Physics - Projectiles

Question

1. The resultant of a number of forces acting on a moving wheelbarrow is 35N due East. Explain what is meant by the resultant of a number of forces.

 

2. The Highway Code indicates that the shortest braking distance for a car stopping from a speed of 18 ms^-1. on a dry road surface is 24m. Calculate the average deceleration for this car.

Answer 1

1. The resultant of a number of forces is the combination of all forces acting on an object. For example in a wheelbarrow you will have the force of gravity, the normal force, the force applied by the user upwards, the force applied by the user forwards, and the frictional force on the wheels. The combination of all these forces comes out to be 35N due east

 

2. So the car has to go from traveling 18 m/s to 0 m/s in 24 m the formula for average acceleration is 

 

The work done by the brakes on the car can be calculated using the formula

 

W = F*d and F = ma

 

so 

 

W = m*a*d

 

W is also equal to the change in kinetic energy, which = 0.5mv²

 

So you have 

 

0.5mv₂² - 0.5mv₁² = mad

 

v₁ = 0 so you have 

 

0.5v₂² = ad  (mass cancels)

 

average acceleration is equal to v₂²/(2d) = a

 

a = 6.75 m/s²