Physics Motions and Forces

Question

A child is sitting in a wagon and is being pulled by a 10N force along a rough surface (friction exists between

the wagon and ground). The weight of the wagon is 150N (mass is 15 kg). The wagon is initially being pulled

at a constant speed of 5 m/s. The wagon is then accelerated to 8 m/s over 3 seconds because the person

pulling the wagon increased their force.

a. Draw a motion diagram showing the wagon initially moving at a constant speed for 2 seconds, and

then increasing its speed over the next 3 seconds.

b. Use a kinematic equation to calculate that the acceleration of the wagon was 1 m/s 2 during the 3

seconds the force was increased.

c Draw a force diagram of the wagon moving at constant speed

d. Draw a force diagram of the wagon while it was accelerating

e.. While the wagon was accelerating at 1 m/s 2 , the person pulling it applied a force of 25N. If they wanted

to accelerate the wagon at 2 m/s 2 , what force would they have to apply?

Answer 1

a) This depends on what is meant by "motion diagram". It could mean a dot diagram (showing the distance between positions for constant time intervals of a series of dots) or a position versus time graph. For the former, think of what would be happening to the distance between the dots (that represent equal time intervals) while it was moving at a constant speed versus when it was accelerating. For the latter, what is happening to the slope of the position-time graph while its speed is constant versus when it's accelerating. (Remember that the slope of the position-time graph is the velocity.)

b) The average acceleration is the change in velocity (final - initial) divided by the time interval.

c&d) Both diagrams have the same forces, what changes is the magnitude of one of the horizontal forces. Think about what the horizontal acceleration is (and when the horizontal forces are/aren't balanced).

e) The tricky thing here is that in order for the acceleration to double, the NET force has to double. Does that mean the pulling force doubles? Careful.