Mechanics questions URGENT HELP PLEASE

Hello I have questions with answers but don't know how to work them out, I tried, but I am stumped.

These are from the A level Physics book by Roger Muncaster.

A railway truck of mass 6.0 tonnes moves with an acceleration of 0.050ms^-2 down a track inclined to the horizontal at an angler A where sin(A)=1/120. Find the resistance to motion, assuming it is constant. (g=10ms^-2, 1 tonne=1.0x10^3kg)

A car of mass 1000kg tows a caravan of mass 600kg up a road which rises 1m vertically for every 20m of its length. There are constant frictional resistances of 200N and 100N to the motion of the car and the motion of the caravan respectively. The combination has an acceleration of 1.2ms^-2 with the engine driving a constant force. Find (a)the driving force (B)the tension in the tow-bar. (g=10ms^-2)

I will put answers in spoilers so you can verify them.

Spoiler

(edited 4 years ago)

Help me out.

@Zacken @have @_gcx @Rohan77642

blud don't ever @ me like we are boys, you don't know me like that G

Reply 9

ThingsFallApart

help me I am in a pickle.

Original post by have

blud don't ever @ me like we are boys, you don't know me like that G

Help

Help me

Original post by ThingsFallApart

Spoiler

I would show you how to attempt the first question. You should draw a free-body diagram to illustrate all the real forces acting on truck and then set up the Newton’s 2^nd law equation to solve for the problem.

There are some unclear points in the question: the truck is moving down the slope under the influence of its weight or a driving force due to the engine.

If there is a driving force, there are 2 unknowns for the questions and we can only set up one useful equation that Newton’s 2^nd law (N2L), so it becomes unsolvable.

Assume that the truck is moving down the slope under the influence of the weight. We would write N2L for the direction along the slope:

mgsin θ – R = ma

R = mgsin θ – ma

Why the component of weight along the slope is mgsin θ? Watch the following videos.

https://www.khanacademy.org/science/ap-physics-1/ap-forces-newtons-laws/inclined-planes-ap/v/inclined-plane-force-components

https://www.youtube.com/watch?v=qvpIv53XkJk

Reply 13

Eimmanuel

Study Forum Helper

The following link has many similar problems with worked solutions and you might want to practise them.
http://problemsphysics.com/forces/inclined_planes_problems.html

Once you are clear with question 1, I would explain question 2. Take care.

Reply 14

ThingsFallApart

Original post by Eimmanuel

I get the first one, I might use this to try the second one and thanks for the videos, although I sort of understood that earlier. Your answer is right too but I don't get why you get ma=mgsintheta - R why does it equal ma? I'll try out those questions.

Reply 15

Eimmanuel

Study Forum Helper

Original post by ThingsFallApart

To understand why

mgsin θ – R = ma

we need to go from F_net = ma and the free-body diagram that you have drawn for the truck that is moving down the slope.

F_net is the vector sum of all the forces acting on the object which is the moving truck in the first problem.

If you draw the free-body diagram of the moving truck, the forces acting on the moving truck parallel to the slope are the component of weight (ie mgsin θ) parallel to the slope and the frictional force (ie R)

See example 5.12 in the following link that is very similar to your question. Figure 5.22 in the link has the free-body diagram that is similar to your problem.
https://cnx.org/contents/[email protected]:dRfHU006@11/5-6-Common-Forces