Spring 1999

MWF 11:30

**Instructor**: Dr. Amit Chakrabarti, CW 213, Tel: 532-1625.

# ANSWERS TO PROBLEMS OF Home Work 3

##
1. Constant velocity means zero acceleration and this, in turn, means zero
net force. Now, the weight of the bear is acting downwards( this is the
force due to gravity) and is given by (w=mg) w= 400 kg x 9.8 m/s/s = 3920
newton. Thus, there must be an upward friction force of
3920 newton, so that the net force is zero.

2. a = net force / m = 2 N /2 kg = 1 m/s/s/.

3. Forces from the engines are acting in the same direction, so
they add up. So the total net force is 4 x 30,000 N = 120,000 N.
Now, a= net force / mass = 120,000 N /30,000 kg = 4 m/s/s/.

4. The reaction force of the wall onto you is also 30 N. your acceleration is
given by a = net force on you / mass = 30 N /60 kg = 0.5 m/s/s/.

5. This problem is like the bike problem I worked out in class. We
know net force = mass x acceleration. So the net force acting on the
firefighter is = 80 kg x 4 m/s/s/= 320 N, downwards. This net force
comes from two contribution; force of gravity (= firefighter's weight)
downwards and the friction force acting upwards. The firefighter's
weight is (w=mg) = 80kg x 9.8 m/s/s = 784 N and this is the force
acting downwards. So we find 784N, downwards - friction force, upwards
= net force of 320 N, downwards. This means the friction force is 464
N, upwards.