help with newton's law of motion

[tolga]

Q)during preseason football training, Matt was required to run with a bag of sand dragging behind him. The bag of mass 50kg was attached to a rope,which made an angle of 25degrees to the horizontal. When Matt ran with a constant speed of 4m/s, a frictional force of 60N was acting on the bag.
a)Calculate the size of the tension force acting in the rope.
b)What was the magnitude of the force that the rope exerted on Matt as he ran?


Q)Phil is standing inside a tram when it starts off suddenly. Len, who was sitting down, commented that Phil was thrown backwards. Is this a correct statement? explain in terms of Newton's law.
The answer states newtons 1st law then finishes of by saying objects will remain at rest unless a unbalanced force acts to change the motion. when it says change the motion doesn't it mean change in position.

[/0]

Q1

a) From the diagram, we can componentize the forces acting on the bag in the horizontal/vertical directions and balance them.
Since Matt is moving at a constant velocity, .

Balancing in the x-direction:







It turns out we didn't need to balance in the y-direction to solve for T.

b) Since tension is the same throughout the rope, the rope exerts a force of on Matt.

Q2.

By change in motion, Newton's 1st Law means acceleration.

i.e. "An object at rest will remain at rest, and an object moving at constant velocity will remain at constant velocity, unless an unbalanced force accerelates it" (by )

In that question, Phil and Len are initially at rest. As the train starts moving, the seat provides an unbalanced force on Len, which accelerates him in the direction of the train's motion, until he is moving with the train.
Phil, who is standing, only experiences an accelerating force through the friction between his shoes and the train floor. This is not enough, however, to accelerate him to the train's speed. In the train's reference frame, it looks as though he has been 'thrown' back. However, in his reference frame, it is the train which has been 'thrown' forward.

In reality, there is no 'backwards' force acting on Phil. This is a fictional force that Len sees because he is accelerating along with the train.

[tolga]

thanks for that

[tolga]

Phil, who is standing, only experiences an accelerating force through the friction between his shoes and the train floor. This is not enough, however, to accelerate him to the train's speed. i dont really understand this one doesn't he have unbalanced forces acting on him as well.

[/0]

Yes, the friction acting on him is an unbalanced force. However, it is not strong enough to accelerate him so he can match the train's accleration. Since he cannot match the train's acceleration and hence speed, he is 'thrown' back in the accelerating reference frame of the train. The normal force of the seat on Len is far greater, so he is able to match the train's acceleration.

[QuantumJG]

Q)during preseason football training, Matt was required to run with a bag of sand dragging behind him. The bag of mass 50kg was attached to a rope,which made an angle of 25degrees to the horizontal. When Matt ran with a constant speed of 4m/s, a frictional force of 60N was acting on the bag.
a)Calculate the size of the tension force acting in the rope.
b)What was the magnitude of the force that the rope exerted on Matt as he ran?


Q)Phil is standing inside a tram when it starts off suddenly. Len, who was sitting down, commented that Phil was thrown backwards. Is this a correct statement? explain in terms of Newton's law.
The answer states newtons 1st law then finishes of by saying objects will remain at rest unless a unbalanced force acts to change the motion. when it says change the motion doesn't it mean change in position.

Firstly Newton's laws are only defined for an inertial reference frame, Len is in a non-inertial frame.

Newton's first law is the definition of a force, in that an object will remain at a constant velocity iff the net force acting on it is 0. In this case Len is in an accelerating frame that will see Phil accelerating, although there is no force acting on Phil! It's Len's accelating frame that is causing this apparent acceleration. Ultimately leaving the statement as false.

As for the first question, that has already been answered.