a.) Initially, the car has a speed of 80 km/hr so has a KE of 1/2 Mv2; we need to be sure to convert the speed into SI units. After 50 m, the car is at rest, so its KE is zero.
b.) The total work done on the car (the work done by the net force) equals the change in KE.
total work done on car = Fnetd = Fnet x 50 mc.) The force on the car causing it to stop is applied at the only point of contact the car has -- the ground! The ground pushes on the car opposite to the car's motion (that's the meaning of the minus sign above), causing it to stop. You can tell this, because a car's tires get hot from the friction forces between them and the road.
total work done on car = change in KE
-> Fnet = -1.2 x 105J / 50 m = -2.4 x 103 Nd.) The KE of the cars motion is turned into heat energy (the car's tires, the brakes and the road get hot) so the work done on the car is not recoverable.
S11 Click here to read the question a.) The ball's potential energy (PE) gets smaller by an amount Mgh, where h is the height through which it falls (10 m). This energy change results from the work done on the ball by gravity. Numerically,change in PE = -Mgh = -(1.0 kg)(9.8 m/s2)(10 m) = -98 Jb.) We can find the speed in one of two ways. The first way is to realize the ball has a constant acceleration (9.8 m/s2 downward), and we can then find the time it takes for the ball to fall. This is harder than the second way. Since the work done by gravity is recoverable we can say that the PE lost equals the kinetic energy (KE) gained by the ball. Initially, the ball's KE is zero. So
c.) It's the force of gravity that does work on the ball.
d.) The work done on the ball equals the energy changed from PE to KE, or 98 J.
e.) Technically no, the work done is not increasing the potential energy, it is decreasing it! We haven't converted energy into a form that we can tap into again.S12 Click here to read the question a.) The total work done on the car is the work done by the net force.
total work = Fnetd = (1,000 N)(50 m) = 5 x 104Jb.) The total work changes the car's kinetic energy (KE). Initially, the car is at rest, so its KE is zero. After traveling a distance of 50 m, the car moves with some unknown speed v, so
c.) When the car moves with constant velocity, its acceleration is zero. By Newton's second law, this means that the net force is zero.d.) The total work is the work done by the net force. After 50 m, the car moves with constant velocity so the net force is zero. This means that the total work done on the car after 50 m is zero. Another way to see this is that the car's speed, hence its KE, isn't changing. This means the total work is zero.Rossing 1.10 Calculate the kinetic energy (KE) of a 1500-kg automobile with a speed of 30 m/s. If it accelerates to this speed in 20 s, what average power has been developed? KE = 1/2 Mv2 = 1/2 (1500 kg)(30 m/s)2 = 6.8 x 105 JThe total work done equals 6.8 x 105 J; i.e., this is the change in kinetic energy. The average power is the total work divided by the time interval, or
average power = 6.8 x 105 J / 20 s = 3.4 x 104 WReturn to P105 Course Schedule
Last updated: 10 Sep 1999
Comments: bland@indiana.edu