PHYS 104 Chapter 7 homework

Show that about 786 W of power is expended when a 500 N barbell is lifted 2.2 m above the floor in 1.4 s

P = W/t = Fd/t = (500 N)(2.2 m)/(1.4 s) = 786 W

Show that the gravitational potential energy of a 1000 kg boulder raised 5 m above ground level is 50,000 J

PE = mgh = (1000 kg)(10 N/kg)(5 m) = 50,000 N.m = 50,000 J

A car that was moving at 50 km/h skids 15 m with locked brakes. How far will the car skid with locked brakes if it was moving at 150 km/h?

At three times the speed, it has 9 times (32) the KE and will skid 9 times as far—135 m. Since the frictional force is about the same in both cases, the distance has to be 9 times as great for 9 times as much work done by the pavement on the car.

Show that 2,500,00 J change in kinetic energy occurs for an airplane that is moved 500 m in takeoff by a sustained force of 5000 N

From W = ∆KE, ∆KE = Fd = (5000 N)(500 m) = 2,500,000 J

An apple hanging from a limb has potential energy because of its height. If it falls, what becomes of this energy just before it hits the ground? When it hits the ground?

Immediately before hitting the ground its initial PE becomes KE. When it hits the ground its energy becomes thermal energy.

Calculate the work done in lifting a 500 N barbell 2.2 m above the floor. (What is the gain of potential energy of the barbell when it is lifted to this height?)

W=Fd=(500N)(2.2m)=1100Nm=1100J which is also the gain in PE

The mass and speed of the three vehicles, A (800kg 1.0 m/s), B (1000 kg 2.0 m/s), and C (90 kg 8.0 m/s), are shown. Rank them from greatest to least for the following: a. momentum b. kinetic energy c. work done to bring them up to their respective speeds from rest

a. B, A, C b. C, B, A c. C, B, A

The roller coaster rise starts from rest at point A. Rank these quantities from greatest to least at each point: a. speed b. KE c. PE

a. D, B, C, E, A b. D, B, C, E, A c. A, E, C, B, D

A force sets an object in motion. when the force is multiplied by the time of its application, we call the quantity impulse, and an impulse changes the momentum of that object. what do we call the quantity force multiplied by distance?

force multiplied by distance is work

When the speed of a moving car is doubled, how much more kinetic energy does it have?

four times as much (velocity is squared)

cite an example in which a force is exerted on an object without doing work on the object

no work is done in pushing a stationary wall

If you push a crate horizontally with 100 N across a 10 m factory floor and the friction between the crate and the floor is a steady 70 N, how much kinetic energy does the crate gain?

∆KE = work done = (100 N - 70 N)(10 m) = (30 N)(10 m) = 300 N⋅m = 300 J