Review

Discussion Question 3

Which equations of equilibrium are correct for a wagon being pulled by a force T, moving at a constant speed? (F=friction, N=normal, W=weight)(A) N = W ; T = F(B) N + Tsinθ = W ; F = Tcosθ(C) N + Tcosθ = W ; F = Tsinθ(D) N = W + Tsinθ ; Fsinθ = T(E) none of the above

T

What do you know…

• About an object in static equilibrium

• About an object in translational equilibrium

• About an object that is accelerating?

• A ball is thrown straight up in the air…• Is there a net force?

• A 2 kg ball is falling. Air resistance acting on the ball in 10N. Has the ball reached terminal velocity?

Example 2

Find the tension in the string connecting the blocks and the acceleration of the system. Ignore any friction. Express answer in terms of givens variables and known constants only.

m MF

• Mr. Moos uses a horizontal force F to pull a 5-kg table across the floor at a constant speed of 3 m/s. The frictional force between the block and the floor is 8 N. What is the value of the coefficient of kinetic friction, µk?

ExampleA block with mass m is at rest on a ramp. Draw a FBD and write the equations for equilibrium.

θ

m

Which object has a net force?

• A ball falls straight down through the air under the influence of gravity. There is a retarding force R on the ball with magnitude given by R = gs, where s is the speed of the ball and g is a positive constant. What is the magnitude of the acceleration a of the ball?

Find tension in horizontal rope

Two blocks are pushed along a horizontal friction less surface by a force of 20 newtons to the right, as shown above. What is the

force that the 2 kilogram block exerts on the 3 kilogram block?‑ ‑

Solution

a= 20N/5kg = 4 m/s/s for the system

However, a 20N force on just the 3kg block should accelerate the block at a higher rate. The difference comes from the force that the 2kg block pushes back which is equal and opposite…

F =ma = 4m/s/s * 2kg = 8N

Good problems to look at

• Homework 23, #2 and #4

• Object accelerating upward on a string

• Atwood machine