk

Fd=1/2mv²