PHYSICS
posted by kim on .
1. A 4.0 N force acts for 3 seconds on an object. The force suddenly increases to 15 N and acts for one more second. What impulse was imparted by these forces to the object?
2. A railroad freight car, mass 15,000 kg, is allowed to coast along a level track at a speed of 2 m/s. It collides and couples with a 50,000 kg second car, initially at rest and with brakes released. What is the speed of the two cars after coupling?
3. Lonnie pictches a baseball of mass .2 kg. The ball arrives at home plate with a speed of 40 m/s and is batted straight back to Lonnie with a return speed of 60 m/s. If the bat is in contact with the ball for .050 s, what is the impulse experienced by the ball?
4. Alex throws a .15 kg rubber ball down onto a steel plate. The ball's speed just before the impact 6.5 m/s, and just after is 3.5 m/s. What is the change in the magnitude of the balls momentum??
ANY HELP WILL BE APPRECIATED.Thanks!

1. Add the two impulses.
2. Apply the law of conservation of total momentum
3. The impulse equals the momentum change. Changing the direction of motion changes the sign
4. Subtract the final momentum from the initial momentum. Make sure you get the sign right on the final momentum
I will be happy to critique your work. 
1. i got 27
2. i got .46
3.still confused
4. " 
1. Correct, but you should include the units, Newtonseconds or kg m/s
2. Correct, but you should include the units of m/s 
could you expand on 3 and 4 please?

3. The momentum change is .2(40+60) = 20 kg m/s. That equals the impulse. You do not need to know the contact time to get the impulse.
4. Do it the same way as 3