# Physics - Motion

• Nov 5th 2013, 05:33 PM
Fratricide
Physics - Motion
1. Police are investigating a car accident to determine if the driver was speeding. There are skid marks on the road that are 37.8m long and a witness reports that she heard the skidding for 4 seconds.
a) Calculate the magnitude of the initial velocity of the car.

I did: d = (1/2)(u + v)t
37.8 = 2u
u = 18.9 ms-1

b) Calculate the acceleration of the car during the braking phase.

I did: v = u + at
0 = 18.9 + 4a
a = -4.725 ms-2

2.
A 650 kg sports car is travelling at a speed of 22 ms-1.
a) What is the kinetic energy of the car?

I did: Ek = (1/2)mv2
= (1/2) x 650 x 222
= 157300 J

b) How much work is done by the net force acting on the car to accelerate it from rest to 22 ms-1?

Because W = change in energy, would the answer simply be 157300 J?

3. A ship of mass 5 tonnes is pulling a barge of mass 2 tonnes with a driving force of 25kN. The resistance forces on the ship total 1000N, while the resistance forces acting on the barge are 800N. Calculate the magnitude of the tension force in the coupling between the ship and barge.

I know that F = ma, but don't know where to go from there.
• Nov 6th 2013, 11:22 AM
ebaines
Re: Physics - Motion
Your answers to questions 1 and 2 are correct, including 2b - very good!

As for question 3 - I suggest you start with sum of forces =ma for the boat + barge system. The sum of forces is the driving force of the engine minus the resistance forces on both boat and barge. From this you can calcualte the acceleration of the boat and barge system. Now apply sum of forces = ma for the barge: the forces acting on it are the tension in the rope minus the boat's resistance force. Its mass and acceeration you already know, so you can solve for tension.